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CTR Tea Seminars

The Center of Turbulence Research organizes tea seminars approximately every other week at 4:00 PM on Friday at CTR conference room, underground floor of CTR building near Building 500, Stanford University. The seminar series presents various speakers with the research areas related to turbulence or flow physics. Light refreshment will be served.

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Date:Friday Jul 12, 2013
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Tianfeng Lu, University of Connecticut, Dept. of Mechanical Engineering
Title:Accommodating Complex Chemistry in Large-Scale Flame Simulations
 
Abstract:Detailed chemistry is an integral component of predictive flame simulations. However, detailed chemical kinetics of realistic fuels can be highly complex due to the large number of species and reactions and the severe chemical stiffness induced by highly reactive radicals. Challenges to accommodate detailed chemistry in large-scale flame simulations are two-fold. First, the large mechanisms must be reduced before they can be applied to multi-dimensional flame simulations, resulting in the need of rigorous and efficient methods for mechanism reduction and stiffness removal. Second, 3-D simulations may involve complex turbulent-flame interactions and output massive simulation data, e.g. in hundreds of terabytes. As such, systematic computational diagnostics are required to extract salient information from the massive simulation results and to understand the complex flame behaviors and the underlying physicochemical processes. For mechanism reduction, a suite of algorithms including directed relation graph (DRG), analytically solved quasi steady state approximations, and dynamic chemical stiffness removal were developed to systematically obtain compact, accurate and non-stiff mechanisms from detailed mechanisms with hundreds or thousands of species, for a variety of fuels ranging from methane to surrogates of gasoline, kerosene, diesel and biodiesel. For computational flame diagnostics, a method of chemical explosive mode analysis (CEMA) was developed to systematically identify limit flame phenomena, such as ignition, extinction and onset of flame instabilities, and other critical flame features, such as premixed flame fronts and non-premixed flame kernels, in complex flame configurations at both laminar and turbulent conditions. In the presentation, the usage of CEMA for flame diagnostics will be demonstrated with Sandia’s recent direct numerical simulations (DNS) of lifted jet flames, homogeneous charge compression ignition (HCCI) combustion, and premixed and non-premixed temporal jet flames.
Date:Friday Apr 12, 2013
Time:4:00pm
Location:CTR Conference Room
Speaker: Dr. Jean-Pierre Hickey, Center for Turbulence Research, Postdoctoral Fellow
Title:Coherent Structures in the Evolution of Sub- and Supersonic Planar Wakes
 
Abstract:Wakes are constitutive components of engineering, aeronautical and geophysical flows. Despite the canonical nature of wakes, many fundamental questions surrounding this simple flow remain unanswered. The present work revisits the nature of archetypal planar splitter-plate wakes in the sub- and supersonic regimes, more specifically addressing the effects of the imbedded coherent structures on the evolution of the flow. In this presentation, three topics will be discussed. Firstly, we will argue that the far wakes maintain a memory of their origin, and the resulting multiplicity of the self-similar states can be tied to a plurality of the large-scale, far wake structural organizations. Secondly, building on linear theory and vortex dynamics, we will infer the concomitant Mach number effects on the emerging structures during the transition of high speed wakes. Finally, we will use critical point theory to explain the possibility of a breakdown mechanism in the inclined vortex ribs of the high speed transitioning wake.
Date:Friday Mar 29, 2013
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Florian Kumer, Fluid Dynamics, Darmstadt University of Technology
Title:Discontinuous Galerkin methods for multi-phase flows
 
Abstract:For the numerical simulation of multi-phase flows, a sharp representation of the fluid interface is a challenging task, especially in the context of thigh-order methods, like the discontinuous Galerkin (DG) method. In order to preserve the hp-convergence property of the DG method, not only an accurate description of the position of the fluid interface is necessary. Furthermore, a special numerical treatment for the jumps and kinks which occur in the pressure and velocity field of the flow is required. Our approach is to track the interface position by a high order level-set method. The discontinuities in the solution will be represented by an adaption of the numerical approximation space. Within the talk, an overview about my research project for developing such a method will be given, and how these attempts are linked to the broader research activities at my home institution in Germany. I will discuss what has been achieved so far, and give an outline on my research activities within the next two years at the CTR.
Date:Friday Mar 15, 2013
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Kourosh Shoele, UC San Diego and Re-Vision Consulting LLC
Title:Fluid Interaction with Structures: From Fish Fins to Hydrokinetic Devices
 
Abstract:Studying the interaction between fluid and structure is a fundamental step in understanding the underpinnings of many engineering and physical phenomena, from energy harvesting to biolocomotion of insects, birds and fishes. The complex nature of these interactions makes the design of computational, experimental, and analytical techniques for modeling such problems challenging. Here I discuss new procedures, both in potential flow and viscous flow, for studying the interactions of a flexible structure with a flow. In particular, I will focus on two particular phenomena, the flow interaction with skeleton-reinforced fish fins and the extraction of ocean energy through oscillating systems. 1) Fins of bony fishes are characterized by a skeleton-reinforced membrane with a soft collagen membrane strengthened by embedded flexible rays. Our results illustrate that the fish’s capacity to control the motion of each individual ray, as well as the anisotropic deformability of the fin, are essential to high propulsion performance. By drawing connections with a set of phenomena, we demonstrate that this structural design is a recurring feature in nature. 2) The ocean offers numerous resources for renewable energy, from winds, waves, currents and tides. The harvesting of ocean energy is a relatively new and emerging research area. I discuss our recent efforts to develop a unified multilevel package for analysis, prediction and control of different floating power systems using Bond graph object-oriented modeling approach. In particular, the results illustrate the benefits of active controlling of wave converters such as a point absorber wave converter and an oscillatory water column (OWC) device.
Date:Friday Mar 8, 2013
Time:4:00pm
Location:CTR Conference Room
Speaker:André Thess, Institute of Thermodynamics and Fluid Mechanics, Ilmenau University of Technology, Germany
Title:Electromagnetic Flow Measurement in Metallurgy, Crystal Growth and Energy Conversion
 
Abstract:The measurement of velocity in liquid metals, semiconductor melts and molten salts is a notoriously difficult problem because these materials are often opaque, hot and aggressive. Hence the development of reliable contactless velocity measurement methods is a challenge with considerable practical impact. Electromagnetic flow measurement is a promising technique to solve this problem. In the first part of the presentation we give a brief overview of the history and new developments of electromagnetic flow measurement starting from Michael Faradays experiments in 1832 to new techniques like eddy current flowmeters, contact less inductive tomography and Lorentz force velocimetry. In the second part of the presentation we discuss several examples that demonstrate the important role of computational fluid dynamics and computational magnetohydrodynamic in the development of such flowmeters. We finally discuss some possible new applications including applications in solar thermal power plants.
Date:Friday Mar 1, 2013
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Sabre Bougrine, Center for Turbulence Research, Postdoctoral Fellow
Title:0-Dimensional Modeling of the Combustion of Alternative Fuels in Spark-Ignition Engines
 
Abstract:The work aimed at proposing a physical combustion chamber system simulation model able to accurately represent the different phenomena associated to all common technologies (GDI, EGR, downsizing, VCR, CAI, etc.) and to the use of alternative fuels (CNG, hydrogen, gasoline, ethanol and liquid or gaseous hydrocarbon mixtures). It has resulted in a new 0D combustion model, CFM1D-TC (Tabulated Chemistry), derived from the 3D CFD ECFM model and integrating several new approaches to describe turbulent premixed flame propagation, auto-ignition and pollutant formation processes.
Date:Friday Feb 15, 2013
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Julian Hunt, University College London, Trinity College Cambridge, TU Delft
Title:Interfacial layers in high Re turbulence-physics and simulations
 
Abstract:Where turbulent flows with or without mean shear are adjacent to regions of very weak turbulence, such as boundary layers wakes, jets, and plumes, or convective layers , quasi-continuous interfacial layers tend to form that separate the turbulent and non-turbulent regions. Measurements and computations show that the mean and variance profiles of velocity and scalars have common global features, such as the outward boundary entrainment velocity’ and discontinuities of the Reynolds stresses .These features vary depending on the strength of the shear . Microscale vortices are common to most interfacial layers. These results can be explained in terms of key mechanisms, including instabilities, shear sheltering, nibbling/engulfment, and energy transfer dynamics in the vicinity of the interfaces. Improvements in parameterisations and in under -resolved simulations are emerging. Computations by Ishihara and Kaneda on the Earth Simulator at Rlamda greater than 1000 and laboratory turbulence at Cambridge at Rlamda up to 450 show how similar thin-layers occur intermittently in the interior of turbulent flows. But there are some important differences. Detailed analysis of the data and a rapid distortion model shows how the eddies impact on the layers leads to a net down scale transport, peak dissipation and microscale velocities of the order of the RMS velocities in the layers. Some of the statistics are consistent with Kolmogorov theory.
Date:Friday Feb 8, 2013
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Charles Tinney, The Univ. of Texas at Austin, Aerospace Engineering and Engineering Mechanics
Title:Cumulative nonlinear distortion of acoustic waves produced by high-speed jet flows
 
Abstract:In supersonic jets, an extended line of distributed sources act in compliance to generate and propagate noise in a complex manner to an observer far away from the jet. Nonlinear distortion of the acoustic waveform is often considered a prerequisite to understanding this propagation process. This is driven by one's capture of 'shock-type' structures or so called 'N-wave' type signatures in the pressure waveform, which are generally attributed to nonlinear wave steepening. Likewise, imperfect collapse of the spectra between an observer signal and a prediction, formed from a linear rescaling of the closer signal (using 1/r2 dependence), is also believed to be caused by nonlinear distortion. Unfortunately, these observations are most often made using measurements acquired in a range-restricted environment where changes to the waveform, due to cumulative nonlinear distortion, are too small to be accurately quantified, and/or without prior knowledge of the sound propagation path. The current work aims to fill this gap by exploring the acoustic field produced by an unheated, perfectly expanded, Mach 3 jet in a laboratory-scale environment. This talk focuses on a time-averaged approach to understanding the degree of non-linearity in the acoustic waveform at several far-field observer positions. Various "off-the-shelf" indicators are explored including a new model for predicting the presence (or lack there of) of cumulative non-linear waveform distortions in the signal.
Date:Friday Jan 18, 2013
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Julien Bodart, Center for Turbulence Research, Postdoctoral Fellow
Title:Wall-modeling for large eddy simulations in complex geometries
 
Abstract:Accurate predictions of the flow field over complex aerodynamic configurations, such as high lift devices is of particular interest for airframe noise predictions as well as wing design with or without active flow control. In particular, computing the flow at high angle of attack and in particular predicting the maximum lift coefficient remains a challenge using RANS solvers. We discuss how large eddy simulation can be used in this context, together with wall-modeling to reduce the grid requirements. I will specifically address two challenges wall-modeling has to face when dealing with complex geometries: i) its implementation in massively parallel, unstructured solvers and ii) its ability to predict transitional flows. The wall-model methodology implemented in the solver CharLES^X relies on flow sensors, to activate the wall model only in turbulent regions of the boundary layers. I will first discuss results obtained for a canonical (H-type) transitional boundary layer, and for the flow around McDonnell-Douglas 30P/30N airfoil, at the realistic Reynolds number (based on the stowed chord) of Re_c=9.10^6.
Date:Friday Dec 14, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Jan Nordström Linkoping University
Title:New Developments for Initial Boundary Value Problems: Time Integration and Dual Consistency for High Order Finite Difference Approximations
 
Abstract:During the last decade, stable high order finite difference methods as well as finite volume methods applied to initial-boundary-value-problems have been developed. The stability is due to the use of so-called summation-by-parts operators(SBP), penalty techniques for implementing boundary and interface conditions, and the energy method for proving stability. In this talk we discuss new aspects of this technique including the relation to the initial-boundary-value-problem. By reusing the main ideas behind the development, new time-integration procedures and increased accuracy as well as new boundary conditions have been derived.
Date:Friday Nov 30, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker: Dr. Ricardo Rossi, Visiting Scholar, Laboratorio di Termofluidodinamica Computazionale, Universita di Bologna, Italy
Title:Scalar Dispersion in Complex Flows: from applied research to an actual case study
 
Abstract:The transport of scalars in turbulent flows is one of the major research topics in many fields of environmental engineering. In this talk, a research project carried out at the Center forTurbulence Research is presented, where scalar dispersion analysis has beenapplied to the transport and mixing of pollutants in the atmosphere and in populated areas. The first part of the talk will provide an overview of research work aimed at improving the understanding and modeling of scalar dispersion in complex flows. Numerical experiments carried out by means of Direct Numerical Simulations (DNS) and then approximated using the Reynolds-Averaged Navier-Stokes (RANS) equations, will be presented to address briefly the role of numerics, scalar flux models and surface topography in dispersion analysis and prediction. In the second part of the talk an actual case study of flow and dispersion on a complex terrain will be presented and discussed, where RANS techniques developed during the research work have been applied to the release of pollutants from the portal of a road tunnel underconstruction in the north of Italy.
Date:Friday Nov 2, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. John Dabiri, Professor, Engineering and Applied Science, Director, Center for Bioinspired Wind Energy, California Institute of Technology
Title:Vortex-enhanced propulsion
 
Abstract:It has been previously suggested that the generation of coherent vortical structures in the near-wake of a self-propelled vehicle can improve its propulsive efficiency by manipulating the local pressure field and entrainment kinematics. We investigated these unsteady mechanisms analytically and in experiments. A self-propelled underwater vehicle was designed with the capability to operate using either steady-jet propulsion or a pulsed-jet mode that features the roll-up of large-scale vortex rings in the near-wake. The flow field is characterized by using a combination of planar laser-induced fluorescence, laser Doppler velocimetry and digital particle-image velocimetry. These tools enable measurement of vortex dynamics and entrainment during propulsion. The concept of vortex added-mass was used to deduce the local pressure field at the jet exit as a function of the shape and motion of the forming vortex rings. The propulsive efficiency of the vehicle was computed with the aid of towing experiments to quantify hydrodynamic drag. Finally, the overall vehicle efficiency was determined by monitoring the electrical power consumed by the vehicle in steady and unsteady propulsion modes. This measurement identified conditions under which the power required to create flow unsteadiness was offset by the improved vehicle efficiency. The experiments demonstrated that substantial increases in propulsive efficiency, over 50% greater than the performance of the steady-jet mode, can be achieved by using vortex formation to manipulate the near-wake properties. At higher vehicle speeds, the enhanced performance was sufficient to offset the energy cost of generating flow unsteadiness. An analytical model explains this enhanced performance in terms of the vortex added-mass and entrainment. The results suggest a potential mechanism to further enhance the performance of existing engineered propulsion systems. In addition, the analytical methods described here can be extended to examine more complex propulsion systems such as those of swimming and flying animals, for whom vortex formation is inevitable.
Date:Friday Oct 19, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Mathias Baekbo Andersen, Center for Turbulence Research, Postdoctoral Fellow
Title:Ion-selective membranes and overlimiting current
 
Abstract:Possible mechanisms for overlimiting current through aqueous ion-selective membranes (exceeding diffusion limitation) have been debated for half a century. Flows consistent with electro-osmotic instability have recently been observed in microfluidic experiments, but the existing theory neglects chemical effects and remains to be quantitatively tested. Eelectro-osmotic instability (EOI) is an electrohydrodynamic instability that appears as a result of a high current inducing an electrically charge boundary layer at an ion-selective surface. The quiescent state is unstable and a small perturbation grows by a positive feedback mechanism until viscous friction limits further growth. At this point the fluid instability consists of a periodic array of vortices reminiscent of Rayleigh–Bénard convection cells. At high enough currents the instability becomes chaotic as evidenced by the appearance of noise in the current through the system. Theoretical models have until now only been able explain the instability qualitatively, partly due to the highly nonlinear nature of the phenomenon and the disparate length scales. Furthermore, it may be that only a full 3D model can obtain quantitatively accurate results. One of our current research objectives is to investigate the combined effect of EOI and membrane chemistry on overlimiting current (OLC). In my talk I will therefore relate EOI to our recent model results [M.B. Andersen et al., PRL 109, 108301 (2012)] showing that chemical charge regulation and water self-ionization can lead to OLC by ‘‘current-induced membrane discharge’’ (CIMD) in ion-selective membranes. Our CIMD model shows a suppression of the extended space charge that leads to EOI and the combination of the two effects is expected to be important. In CIMD, and as in EOI, an electric current passes through the membrane, which leads to salt depletion and a locally large electric field that enhances the rate of water splitting. The resulting hydronium and hydroxide ions induce large pH variations in the system and in the membrane. Depending on its chemical nature the membrane loses its ion-selectivity and thereby allows for current above the nominal diffusion limit. CIMD is likely to act in parallel with EOI and a combined model is currently being pursued. Understanding the mechanisms related to OLC is important in technologies such as electrodialysis (water de-ionization), energy storage in supercapacitors, production of acids and bases by bipolar membranes and fuel cells.
Date:Friday Oct 5, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Carlo Scalo, Center for Turbulence Research, Postdoctoral Fellow
Title:Large-eddy simulation and modeling of oxygen transport and depletion in water bodies
 
Abstract:The prediction of dissolved oxygen (DO) concentrations is critical for managing and monitoring marine ecosystems. Oxygen evolves in water bodies as a high-Schmidt-number passive scalar. It is entrained at the surface and transported down the water column by turbulent motions. Many natural factors can interfere with this mixing process such as stratification, which damps the turbulence, reducing the supply of oxygen to the near-bed region.Here, in the sediment layer, decomposition of dead organic matter (resulting from eutrophication, i.e. the ecosystem response to the introduction of excess artificial or natural nutrients) by oxygen-consuming bacteria can cause the DO concentration to drop to unsustainable levels for aquatic life making the sediment-oxygen uptake (SOU) the most important factor affecting oxygen depletion. Despite the considerable environmental and economical impacts of oxygen depletion the interaction among all the physical processes involved is still not well understood, with obvious consequences on the quality of modeling. A computational model for DO transfer from water to underlying flat and cohesive sediment beds populated with DO-absorbing bacteria has been developed and will be discussed in this talk. The model couples Large-Eddy Simulation of turbulent transport in the water-column, a biogeochemical model for DO transport and consumption in the sediment, and Darcy's Law for the pore water-driven solute dispersion and advection. The model's predictions compare well against experimental data for low friction-Reynolds numbers, Ret. The disagreement for higher Ret is investigated by progressively increasing the complexity of the model. A sensitivity analysis shows that the SOU is approximately proportional to the bacterial content of the sediment layer, and varies with respect to fluid dynamics conditions, in accordance to classic high-Schmidt-number mass-transfer laws. The non-linear transport dynamics responsible for sustaining a statistically steady SOU are investigated by temporal-and-spatial correlations and with the aid of instantaneous visualizations: the near-wall coherent structures modulate the diffusive sublayer, which exhibits complexspatial and temporal filtering behaviors; its slow and quasi-periodic regeneration cycle determines the streaky structure of the DO field at the sediment-water interface, retained in the deeper layers of the porous medium.– Finally, the model has been tested in a real geophysical framework, reproducing published in-situ DO measurements of a transitional flow in the bottom boundary layer of lake Alpnach (Switzerland). A simple model for the SOU is then derived for eventual inclusion in RANSE biogeochemical management-type models for similar applications.
Date:Monday Jul 30, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Martin Bazant, Departments of Chemical Engineering and Mathematics, Massachusetts Institute of Technology, Cambridge, MA
Title:Overlimiting Current and Shock Electrodialysis in Porous Media
 
Abstract:Salt transport in bulk electrolytes occurs by diffusion and convection, but in microfluidic devices and porous media, surface conduction and electro-osmotic flow also contribute to ionic fluxes. The classical theory of electrokinetic phenomena in porous media assumes linear response to a small voltage, where the electrolyte concentration is only weakly perturbed. When a large voltage or concentration gradient is imposed, some surprising nonlinear electrokinetic phenomena result from the competition between bulk and interfacial transport in a microstructure. At constant voltage, the microstructure can sustain an over-limiting current (exceeding diffusion limitation) without any hydrodynamic or chemical instability. At constant current, a “deionization shock” can propagate through the microstructure, leaving behind a macroscopic region depleted of ions and particles. This talk will present the mathematical theory and new experimental evidence for surface-driven overlimiting current and deionization shocks in porous glass frits, as well as applications to water deionization by ``shock electrodialysis”. This work builds on the seminal 2009 paper of Mani, Zangle and Santiago at Stanford on concentration shocks in microchannels by extension to porous media, where current and flow directions are decoupled.
Date:Tuesday Jul 17, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Yury S. Kachanov, Institute of Theoretical and Applied Mechanics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russian Federation
Title:The Deterministic Turbulence – Philosophical Abstraction or Reality?
 
Abstract:This paper concentrates on physical and philosophical aspects of the problem of deterministic turbulence, the idea of which has been suggested by the author about a decade ago and realized later experimentally together with two younger colleagues. The answer to the question formulated in the paper title is: “The deterministic turbulence is a reality.” In contrast to the usual (random) turbulence, the deterministic turbulent flows have reproducible instantaneous structure, representing one particular realization from infinite number of possible ones. The discovery of the deterministic turbulence is a breakthrough in the fields of: fluid dynamics, nonlinear science on turbulence and chaos, and in philosophy. It is shown in the present paper that the deterministic turbulence can be regarded as an inherent consequence of the idea on determinism of laws of nature and predictability of real physical systems, even unstable ones, occurred at least in cases of relatively short time intervals. These features enable reproducibility of behavior of even very complex real systems having infinite, practically, number of degrees of freedom. Such behavior only appears chaotic, due to its great complexity, but, simultaneously, can be deterministic, i.e. reproducible, in the main, from one realization to another. One has note that the deterministic turbulence can exist (and can be produced) not in all flows (in the majority of them only usual, stochastic turbulence is possible).
Date:Thursday Jul 5, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Barry Koren, Numerical Mathematics, Mathematical Institute, Leiden University
Title:Runge-Kutta methods for the incompressible Navier-Stokes equations
 
Abstract:Time integration of the incompressible Navier-Stokes equations with Runge-Kutta methods is not straightforward due to the differential-algebraic nature of the equations.We consider the temporal order of accuracy of velocity and pressure and focus on one specific class of Runge-Kutta methods, namely symplectic Runge-Kutta methods. We apply existing theory on Runge-Kutta methods for differential algebraic equations [1] to the incompressible Navier Stokes equations. Explicit Runge-Kutta methods are in general a good choice for integrating non-stiff differential equations. They combine good stability, high accuracy and low cost. If explicit Runge-Kutta methods are used, both velocity and pressure can be computed to the classical (‘ODE’) order of accuracy, except in two important cases [2]. First, if the boundary conditions for the normal velocity component depend on time, the order of accuracy of the pressure is affected. Second, if the mesh is time-dependent, the order of accuracy of both velocity and pressure is affected.We propose a number of new explicit Runge-Kutta methods for these cases that are third- and fourth-order accurate for the velocity and second-order for the pressure. The second-order accuracy of the pressure is obtained with a new technique based on reconstruction of instantaneous pressure values from time-averaged values. This technique is also applicable to obtain a second-order accurate pressure when implicit Runge-Kutta methods are used. Implicit Runge-Kutta methods are better suited for stiff problems, as is for example the case with flows featuring boundary layers. Another attractive feature is that they can be constructed such that the (kinetic) energy of the flow is conserved (in absence of diffusion) [3]. Such ‘energy-conserving’ or ‘symplectic’ Runge-Kutta methods [1] do not distort the energy cascade from large to small scales, an important property for turbulence simulations. Moreover, these Runge-Kutta methods, combined with energy-conserving space discretizations [4, 5], lead to methods that are stable for any mesh size and any step size. As a test case we consider the roll-up of an inviscid shear layer, see figure 1. The spatial discretization is a second-order finite volume method on a staggered grid, which is energy-conserving in absence of viscosity. Figure 1a shows the vorticity field at t = 8. Small wiggles appear because there is no viscosity to damp the smallest scales, but the wiggles do not lead to blow-up of the solution if the initial energy of the flow is bounded. Figure 1b shows that the symplectic single-stage Gauss 2 (implicit midpoint, second-order), two-stage Gauss 4 (fourth-order) and two-stage Radau IIB (third-order) methods conserve energy to machine precision, independent of the time step. Well known implicit methods such as Crank-Nicolson (second-order) or two-stage Radau IIA (third-order) do not have this favorable property. In ongoing work we combine these energy-conserving Runge-Kutta methods with methods that are suitable for handling very stiff diffusive terms. This results in a new class of additive Runge-Kutta methods, of which the two-stage, third-order Radau IIA/B method is an example. Other test cases involving both convection- and diffusion-dominated flow regions will show the effectiveness of these new methods.
Date:Friday Jun 22, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. H. Dogus Akaydin. The Grove School of Engineering, The City College of The City University of New York
Title:Flow-Powered Piezoelectric Energy Harvesters: A Case of Aeroelectromechanics
 
Abstract:"Smart structures” that respond to electrical stimuli are used in fluidic environments for many purposes including energy harvesting, sensing, active flow control and bio-inspired locomotion. In this talk, the use of flexible piezoelectric structures for flow-powered energy harvesting will be introduced. The design and performance of several harvesters, which exploit the wakes of bluff bodies, turbulent boundary layers, vortex rings and self-excited aeroelastic oscillations, will be discussed. Due to their mechanical simplicity and low speed operation, the aeroelastic harvesters can become an alternative to micro-turbines for powering small wireless sensor nodes. According to our wind tunnel tests, a self-excited harvester utilizing vortex-induced vibrations on circular cylinder generated a maximum of 0.1 mW of electrical power while resonating at a flow speed of 1.2 m/s. However, when the shape of the cylinder is changed to induce galloping instability, the harvested power rapidly increased with flow speed from 0.11 mW at 1.2 m/s to nearly 2mW at 2.7 m/s. The inclusion of a flexible piezoelectric structure in a fluid flow adds an electrical component to the complexity of the fluid-structure interaction problem and results in an “aeroelectromechanical” system. In the remainder of the talk, a computational framework to model the behavior of aeroelectromechanical systems will be introduced. The application of this framework to model the behavior of a piezoelectric beam in the wake of a circular cylinder will be demonstrated. Finally, future extensions of the framework for modeling other aeroelectromechanical configurations, including active flow control and bio-inspired locomotion, will be briefly discussed.
Date:Friday Jun 8, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Alexander Bihlo, Postdoctoral Fellow, Centre de recherches mathematiques at the Universite de Montreal, Canada
Title:Invariant turbulence modeling
 
Abstract:Symmetries are among the most successfully employed concepts in science and mathematics. They form a cornerstone of various branches of physics, such as classical and quantum mechanics, particle physics and relativity. The governing equations of hydrodynamics generally possess wide symmetry groups and therefore there is a great potential to exploit these symmetries so as to derive similarity solutions, conservation laws and invariants or to study the effects of symmetry breaking due to the presence of boundaries or additional body-forces. However, to date, symmetries are often used in a non-explicit or indirect way in hydrodynamics and turbulence theory. On the other hand, there exist powerful and general methods introduced in the field of group analysis of differential equations which, when suitably adapted, can be readily applied to the aforementioned fields. In this talk we will introduce an algorithmic method which allows associating with a given object its invariant counterpart. The object under consideration can be, e.g., a turbulence closure model or a finite-difference discretization of a differential equation, which can then be invariantized to yield a turbulence model or a finite-difference discretization that is invariant under the same Lie point symmetry group as admitted by the original governing equations of hydrodynamics. This method can therefore be used to correct artificial symmetry breaking due to non-appropriately designed turbulence models. As an example it is shown that classical hyperdiffusion as used in two-dimensional (decaying) turbulence simulations violates the symmetries of the incompressible Euler equations. Invariantization of these hyperdiffusion terms yields symmetry-preserving but nonlinear diffusion-like terms. Using the notion of differential invariants it is demonstrated that the invariantized hyperdiffusion models can be modified with quite some flexibility while still preserving their desired invariance characteristics. First numerical tests show that the invariant hyperdiffusion schemes which can be obtained by this method might be able to reproduce the -3 slope of the energy spectrum in the enstrophy inertial range.
Date:Thursday Jun 7, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker: Dr. Ahmad Shakibaeinia, Research Associate, Faculty of Engineering & Applied Science University of Regina, Canada
Title:Mesh-free particle modeling of multiphase flows
 
Abstract:The major challenge for numerical modeling of the multiphase systems has long been handeling the large deformation and fragmentation of the interfaces and boundaries. A recent strong interest in the field of computational fluid dynamics is focused on development of the next generation of numerical methods, the mesh-free particle (Lagrangian) method. The main goal of these methods is to facilitate the simulation of increasingly demanding problems that are involved large interfacial deformations, complex geometries and discontinuities. Mesh-free particle methods (also called particle methods) use a set of particles without any connectivity to represent the continuum and record the state and movement of the system. Each particle possesses a set of field variable (e.g., mass, momentum) and moves in a Lagrangian frame based on its velocity. The objective of this study is to develop a mesh-free particle methods based on the Moving particle Semi-implicit (MPS) method for modeling of multiphase systems in a broad verity of conditions. In this talk the MPS mathematical fundamentals and numerical algorithms for modeling of single-phase and multiphase fluid flow systems are presented. Moreover, the accuracy and performance of the MPS method is addressed through some fundamental and practical sample problems. Finally, the future works on MPS and mesh-free particle modeling of multiphase flows are briefly explained.
Date:Friday May 18, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Ricardo Garcia-Mayoral, Center for Turbulence Research, Postdoctoral Fellow
Title:THE INTERACTION OF RIBLETS WITH WALL-BOUNDED TURBULENT FLOWS
 
Abstract:Riblets are small surface protrusions that can reduce turbulent friction when aligned in the direction of the overlying flow. The reduction is a function of their size, measured in wall units. For vanishingly small sizes, the reduction is linear, but this linear behavior breaks down for a given, geometry-dependent size, limiting riblet performance. The mechanism of this breakdown was pre- viously not understood, and it was thus difficult to predict riblet optimum size and performance. Analyzing the available experimental data, the linear and breakdown regimes are found to collapse well for diverse configurations, when the size is scaled with the length ℓ+g = (A+g )1/2, based in the groove cross-section Ag. The linear drag reduction is found to break down for an optimum ℓ+g ,opt 11. To understand the breakdown, we analyze DNSs of turbulent channels with riblets, with sizes covering the full drag reducing range, and up to Re 550. As the size increases from ℓ+g,opt, coherent spanwise rollers begin to appear immediately above the riblets, growing rapidly in intensity. The extra Reynolds stress that they generate accounts quantitatively for the drag degradation. The rollers are similar to those found over porous surfaces and plant canopies, and can be traced to a Kelvin–Helmholtz-like instability, associated with the relax- ation of the impermeability condition for the wall-normal velocity. An inviscid model for the instability confirms its nature, agreeing well with the observed perturbation wavelengths and shapes. Using this model, the key parameter for the onset of the instability is identified as a ‘penetration’ length L+w , which for conventional riblet geometries is proportional to ℓ+g . The instability onset, L+w 4, actually corresponds to the empirical breakdown size ℓ+g 11.
Date:Friday May 11, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Hiroyuki Abe, Japan Aerospace Exploration Agency
Title:DNS and modeling of a separated turbulent boundary layer
 
Abstract:Separation of a turbulent boundary layer is one of the most challenging research topics in aeronautics. The data, which can serve as reference data for developing turbulence models, are however still limited. Over the past several years, we have performed DNS of a separated turbulent boundary layer, where a separation bubble is created by imposing blowing and suction at the upper boundary. The inlet Reynolds numbers based on the freestream velocity and the momentum thickness are 300, 600 and 900, the latter value being three times larger than that of the seminal DNS works (Spalart and Coleman 1997; Na and Moin 1998). The objectives of the present study are to clarify effects of the Reynolds number in a separation bubble and also to develop turbulence models. In this talk, some representative DNS results are first described (e.g. negative production of the turbulent kinetic energy, large-scale spanwise meandering of the separation line and large-scale structures in the bubble). Next, results regarding the RANS (low Re k-epsilon) model testing are presented; the model tends to predict the separation point earlier than the DNS. Finally, the future works on DNS and modeling (RANS and LES) are briefly described.
Date:Friday Apr 20, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Takao Suzuki, The Boeing Company
Title:Hybrid unsteady-flow simulation combining PIV/PTV and DNS: Introduction to the reduced-order Kalman filter
 
Abstract:Capability of state-of-the-art techniques integrating experimental and computational fluid dynamics is recently expanding. In particular, time-resolved particle image velocimetry (PIV) has opened the door to hybrid simulation driven by experimental data. We have developed a hybrid unsteady-flow simulation technique combining particle tracking velocimetry (PTV) and direct numerical simulation (DNS), and demonstrated its capability by solving laminar planar-jet flows. Unsteady velocity fields on a laser sheet in a water tunnel are measured with time-resolved PTV; subsequently, PTV velocity fields are rectified in a least-squares sense so that the equation of continuity is satisfied, and they are transplanted to a two-dimensional incompressible Navier-Stokes solver by setting a multiple of the computational time-step equal to the frame rate of the PTV system. In this parental algorithm, the hybrid velocity field is given by a linear combination between the PIV/PTV velocity field and that marched from a previous time-step with the DNS. The constant weight between them is determined such that all growing modes of the numerical system are suppressed. As a result, the unsteady hybrid velocity field is converged to that of the measured one over time, and the unsteady pressure field can be simultaneously acquired. The resultant set of flow quantities satisfies the governing equations, and their resolution becomes comparable to that of numerical simulation with the noise level much lower than the original PTV data. To further improve the filtering capability for noisy PIV/PTV data at higher Reynolds numbers, this study introduces the reduced-order Kalman filter, which is widely used for data assimilation. The new algorithm temporally and spatially optimizes the weight function for the time-resolved PTV feedback; thus, it improves the spatial-filtering function, and the hybrid velocity fields converge faster on time-step bases, while the drawback is huge computational time and memory. 
Date:Friday Apr 6, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Jan Nordström, Dept. of Mathematics, Linkoping University, Sweden
Title:Recent Developments in Computational Mathematics at Linköping University
 
Abstract:The current activities in computational mathematics at the mathematics department of Linköping University regarding methods for fluid flow, wave-propagation, heat conduction and signal propagation will be discussed. The talk will give a brief presentation of the specific theoretical problems, the specific research issues and possibly provide a ground for a discussion about future collaborations in related application fields.
Date:Friday Mar 23, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Brian Launder, Turbulence Mechanics Group, University of Manchester, UK
Title:Back to the Future: Flettner-Thom Rotors for low-carbon maritime propulsion
 
Abstract:The seminar will first review the major steps taken over the past 90 years to exploit the Magnus effect for ship propulsion (i.e. the lift imparted to a rotating cylinder past which a fluid stream passes) and the ambitious plan to use such vessels to combat the effects of global warming. The current computational work being undertaken by the Turbulence Mechanics Group at Manchester to examine this problem is then summarized. A range of strategies is being pursued from steady RANS to LES. The seminar will, however, focus on an unsteady RANS approach. The effect of the pure Flettner rotor is first examined and results compared with the available DNS, LES and experimental data (the first two being at Reynolds numbers orders of magnitude below those at which Flettner rotors would actually operate). The effect of adding discs to the rotor as first proposed by Alexander Thom is next examined. Current results have failed to reproduce the extraordinary increase in lift coefficient that Thom reported though they have captured the negative drag coefficients that he found occurred when the rotor speed exceeds four times the air speed. Finally an indication is given of immediate and medium-term further research needed.
Date:Friday Mar 9, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Dmitry Kotov, Postdoctoral Scholar, Stanford University, Center for Turbulence Research
Title:Performance of High Order Shock-Capturing Schemes for Stiff Source Terms & Discontinuities
 
Abstract:In compressible turbulent combustion/nonequilibrium flows, the constructions of numerical schemes for (a) stable and accurate simulation of turbulence with strong shocks, and (b) obtaining correct propagation speed of discontinuities for stiff reacting terms on ``coarse grids" share one important ingredient - minimization of numerical dissipation while maintaining numerical stability.  This dual requirement to achieve both numerical stability and accuracy with zero or minimal use of numerical dissipation is most often conflicting for existing schemes that were designed for non-reacting flows. The goal of this paper is to relate numerical dissipations that are inherited in a selected set of high order shock- capturing schemes with the onset of wrong propagation speed of discontinuities for two representative stiff detonation wave problems.
Date:Friday Feb 24, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker: Prof. Rodney O. Fox, Anson Marston Distinguished Professor of Engineering, Iowa State University
Title:Quadrature-Based Moments Methods
 
Abstract:Kinetic theory is a useful theoretical framework for developing multiphase flow models that account for complex physics (e.g., particle trajectory crossings, particle size distributions, etc.). For most applications, direct solution of the kinetic equation is intractable due to the high-dimensionality of the phase space. Thus a key challenge is to reduce the dimensionality of the problem without losing the underlying physics. At the same time, the reduced description must be numerically tractable and possess the favorable attributes of the original kinetic equation (e.g. hyperbolic, conservation of mass/momentum, etc.) Starting from the seminal work of McGraw (1) on the quadrature method of moments (QMOM), we have developed a general closure approximation referred to as quadrature-based moment methods (2; 3; 4). The basic idea behind these methods is to use the local (in space and time) values of the moments to reconstruct a well-defined local distribution function (i.e. non-negative, compact support, etc.). The reconstructed distribution function is then used to close the moment transport equations (e.g. spatial fluxes, nonlinear source terms, etc.). In this seminar, I will present the underlying theoretical and numerical issues associated with quadrature-based reconstructions. The transport of moments in real space, and its numerical representation in terms of fluxes, plays a critical role in determining whether a moment set is realizable. Using selected multiphase flow examples, I will introduce recent work on realizable high-order flux reconstructions developed specifically for finite-volume schemes (5).
Date:Friday Feb 17, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Mihailo Jovanovic, Dept. of Electrical and Computer Engineering, University of Minnesota
Title:Model-based design of transverse wall oscillations for turbulent drag reduction
 
Abstract:Over the last two decades, both experiments and simulations have demonstrated that transverse wall oscillations with properly selected amplitude and frequency can reduce turbulent drag by as much as 40 percent. In this talk, we will describe a model-based approach for designing oscillations that suppress turbulence in a channel flow. We utilize judiciously selected linearization of the flow with control in conjunction with turbulence modeling to determine skin-friction drag in a simulation-free manner. The turbulent viscosity hypothesis is used to quantify the effect of fluctuations on the mean velocity in the flow subject to control. In contrast to the traditional approach, we determine the turbulent viscosity from the second order statistics of the linearized model driven by white-in-time stochastic forcing. The spatial power spectrum of the forcing is selected to ensure that the linearized model for the flow with no control reproduces the turbulent energy spectrum. The resulting correction to the turbulent mean velocity induced by small amplitude wall movements is then used to identify the optimal frequency of drag reducing oscillations. In addition, the control net efficiency and the turbulent flow structures that we obtain agree well with the results of direct numerical simulations and experiments. This demonstrates the predictive power of our model-based approach to controlling turbulent flows and is expected to pave the way for successful flow control at higher Reynolds numbers than currently possible.
Date:Friday Jan 27, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Jerome Dombard, Postdoctoral Scholar, Stanford University, Center for Turbulence Research
Title:Direct Numerical Simulation of non-isothermal dilute sprays using the Mesoscopic Eulerian Formalism
 
Abstract:This talk presents some aspects of my PhD which addressed the Direct Numerical Simulation of non-isothermal turbulent flows laden with solid particles in the dilute regime. The focus is set on the accurate prediction of heat transfer between phases and of particle dispersion. The dispersed phase is described by an Eulerian approach: the Mesoscopic Eulerian Formalism, recently extended to non-isothermal flows. The main objective of this work is to assess the ability of this formalism to account accurately for both dynamic and thermal inertia of particles in turbulent sheared flows. The CFD code used in this work is AVBP, developed at CERFACS and IFPen. The numerical simulation of dilute sprays with an Eulerian approach calls for specific modeling and raises additional numerical issues. First, the numerical methods implemented in AVBP for two-phase flows will be presented and tested in a series of canonical test cases. The objective is to propose an accurate and robust numerical strategy that withstands the steep gradients of particle volume fraction due to preferential concentration with a limited numerical diffusion. For example, the dissipations due to numerics and physical effects are explicitly extracted and quantified in a configuration of homogeneous isotropic turbulence laden with particles. Then, the issue of the correct prediction of particle dispersion in configurations with a mean shear will be addressed. A new Random Uncorrelated Motion (RUM) model was tested in a three-dimensional non-isothermal jet laden with particles. Contrary to the former RUM models, the main statistics of the dispersed phase were recovered at both the center and the edges of the jet. Finally, the impact of the thermal inertia of particles on their temperature statistics will be investigated. The results show a strong dependency of these statistics on thermal inertia, pinpointing the necessity of the numerical approaches to account for this phenomenon. Therefore, the extension of the MEF to non-isothermal conditions, i.e. the RUM heat fluxes, has been implemented in AVBP. Results show a strong positive impact of the RUM heat fluxes on the fluctuations of mesoscopic temperature, and to a lesser extent on the mean mesoscopic temperature depending on the configuration. Neglecting the RUM heat fluxes leads to erroneous results whereas the Lagrangian statistics are recovered when they are accounted for.
Date:Friday Jan 13, 2012
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Remi Zamansky, Postdoctoral Scholar, Stanford University, Center for Turbulence Research
Title:Acceleration in turbulent channel flow: statistics, stochastic sub-grid models and application to simulation of solid particle transport
 
Abstract:This talk will focus on characterization and stochastic modeling of the fluid acceleration in a turbulent channel flow, with emphasis on long-range interactions across the channel. In the first part, a result concerning the acceleration statistics obtain by direct numerical simulation (DNS) at three Reynolds numbers (180, 590 and 1000) are presented. It is observed that the norm of acceleration is log-normal whatever the wall distance. Universal form of scaling law for the acceleration is proposed by dimensional analysis. In the second part, the acceleration is modeled stochastically, assuming the norm / orientation decomposition. The stochastic model for the norm is based on fragmentation processes in order to represent the long-range interactions across the channel. The orientation is simulated by random walk on a sphere in order to reproduce the relaxation towards isotropy with increasing the wall distance, as it as was observed preliminary in our DNS. These models are applied, in the third part, to the LES-SSAM framework (Large Eddy Simulation with Stochastic sub-grid Acceleration Model), which was introduced by Sabel'nikov, Chtab and Gorokhovski (Euro. Phys. J. B, vol. 80, 2011, p. 177) and assessed in the case of the box turbulence. Our computations showed that the mean velocity, the energy spectra, the viscous and turbulent stresses, as well as the non-gaussianity of acceleration statistics can be considerably improved in comparison with standard LES. The advantage of the LES-SSAM approach, which accounts for intermittency at sub-grid scales, for the simulation of multiphase flows is demonstrated in the last part of this talk. Here simulations of the transport of inertial pointwise particles by DNS, by LES and by LES-SSAM are compared.
Date:Monday Nov 28, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Richard J.A.M. Stevens, Department of Science and Technology, Twente University, P.O Box 217, 7500 AE Enschede, The Netherlands
Title:Rayleigh-B´enard turbulence
 
Abstract:Turbulence is seen as one of the last outstanding unsolved problems in classical physics. In the last century, great minds as Heisenberg, von Weizs¨acker, Kolmogorov, Prandtl, and G.I. Taylor had worked on it, and Einstein put his last postdoc Bob Kraichnan on the subject of turbulence – a task which Kraichnan never finished. The rapid development of experimental and numerical techniques in this area and the growth of computing power creates a lot of activity on turbulence research. In turbulence problems encountered in the real world the influence of walls is very important and one of the classical systems to study concepts in fluid dynamics is the Rayleigh-B´enard (RB) system, i.e. the buoyancy driven flow of a fluid heated from below and cooled from above, see figure 1a and figure 1b. Also from an applied viewpoint, thermally driven flows are of utmost importance. Examples are thermal convection in the atmosphere, in the ocean, or in process technology. Rotating turbulent flow is of utmost importance to optimize industrial applications such as the efficient separation of carbon dioxide (CO2) from nitrogen in the emission gases of conventional carbon-based power plants to enable long term CO2 storage or the separation of CO2 from natural methane gas. In both cases the method of choice is pressurization and cooling down of the gas mixture so that finally CO2 condensates into droplets and can be separated in so-called rotational phase separators. Due to the droplet condensation, considerable heat transfers emerge in this process which are strongly affected by rotation. In this presentation experimental I will present experimental, numerical and theoretical results on RB convection. Simulations and experiments on RB convection are complementary. In accurate experimental measurements of the heat transfer a completely isolated system is needed. Therefore, one cannot visualize the flow while the heat transfer is measured. On the positive side, in experiments one can obtain very high Ra numbers and long time averaging. In direct numerical simulations (DNS), on the other hand, one can simultaneously measure the heat transfer while the complete flow field is available for analysis. Unfortunately, up to recently, there was a major disagreement between experimental and numerical measurements of the heat transfer. We showed that this disagreement was due to insufficient numerical resolution in the simulations. Our new high resolution simulations agree excellently with the experimental result, see figure 1c. In addition, we showed some very unexpected transitions between different turbulent states in RB convection. We find that very small changes in the control parameters can completely change the flow structure or can drastically alter the flow dynamics that are observed.
Date:Friday Nov 4, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:M. Massot, Laboratoire EM2C, UPR CNRS 288 – Ecole Centrale Paris – marc.massot@ecp.fr and Center for Turbulence Research – Stanford University –mmassot@stanford.edu
Title:Adaptive time-space algorithms for the simulation of multi-scale reaction waves
 
Abstract:Numerical simulations of multi-scale phenomena are commonly used for modeling purposes in many applications such as combustion, chemical vapor deposition, or air pollution modeling. In general, all these models raise several diffculties created by the high number of unknowns, the wide range of temporal scales due to large and detailed chemical kinetic mechanisms, as well as steep spatial gradients associated with very localized fronts of high chemical activity. Furthermore, a natural stumbling block to perform 3D simulations with all scales resolution is either the unreasonably small time step due to stability requirements or the unreasonable memory requirements for implicit methods. In this work, we introduce a new resolution strategy for multi-scale reaction waves based mainly on time operator splitting and space adaptive multiresolution, in the context of very localized and stiff reaction fronts. It considers high order time integration methods for reaction, diffusion and convection problems, in order to build a time operator splitting scheme that exploits efficiently the special features of each problem. Based on theoretical studies of numerical analysis, such a strategy leads to a splitting time step which is not restricted neither by fast scales in the source term nor by restrictive stability limits of diffusive or convective steps, but only by the physics of the phenomenon. Moreover, this splitting time step is dynamically adapted taking into account a posteriori error estimates, carefully computed by a second embedded and economic splitting method. The main goal is then to perform computationally very efficient as well as accurate in time and space simulations of the complete dynamics of multi-scale phenomena under study, considering large simulation domains with conventional computing resources and splitting time steps purely dictated by the physics of the phenomenon and not by any stability constraints associated with mesh size or source time scales. The main results of this Tea Seminar are originally the Ph.D. work of M. Duarte, who will graduate in France in December 2011.
Date:Friday Oct 28, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Damien KAH, Center for Turbulence Research, Stanford University, office 501K
Title:Taking into account polydispersity for the modeling of liquid fuel injection in internal combustion engines
 
Abstract:Multiphase Flow modeling is a critical aspect involved in the study of fuel injection in internal combustion engines, or more generally any industrial device involving a multiphase flow made of liquid fuel injected in a chamber filled with gaz. In and of itself, it is possible to simulate this flow in the context of a direct numerical simulation. However the small structures created during injection (droplets of diameter until 10 μm or less) lead to a prohibitive computational cost for any industrial application. Therefore modeling is necessary. In this context, two areas are formally distinguished: the dense liquid core close to the injector called separate-phase flow, and the spray made of a polydisperse droplet population (i.e. droplets with different sizes) generated after the atomization processes downstream of the injector. This talk investigates Eulerian models for the description of polydisperse evaporating sprays. They represent a potential alternative to Lagrangian models, widely used at present, yet suffering from major drawbacks. Thus, the Multi-Fluid model ([1] and references herein) has been assessed. Although it is very promising, two difficulties are highlighted: its cost for a precise description of polydispersity, and its inability to describe particle trajectory crossing (PTC). Solutions to these two limitations are studied in [3], relying both on high order moment methods. This talk addresses the first aspect with the presentation of the Eulerian Multi Size Moment (EMSM) model, offering a more efficient resolution of polydisperse evaporating sprays than the Multi-Fluid model does. Mathematical tools are used to close the model and are combined with original finite volume kinetic-based schemes in order to preserve the moment-set integrity, for evaporation and advection. In order to assess its potential, the EMSM model and numerical tools are implemented in the MUSES3D code, an academic DNS solver that provides a framework devoted to spray method evaluation [1]. The extension of the EMSM model to an industrial context is then considered, with its implementation in the IFP-C3D code, a 3D unstructured reactive flow solver with spray. In order to perform computations within a moving domain (due to the piston movement) the Arbitrary Lagrangian Eulerian (ALE) [2] formalism is used. The robustness of the EMSM model in the IFP-C3D code has been successfully demonstrated. Moreover, very encouraging results demonstrate the feasibility of the EMSM model for spray injection. References [1] S. de Chaisemartin, Eulerian models and numerical simulation of turbulent dispersion for polydisperse evaporation sprays, Ecole Centrale Paris, 2009. [2] J. Donea et al., Arbitrary Lagrangian-Eulerian methods, Encyclopedia of Computational Mechanics pp 413 − 437, 2004 [3] D. Kah, Taking into account polydispersity for the modeling of liquid fuel injection in internal combustion engines, Ecole Centrale Paris, 2010.
Date:Friday Sep 9, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:M. Massot, Laboratoire EM2C, UPR CNRS 288 – Ecole Centrale Paris – marc.massot@ecp.fr and Center for Turbulence Research – Stanford University –mmassot@stanford.edu
Title:Eulerian moment methods for evaporating polydisperse sprays: from mathematical issues to HPC
 
Abstract:The modeling and simulation of multiphase reacting flows covers a large spectrum of applications ranging from combustion in automobile and aeronautical engines to atmospheric pollution as well as biomedical engineering. In the framework of this talk, we will mainly focus on a disperse liquid phase carried by a gaseous flow field which can be either laminar or turbulent; however, this spray can be polydisperse, that is constituted of droplets with a large size spectrum. Thus, such flows involve a large range of temporal and spatial scales which have to be resolved in order to capture the dynamics of the phenomena and provide reliable and eventually predictive simulation tools. Even if the power of the computer ressources regularly increases, such very stiff problems can lead to serious numerical difficulties and prevent efficient multi-dimensional simulations. The purpose of this talk is to show that all the necessary steps in order to develop a new generation of computational code have to be designed at the same time with a high level of coherence: mathematical modeling through Eulerian moment methods, development of new dedicated stable and accurate numerical methods, implementation of optimized algorithms as well as validations of both model and methods using either experimental measurements or other codes. We will introduce both a new class of models for the direct numerical simulation of spray dynamics [1, 2, 3, 4, 5], as well as a set of dedicated numerical methods [6, 7, 2] and prove that such an approach has the ability, once validated [8, 9], to lead to high perfomance computing on parallel architectures [10], obtained during the Summer Program 2010 of the Center for Turbulence Research at Stanford University.
Date:Thursday Aug 11, 2011
Time:2:15pm
Location:BLDG 530 ROOM 127
Speaker:Amable Liñán, Escuela de Ingeniería Aeronáutica, Universidad Politécnica de Madrid, Spain.
Title:The initiation of self-sustained detonations in reactive gases.
 
Abstract:The presentation will be devoted to the analysis of the initiation, by concentrated external energy sources, of self-sustained cylindrical or spherical detonation waves in gaseous reactive mixtures. The dynamics of the detonation front will be described in the fast reaction limit, when the thickness of the reaction layer that follows the shock front is very small compared with the shock radius. At early times, after the initiation of the external thermal energy deposition, the detonation front, which generates a strongly expanding flow, is overdriven; so that it is reached by expansion waves that decrease its velocity to the Chapman-Jouguet value, for which the expansion waves can no longer reach the front. In cylindrical and spherical detonations, the transition to the constant Chapman-Jouguet velocity occurs at a finite value of the detonation radius, which is scaled by the radius for which the energy released by the external source equals the heat released by the chemical reaction. A brief discussion will be given on how the reaction may be quenched by the expansion waves if the initiating energy is smaller than a critical value, thus failing to generate a self-propagating detonation wave (first described, independently, by Ya.B. Zeldovich and G.I.
Date:Tuesday Aug 2, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Chi-Wang Shu, Division of Applied Mathematics, Brown University
Title:High-order finite difference methods with subcell resolution for hyperbolic conservation laws with stiff reaction terms
 
Abstract:In this talk we will present the result of the joint research with Wang, Yee and Sjogreen on high order finite difference WENO schemes with subcell resolution for hyperbolic conservation laws with stiff reaction terms. For chemical reaction problems, when the reaction time scale is very small, e.g., orders of magnitude smaller than the fluid dynamics time scales, the problems will become very stiff. Wrong propagation speed of discontinuity may occur due to the under resolved numerical solutions in both the space and time. Our proposed method is a modified fractional step method which solves the convection step and reaction step separately. In the convection step, any high-order shock-capturing method can be used, e.g. a fifth-order finite difference WENO scheme is considered in this work. In the reaction step, a modified ODE solver is applied but with the flow variables in the shock region modified by the subcell resolution idea. This method maintains high order accuracy in space for smooth flows and it can capture the correct wave speeds in very coarse meshes with regular time steps. The properties of the proposed method are demonstrated by a wide range of numerical examples for scalar problems and reactive Euler problems in both one and two dimensions.
Date:Monday Aug 1, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Joseph Powers, University of Notre Dame
Title:Verified Calculation of Multiscale Combustion in Gaseous Mixtures
 
Abstract:Prediction of combustion of gaseous mixtures described by detailed kinetics and multicomponent transport is notoriously difficult due the multiscale nature of the phenomena. For common mixtures such as hydrogen-air at atmospheric pressure, it will be rigorously shown that the finest continuum reaction length scales are a few microns and are well predicted by a simple estimate from Maxwellian diffusion analysis. The coarsest reaction length scales are typically several orders of magnitude larger. Both fine and coarse reaction scales are usually dwarfed by geometric scales of an associated engineering device. For flows characterized by a finite number of thin fronts, a computational strategy for achieving a fully verified, direct numerical simulation (DNS) solution in a demanding multiscale environment is demonstrated. The strategy is based upon a massively parallel implementation of an adaptive algorithm built with the aid of a wavelet-based representation of all state variables in the flow field. Results are shown for inert and reactive compressible multi-dimensional Navier-Stokes models.
Date:Thursday Jul 28, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Shankar Ghosh, Postdoctoral Associate, Aerospace Engineering and Mechanics University of Minnesota, Minneapolis, MN
Title:Towards numerical simulation of high enthalpy turbulent flows.
 
Abstract:The first part of the presentation describes numerical simulation of a laser-induced plasma and it's interaction with isotropic turbulence. The numerical method developed for this problem uses Fourier spectral spatial derivatives, characteristic based shock capturing and logarithm based formulation of the continuity equation. Local thermodynamic equilibrium conditions are assumed to apply. The post-energy deposition flow field is divided into shock formation, propagation and core roll up stages. Each stage is studied in detail and a mechanism is suggested for core roll up. Vorticity is found to be produced in the flow at short and long times due to different mechanisms. For the turbulent simulations, the effect of turbulence on the flow is studied for compressible and incompressible limits. Compressible turbulence is found to slow down the mean shock wave during formation and propagation stages and suppress core roll up in the mean. Also turbulence levels are found to be amplified in the vicinity of the shock wave due to presence of mean compression there. This effect is spatially inhomogeneous and non-stationary in time. Turbulent kinetic energy budgets are computed to explain the mechanism of transfer of energy between the mean flow and the background turbulence. The second part of the presentation describes a numerical method for simulation of high-enthalpy turbulent flows. A non-dissipative algorithm is used for accurate flux reconstruction at the cell faces. A predictor corrector based shock capturing scheme is used to simulate strong shock waves. The Navier-Stokes equations are suitably modified to represent various thermo-chemical processes occurring in high enthalpy flows. A five species model for air is considered. To account for finite rate chemical reactions, individual mass conservation equations are solved for every species. An equation for conservation of vibrational energy is also solved to account for vibrational excitation. The numerical method is evaluated using test problems.
Date:Monday Jul 18, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:ANTHONY RUIZ, CERFACS/Snecma, Toulouse, France
Title:DNS AND LES OF TURBULENT COMBUSTION IN LIQUID ROCKET ENGINES
 
Abstract:In a rocket engine at nominal operating point, combustion takes place at high pressure (typically 100 bar), well above the thermodynamic critical point of the reactants. At these supercritical conditions, the dense reactants injected into the combustion chamber do not follow the classical atomization, evaporation, and burning path. Due to vanishing surface tension and latent heat of vaporization, this reactive flow can be modeled as a single variable-density fluid and developing DNS or LES for such flows raises new challenges. In this talk, we first present the thermodynamic equations needed for the use of a real gas equation of state in an explicit variable-density solver. Then the stabilization of a 2D LOX/GH2 jet flame behind a splitter plate is studied using DNS of the reacting flow with realistic H2/O2 chemistry to gain insight into the combustion physics for this type of flame. Finally, a 3D LES of a coaxial LOX/GH2 jet flame, correspondingto the C60 Mascotte experiment performed at ONERA is presented.
Date:Friday Jul 15, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Thierry Poinsot, Université de Toulouse, CNRS and CERFACS
Title:LES AND ACOUSTIC STUDIES OF COMBUSTION INSTABILITIES IN ANNULAR COMBUSTION CHAMBERS
 
Abstract:Annular chambers found in most gas turbines exhibit strong unstable modes where the acoustics of the combustor interact with the flames and can destroy the whole gas turbine. We will describe the physics of these instabilities and present two approaches to study them: (1) brute force compressible LES on massively parallel machines and (2) acoustic solvers where the flow is frozen and only unstable acoustic modes are searched in frequency domain. The nature of these modes will be studied in two configurations: a small helicopter chamber (Turbomeca) and a large industrial gas turbine (Ansaldo). Validations will be discussed as well as possible path to use simulation results to control unstable modes.
Date:Friday Jul 8, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Haecheon Choi (visiting professor, CTR, Stanford University)
Title:Grid-point requirement for large eddy simulation and wall boundary condition for wall-modeled LES
 
Abstract:We revisit Chapman's estimate on the required numbers of grid points for wall-modeled and wall-resolving LES. Using more accurate correlations of the skin friction coefficient and boundary layer thickness for high Reynolds number boundary layer flow, we present new Reynolds-number dependence of required numbers of grid points. The importance of providing proper mean wall shear stress is examined for wall-modeled LES and tested for turbulent channel flow using very coarse grids. It is shown that mean velocity profile is predicted reasonably well even without wall modeling once the mean shear stress at the wall is accurately given. An extension of this approach to turbulent boundary layer flow is underway and is briefly discussed.
Date:Friday Jul 1, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:van Oijen - Combustion Technology, Mechanical Engineering Eindhoven University of Technology (TU/e) The Netherlands
Title:Modeling turbulent combustion with flamelet-generated manifolds
 
Abstract:Numerical simulation of combustion has become an important tool in the design and optimization process of modern gas turbines and engines. The ever more strict emission limits are the main driving force for the development of new combustion concepts based on future sustainable fuels. Numerical modeling of combustion is a challenging task. The many different chemical components and reactions introduce a wide range of length and time scales, which demand special numerical solvers. Due to the enormous computational costs, application of detailed reaction models is usually limited to simple academic flow problems. In order to simulate turbulent combustion in real engine geometries, reduction techniques have to be used which simplify the chemistry model without losing (too much) accuracy. A very efficient method is the Flamelet-Generated Manifold (FGM) method developed at TU/e. Since the FGM method was introduced by Van Oijen, it has been systematically analyzed and its application range has been extended to cover almost all combustion modes in existing engines, furnaces and gas-turbines. FGM and very similar methods have been adopted by many academic and industrial research groups, because of their simplicity, efficiency and accuracy. The concentrations of both major and minor species are accurately predicted while the computation time is reduced by approximately two orders of magnitude. The method is valid for laminar and turbulent combustion up to the thin reaction zone regime, in which the internal structure of the chemical reaction layer is not perturbed by turbulent flow structures. In the presentation, several aspects of FGM will be discussed. The procedure to generate FGMs and an analysis of their performance in several test cases will be presented. Special attention will be given to the modeling of preferential diffusion effects and stratification in premixed turbulent flames.
Date:Friday Jun 10, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr Danielle Moreau, Postdoctoral Research Associate, Aerospace, Acoustics and Autonomous Systems Research Group, School of Mechanical Engineering, The University of Adelaide, Australia
Title:An experimental investigation of flat plate trailing edge flow and noise at low-to-moderate Reynolds number
 
Abstract:Trailing edge noise is a major sound source in many noise sensitive applications that use airfoil shapes such as fans, rotors and propellers, wind turbines and underwater vehicles. This seminar presents results of a comprehensive experimental study on the flow and noise generated by the sharp trailing edge of a flat plate at low-to-moderate Reynolds number (Rec = 0.7 × 105 to 5.0 × 105, based on chord). Simultaneous measurements of the flow and farfield noise have been taken for three flat plate models with symmetric and asymmetric bevelled trailing edge geometries in an anechoic wind tunnel at the University of Adelaide. Far-field acoustic data are scaled according to existing theory and compared with predictions of trailing edge noise obtained using current semi-empirical methods. Mean and unsteady velocity data measured in the very near wake with hot-wire anemometry are related to the far-field noise measurements. One of the flat plate models with an asymmetric bevelled trailing edge is observed to radiate high amplitude tonal noise at low-to-moderate Reynolds number. Flow and far-field noise data reveal that, in this particular case, the tonal noise appears to be governed by vortex shedding processes at the trailing edge. Also related to the existence of the tonal noise is a region of separated flow slightly upstream of the trailing edge. Hydrodynamic fluctuations at selected vortex shedding frequencies are strongly amplified by the inflectional mean velocity profile in the separated shear layer. The amplified hydrodynamic fluctuations are diffracted by the trailing edge, producing strong tonal noise.
Date:Friday May 27, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Jan Nordstrom, Professor in Scientific Computing Department of Mathematics, Linkoping University, Sweden
Title:Initial Boundary Value Problems, Summation-By-Parts Operators and Weak Boundary Conditions with Multi-Physics Applications
 
Abstract:During the last decade, stable high order finite difference methods as well as finite volume methods applied to initial-boundary-value-problems have been developed. The stability is due to the use of so-called summation-by-parts operators (SBP), penalty techniques for implementing boundary and interface conditions, and the energy method for proving stability. In this talk we discuss some aspects of this technique including the relation to the initial-boundary-value-problem. By reusing the main ideas behind the recent development, new coupling precodures for multi-physics applications have been developed. We will present the theory by analyzing simple examples and apply to very complex multi-physics problems such as fluid flow problems, elastic and electromagnetic wave propagation, fluid-structure interaction and conjugate heat transfer.
Date:Friday May 20, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Professor Patrick Jenny, Institute of Fluid Dynamics, Zurich, Switzerland
Title:Modeling Premixed Turbulent Combustion Using a PDF Method and Scale-Separation
 
Abstract:Probability density function (PDF) methods for turbulent reactive flows have the well known advantage that averaging of reaction source terms requires no additional modeling. Moreover, if one solves for joint velocity-scalar PDFs, also turbulent convection appears in closed form, which is in particular attractive in situations where counter gradient diffusion may occur. So far, however, most of the success of PDF methods is attributed to their applications to non-premixed turbulent combustion. Here, a closure for premixed turbulent combustion in the corrugated flamelet regime is presented, which is based on scale separation and where a PDF method is employed to provide the required joint statistics of velocity, turbulence frequency, and a scalar vector. Unlike in most other PDF methods, the scalar source term not only describes reaction rates, but accounts for ignition of reactive unburnt fluid elements due to propagating embedded quasi laminar flames within a turbulent flame brush. If one assumes unperturbed embedded flame structures and a constant laminar flame speed (as more or less expected in the corrugated flamelet regime), then the mean flame surface density, and thus the probability for an individual fluid element (represented by a Lagrangian particle in the PDF method) to ignite during a time step, can rigorously be calculated within the presented modeling framework. This is achieved by introducing the following particle properties: a flag indicating whether a particle represents the unburnt mixture; a flame residence time, which allows to resolve the embedded quasi laminar flame structure; and a flag indicating whether the flame residence time lies within a specified range. Latter, together with precomputed one dimensional laminar flames, allows to accurately estimate the flame surface density and thus the ignition probability. In most flames, in addition to the turbulent flame brush, also molecular mixing of the products with a co-flow has to be modeled by a mixing model. It has to be emphasized however, that in the presented modeling approach latter is not critical for the turbulent premixed flame propagation. To validate the new model, PDF simulation results of three piloted methane-air Bunsen flames (Aachen flames) are compared with experimental data. At the end of the presentation, the necessary modeling steps to extend this approach for also non- premixed and partially premixed turbulent combustion are discussed.
Date:Friday May 13, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Stanley Ling, Department of Mechanical and Aerospace Engineering, University of Florida
Title:Particle dispersal in multiphase explosions
 
Abstract:Particle dispersal in multiphase explosions is an interesting and complicated phenomenon. Modeling and simulation of this problem are challenging due to the complex interactions between the particles and the compressible flow features. Previous experiments and direct numerical simulations (DNS) have shown that the particle force and heating during the interaction with shock waves can be much larger than those predicted by the standard quasi-steady correlations. However, little work has been done to improve the inter-phase interaction models. Based on recent advances in our understanding of particle force and heating in compressible flows, this work proposes a rigorous inter-phase interaction model for unsteady compressible multiphase flows that includes unsteady force and heating contributions. The model is first validated by comparing with DNS results for particle interaction with planar shock and detonation waves. The importance of unsteady contributions to force and heating are evaluated by measuring their peak values and net effects. The model is applied to investigate the particle dispersal in the classical explosion problem considered by Brode (1955). The simulation results show that ignoring compressibility and unsteady force and heating contributions in the inter-phase interaction model introduces significant errors.
Date:Thursday May 12, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:V. Olshevsky, Postdoctoral Scholar, Stanford University Center for Turbulence Research
Title:Solar convection and differential rotation: a need for a new model.
 
Abstract:Present knowledge of solar differential rotation and global-scale convection is reviewed. Theory of these phenomena has been developing for more than half a century, but still our understanding is far from the complete picture. Mean-field models are too simplified and have too many free parameters to pretend to be the solution. Only a decade ago a realistic numerical modeling of the global-scale convection in rotating spherical shells became possible. Now the problem is generally considered as solved, however... it is not! There is a demand for realistic, three-dimensional simulations of global-scale solar convection and differential rotation. I think, unstructured LES solvers provide such opportunity, and necessary computational resources are already available.
Date:Friday May 6, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Robert Martin, Staff Research Associate, Center for Energy Research, University of California San Diego
Title:Modeling and Validation of Mixture Separation Effects in Compressible Gasses
 
Abstract:For complex integrated systems such as hypersonic aircraft, high fidelity direct numerical simulations play a critical role in linking experimentally accessible physical phenomena to the tractable reduced order models necessary for design. However, in compressible turbulence modeling, gas kinetic effects resulting from disparate mass mixture components are often neglected even at the level of direct numerical simulation. In addition to Fickian diffusion driven by concentration gradients, kinetic theory predicts mass fluxes relative to mixture average velocity due to gradients in pressure and temperature as well as external fields. In this talk, the incorporation and validation of thermal- and baro-diffusive effects within the framework of an adaptively refined multi-dimensional shock capturing mixture fluid algorithm will be discussed. Through validation studies with respect to experimental data as well as prior analytical and numerical models, the impact these effects have on the composition and structure of inert shocks and under-expanded jets will be demonstrated. Comparison with Monte Carlo kinetic simulations also enables investigation of continuum breakdown for mixtures involving high Mach numbers, low density, and small spatial scales. Finally, potential regimes for which macroscopic turbulent mixing properties may be impacted by these effects will be briefly considered with particular emphasis on rarefied supersonic hydrogen injection as an area of future study.
Date:Friday Apr 15, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Alessandro Gomez, Prof. of Mechanical Engineering at Yale Center for Combustion Studies Department of Mechanical Engineering, Yale University
Title:Highly Turbulent Strained Flames: a Benchmark for Computational Studies?
 
Abstract:I will review ongoing experimental research in my group on turbulent nonpremixed and premixed flames in the counterflow configuration. This type of system was originally pioneered in the ‘60s for laminar flows and has been widely used in the combustion community in laminar flame studies. Recently developments in our laboratory have shown that it is also a promising benchmark for highly turbulent flames, at turbulent Reynolds numbers on the order of one thousands. The system may offer significant advantages from a computational viewpoint, including the following ones: a) the flame stabilization is aerodynamic, near the stagnation plane between two opposed jets of either reactants or reactants and products, with ensuing simplifications in the prescription of boundary conditions; b) the domain of interest is dramatically more compact as compared to conventional jet flames at the same Reynolds number; and c) the mean residence time is on the order of one millisecond, which is particularly useful from a DNS/LES standpoint, and to avoid soot complications in the combustion of practical fuels. Case studies will demonstrate the versatility of the system in mimicking real flame effects such as heat loss and flame stratification under well controlled conditions. Speaker’s biographical sketch: Professor Gomez received a Laurea in Ingegneria Aeronautica from University of Naples (Italy) in 1980, and a Masters and Ph.D. in Mechanical and Aerospace Engineering from Princeton University in 1982 and 1986, respectively. After a postdoctoral and lectureship experience in the Department of Chemical Engineering at Yale, he joined the Faculty of the Department of Mechanical Engineering in 1989, where he raised through the ranks to his current position as Professor. His research interests focus on fundamentals of combustion and of electrostatic spray processes with applications. Coauthor of more than 90 articles in the peer-reviewed literature, he has been the recipient of a NSF Young Investigator Award, the Whitby Award from the American Association for Aerosol Research and awards from the Fulbright Foundation, the Von Karman Institute of Fluid Mechanics, ATA Fiat Research Center, and Aeritalia. He is Associate Editor of Combustion Science and Technology and Director of the Yale Center for Combustion Studies. For further details see http://www.eng.yale.edu/gomez-lab/
Date:Friday Apr 8, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Haecheon Choi, Professor School of Mechanical and Aerospace Engineering Seoul National University, Korea. Also Visiting Professor Department of Mechanical Engineering Stanford University
Title:Flow over a Sphere and Its Controls
 
Abstract:The flow over a sphere contains complicated flow phenomena, although its geometry is simple. We simulate the flow over a sphere in the subcritical regime (Re = 3700, 10,000 and 100,000) using large eddy simulation with an immersed boundary method. The computational results are compared with the experimental ones and some of the characteristics of flow over a sphere are described. Next, we briefly introduce passive and active controls applied to flow over a sphere. These controls include the dimples, surface trip wires, background turbulence, and surface blowing/suction. These controls provide more than 50% drag reduction, and we show that the drag-reduced flows are very similar to the flow over an uncontrolled sphere under drag crisis, suggesting an important flow control strategy for drag reduction. Finally, we present our on-going efforts on new design of golf ball.
Date:Friday Mar 4, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Julien Bodart, Postdoc, Stanford University Center for Turbulence Research
Title:DNS of shearless turbulent boundary layers
 
Abstract:The main purpose of studying shearless boundary layers is to understand how energy is transferred between normal and tangential velocity components in the near-wall region. In the context of wall-bounded turbulent flows, this remains one of the main issues to be solved to build appropriate second-order closure turbulence models. We revisit the problem of Perot and Moin (1995) where decaying homogeneous turbulence is interacting with a wall/free-slip surface. In the present case, the shearless turbulent boundary layer is sustained by applying an outer random forcing acting in a central layer of the domain. The produced shearless turbulence self-diffuses and interacts with a solid wall/free-slip surface on both sides, which makes the configuration comparable to oscillating grid experiments. The proposed DNS database of this case brings new conclusions: in addition to the viscosity, we identify the skewness of the velocity field in the outer region as the main responsible of the energy redistribution close to the wall and we propose new length scales associated with this transfer. Rotational events are also shown to play a role.
Date:Friday Feb 11, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Ivo F. Sbalzarini, Assistant Professor of Computational Science ETH Zurich
Title:From stochastic optimization to more efficient simulation algorithms (Ivo F. Sbalzarini, MOSAIC Group, ETH Zurich, Switzerland)
 
Abstract:Stochastic dynamics provides a rich framework for analyzing and designing optimization and simulation methods. In continuous black-box optimization, bio-inspired concepts from Darwinian evolution have triggered the development of numerous algorithms, including Genetic Algorithms and Evolution Strategies. Despite the success of these algorithms in real-world applications, however, the underlying stochastic dynamics remains poorly understood, and guarantees of solution quality and convergence are not available for any but the simplest toy cases. We analyze stochastic optimization algorithms as probability processes based on Gaussian Adaptation and illustrate several analogies and connections with stochastic processes in chemistry and physics, as well as with adaptive MCMC sampling. Exploiting these connections helps understand stochastic optimization algorithms, but also inspires more efficient algorithms for adaptive sampling and stochastic simulations. The latter is shown for chemical kinetics on the example of the PDM exact Stochastic Simulation Algorithm.
Date:Friday Feb 4, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Claude Cambon, Laboratoire de Mécanique des Fluids et d’Acoustique, École Centrale de Lyon, France
Title:What remains to be done using Rogallo’s techniques. Some proposals for Geophysical and Astrophysical Flows.
 
Abstract:Unbounded turbulent shear flows subjected to large-scale effects of shear, rotation, stratification, MHD, can be studied using pseudo-spectral DNS in a comoving frame (Rogallo 1981). A brief historical survey is given, including connection with linear theory, sometime called "Rapid Distortion Theory", towards a fully nonlinear approach. Different communities use this approach, namely engineering, applied mathematics, geophysics and astrophysics. In the last community, numerous studies, even recent, are performed in the context of thin radially stratified accretion disks, but they use their own terminology: This is clarified here as far as possible. I will present studies in my team, which all take into account the gyroscopic torque induced by the misalignment of system vorticity, in a rotating frame, and vorticity of the rotating mean flow. The case of precessing rotating flow is first addressed. The gyroscopic torque induced by a precession can be exactly balanced by a mean shear, which in turn can trigger an instability very close to the "elliptical flow instability" (Kerswell 1994, Salhi & Cambon 2009). Just recall that turbulence forced by the basic eliptical flow instability was studied at CTR by Blaisdell & Shariff, and Mansour & Lundgren, namely. The second case is the turbulence forced by a baroclinic instability. In this case, the starting point is a mean shear flow rotating in the vertical direction and subjected to a vertical stabilizing mean density gradient. In this case, the gyroscopic torque resulting from vertical system vorticity and spanwise shear-induced vorticity is exactly balanced by an horizontal new component of the density gradient: This yields tilting the mean isopycnal lines vs. their horizontal direction, so that a baroclinic instability is triggered. Typical results from "RDT" and DNS are presented, following Salhi & Cambon (2006) Simon et al., Pieri et al.
Date:Wednesday Jan 26, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Robert Rosner – William E. Wrather Distinguished Service Professor Departments of Astronomy & Astrophysics and Physics, Enrico Fermi Institute, and Computational Institute The University of Chicago
Title:Key Problems of Plasma Astrophysics: A Foray into new Territories
 
Abstract:
Date:Friday Jan 14, 2011
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Eric Darve
Title:Fast linear algebra using low rank properties of matrices
 
Abstract:
Date:Friday Dec 10, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Riccardo Rossi, Visiting Scholar – Research Assistant, Laboratori di Termofluidodinamica Computazionale Università di Bologna and Visiting Scholar, Center for Turbulence Research Stanford University
Title:Scalar mixing from concentrated sources in urban-like roughness
 
Abstract:The prediction of scalar mixing in urban areas at the neighborhood/street-scale is challenging because of the presence of obstacles which lead to very complex flow features. Gaussian dispersion models are inappropriate in the near-field, where the scalar plume is typically confined within the roughness-sublayer and therefore more refined computational models, such as LES and RANS are required. The present study is motivated by the need for a coherent framework for the evaluation of computational models for dispersion in urban environments and it is the final step in a process involving flows around 2D and 3D isolated obstacles. We investigate the scalar release from a concentrated source located within a staggered array of wall-mounted cubical obstacles of random height. In this talk we present some preliminary results from a DNS study of the flow setup which is carried out to obtain a characterization of the mixing process in the presence of discrete roughness as well as to provide a reliable and comprehensive database for model evaluation and improvement.
Date:Friday Dec 3, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Marie Farge, LMD-CNRS, Ecole Normale Superieure, Paris, France Kai Schneider, CMI Universite de Provence, Marseille, France
Title:Wavelets and Turbulence: an Update
 
Abstract:We developed wavelet based method to extract coherent fluctuations out of fully developed turbulent flows, called Coherent Vorticity Extraction (CVE). After recalling the principles of this method we will first present recent applications for 3D homogeneous MHD turbulence and 2d wall-bounded turbulence. In a second part we will present results obtained during the CTR 2010 summer program, in collaboration with George Khujadze, Romain Nguyen van yen and Martin Oberlack, where we have applied CVE to high-resolution DNS of 3D turbulent boundary layers. In a third part we will show adaptive wavelet-based computations, in 2D for compressible Euler equations, and in 3D for weakly compressible mixing layers. The results thus obtained will be compared with adaptive mesh refinement (AMR) computations in terms of both CPU time and memory savings.
Date:Monday Nov 29, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Christophe Josserand, Institut D'Alembert, CNRS & Univ. Paris VI
Title:Air effects in drop impact dynamics
 
Abstract:Drop impacts are ubiquitous in many processes, ranging from ink-jet printing to combustion, particles dissemination or rain drop erosion for instance. Despite this wide range of applications, the dynamics of drop impacts remains poorly understood, in particular, when splashing occurs. Recently the influence of the surrounding gas (often neglected) has been exhibited in experiments on solid surface. I will discuss in the seminar the role of the surrounding gas in drop impact for the two different situations of impact on solid surfaces and on thin liquid films. I will focus in particular in the entrapment of a gas bubble at the impact and on its influence on the splashing dynamics.
Date:Friday Aug 13, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Chi-Wang Shu, Brown University
Title:High Order Schemes for Hyperbolic Systems with Source Terms and Applications to Reactive Flows
 
Abstract:In this talk we will summarize the joint research over the past two years with Sjogreen, Wang and Yee on high order finite difference WENO schemes and filtered schemes for solving hyperbolic systems with source terms. The main objectives of this research are two-folds. The first is to obtain high order accurate, non-oscillatory well balanced schemes for reactive flows. The well balancedness property allows the schemes to resolve non-polynomial equilibrium solutions of the nonlinear system exactly, thus allowing good resolution of small perturbations from these equilibrium solutions using coarse meshes. The second is to obtain high order accurate, non-oscillatory schemes which may resolve systems stiff source terms with correct shock propagation speed using coarse meshes. We apply a sub-cell resolution idea together with an anti-diffusive technique to achieve this purpose without compromising
Date:Sunday Aug 8, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Alexander Bihlo, Postdoctoral Fellow, Centre de recherches mathematiques at the Universite de Montreal, Canada
Title:Invariant turbulence modeling
 
Abstract:Symmetries are among the most successfully employed concepts in science and mathematics. They form a cornerstone of various branches of physics, such as classical and quantum mechanics, particle physics and relativity. The governing equations of hydrodynamics generally possess wide symmetry groups and therefore there is a great potential to exploit these symmetries so as to derive similarity solutions, conservation laws and invariants or to study the effects of symmetry breaking due to the presence of boundaries or additional body-forces. However, to date, symmetries are often used in a non-explicit or indirect way in hydrodynamics and turbulence theory. On the other hand, there exist powerful and general methods introduced in the field of group analysis of differential equations which, when suitably adapted, can be readily applied to the aforementioned fields. In this talk we will introduce an algorithmic method which allows associating with a given object its invariant counterpart. The object under consideration can be, e.g., a turbulence closure model or a finite-difference discretization of a differential equation, which can then be invariantized to yield a turbulence model or a finite-difference discretization that is invariant under the same Lie point symmetry group as admitted by the original governing equations of hydrodynamics. This method can therefore be used to correct artificial symmetry breaking due to non-appropriately designed turbulence models. As an example it is shown that classical hyperdiffusion as used in two-dimensional (decaying) turbulence simulations violates the symmetries of the incompressible Euler equations. Invariantization of these hyperdiffusion terms yields symmetry-preserving but nonlinear diffusion-like terms. Using the notion of differential invariants it is demonstrated that the invariantized hyperdiffusion models can be modified with quite some flexibility while still preserving their desired invariance characteristics. First numerical tests show that the invariant hyperdiffusion schemes which can be obtained by this method might be able to reproduce the -3 slope of the energy spectrum in the enstrophy inertial range.
Date:Wednesday Jul 14, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Igor Rogachevskii, Ben-Gurion University of the Negev, Beer-Sheva, Israel
Title:New Phenomena in Turbulent Transport of Particles and Droplets: Theory, Experiments and Numerical Simulations
 
Abstract:We discuss new effects related to large-scale and small-scale clustering of particles and droplets in stratified turbulent flows. The large-scale clustering is caused by phenomenon of turbulent thermal diffusion which has been predicted theoretically and detected in our laboratory experiments, atmospheric observations and direct numerical simulations. The essence of this phenomenon is the appearance of a non-diffusive mean flux of particles in the direction of the mean heat flux. This effect results in formation of large-scale inhomogeneities in the spatial distribution of particles that accumulate in the regions of minimum mean temperature of the surrounding fluid. The phenomenon of turbulent thermal diffusion can be significant, e.g., in dispersion of atmospheric aerosols. We also discuss a new type of small-scale particle clustering (namely, tangling clustering of inertial particles) in a stably and unstably stratified turbulence with imposed mean vertical temperature gradient. This phenomenon have been predicted theoretically and detected in our laboratory experiments. In the stratified turbulence a spatial distribution of the mean particle number density is nonuniform due to the phenomenon of turbulent thermal diffusion, that results in formation of a non-zero gradient of the mean particle number density. It causes generation of fluctuations of the particle number density by tangling of the large-scale gradient by velocity fluctuations. In addition, the mean temperature gradient produces the temperature fluctuations by tangling of the large-scale gradient by velocity fluctuations. The anisotropic temperature fluctuations increase the rate of formation of the particle clusters in small scales. In stratified turbulence this tangling clustering can be much more effective than a pure inertial clustering (preferential concentration) that has been observed in isothermal turbulence. The tangling clustering of droplets can increase the rate of rain formation in turbulent clouds and can be significant in various industrial multi-phase turbulent flows (e.g., internal combustion engines).
Date:Wednesday Jun 30, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Suk Ho Chung, Professor of Mechanical Engineering and Director of Clean Combustion Research Center, King Abdullah University of Science and Technology (KAUST)
Title:Electric-field effect on Jet Flame Stabilization and Instability
 
Abstract:Electric field or plasma assisted combustion has been studied with regard to flame speed, flame stabilization, and pollutant emission. In case of electric field assisted combustion, lower power consumption and less flow disturbance could be expected as compared to plasma- assisted combustion. To improve the stabilization characteristics of jet flames, the effects of AC and DC electric fields on the stabilization of jet flames have been investigated, including the influences on liftoff, blowout, blowoff, and reattachment for nonpremixed laminar jet flames, on the propagation speed of tribrachial (or triple) flames, and on the liftoff of nonpremixed turbulent jet flames. Together with these experimental results, oscillation behavior of a diffusion flame in a counterflow burner in response to applied AC electric fields will be discussed to identify the ionic wind effect.
Date:Friday Jun 25, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Sang Lee, University of Illinois at Urbana-Champaign
Title:Turbulence and high Mach number flow control
 
Abstract:In this talk, two topics are presented: intermittent accelerations in turbulence and flow control of shock boundary layer interactions. The intermittency of acceleration is investigated for isotropic turbulence using Direct Numerical Simulation. Intermittently found acceleration of large magnitude always points towards the rotational axis of a vortex filament, indicating that the intermittency of acceleration is associated with the rotational motion of the vortices that causes centripetal acceleration. Further investigations on movements of such vortex filaments provide some insights into the dynamics of local dissipation, enstrophy and acceleration. Strong dissipation partially covering the edge of a vortex filament shows weak correlation with enstrophy, while it is strongly correlated with acceleration. The second part of the talk discusses the high Mach number flow control with passive devices using Large Eddy Simulation. The performance of supersonic engine inlets and external aerodynamic surfaces can be critically affected by shock wave / boundary layer interactions, whose severe adverse pressure gradients cause flow separation leading to poor boundary layer recovery. This study investigates a novel type of flow control device called micro-vortex generators which alter the near-wall structure of compressible turbulent boundary layer to provide increased mixing of high speed fluid. This contributes to the enhancement of the boundary layer health downstream of the shock interaction, some of which include: improved pressure recovery, reductions in displacement thickness and increased wall shear stress. Future plans to extend this study with active control strategies at hypersonic regime are briefly addressed in this presentation.
Date:Monday May 17, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Annick Pouquet, Senior Research Scientist at NCAR
Title:Combining rotation and helicity in turbulent flows and the emergence of strong and persistent cyclonic columnar vortices
 
Abstract:Rotation, as measured by the Rossby number Ro = U0/[L0 ] with U0,L0 characteristic velocity and scale, and the rotation rate is present in many astro- geophysical flows. For rapid rotation, significant progress has been made by developing resonant wave interactions, two-point spectral closures, and the theory of weak turbulence. However, at high Reynolds number Re, small scales are excited resulting in a break-down of the weak turbulence regime because the scale-dependent Rossby number increases. Invariance properties of a physical system are known to govern its behavior but the role of helicity, which measures departures from mirror symmetry and which is an invariant of the ideal Euler equations, remains unclear since it does not alter the energy distribution. However, the interplay of rotation and helicity leads to significant differences as we show using direct numerical simulations (DNS) up to a grid of 15363 points and down to Ro  0.06. Long-lived laminar cyclonic vortices together with turbulent vortices are found to co-exist, somewhat reminiscent of recent tornado observations but in a simpler physical context. The energy undergoes both a large-scale and a small-scale cascade. In the latter case, it is self-similar with spectrum E(k)  k−e and transfer rate  with no deviations from Gaussianity and dominated by the helicity cascade (with spectrum H(k)  k−h and transfer rate ˜). This result points to the discovery of a new small parameter in helical turbulence with solid body rotation, namely /[L0˜]. We also find that the spectral indices obey the scaling law e + h = 4 when taking into account the inertial wave mediation of nonlinear transfer to small scales. In view of the cost of such a massive DNS, resorting to Large Eddy Simulation (LES) modeling is in order. Using an isotropic model based on two-point closures of turbulence, and taking into account the contribution of helicity to eddy viscosity and eddy noise, we show that we can recover the DNS results at substantially lower costs, up to almost four order of magnitude less: the DNS run on a grid of 15363 points can be reliably modeled using grids of 963 points. Performing with the model a parametric study, we find that, at fixed Re, strong rotation leads to this new e + h = 4 regime, whereas one may be recovering the classical Kolmogorov law when increasing the Reynolds number at fixed rotation rate.
Date:Tuesday May 11, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Hiroyuki Abe, Japan Aerospace Exploration Agency
Title:DNS and modeling of a separated turbulent boundary layer
 
Abstract:Separation of a turbulent boundary layer is one of the most challenging research topics in aeronautics. The data, which can serve as reference data for developing turbulence models, are however still limited. Over the past several years, we have performed DNS of a separated turbulent boundary layer, where a separation bubble is created by imposing blowing and suction at the upper boundary. The inlet Reynolds numbers based on the freestream velocity and the momentum thickness are 300, 600 and 900, the latter value being three times larger than that of the seminal DNS works (Spalart and Coleman 1997; Na and Moin 1998). The objectives of the present study are to clarify effects of the Reynolds number in a separation bubble and also to develop turbulence models. In this talk, some representative DNS results are first described (e.g. negative production of the turbulent kinetic energy, large-scale spanwise meandering of the separation line and large-scale structures in the bubble). Next, results regarding the RANS (low Re k-epsilon) model testing are presented; the model tends to predict the separation point earlier than the DNS. Finally, the future works on DNS and modeling (RANS and LES) are briefly described.
Date:Friday May 7, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Roberto Battiti, University of Trento, Italy and Reactive Search SrL
Title:Reactive Search Optimization: Incorporating User Preferences and Feedback in the Design Process
 
Abstract:Reactive Search Optimization (RSO) advocates the integration of sub-symbolic machine learning techniques into search heuristics for solving complex optimization problems. The word reactive hints at a ready response to events during the search through an internal online feedback loop for the self-tuning of critical parameters. Methodologies of interest for Reactive Search include machine learning and statistics, in particular reinforcement learning, active or query learning, neural networks, and meta-heuristics (although the boundary signaled by the "meta" prefix is not always clear). We will discuss issues related to interactive multi-objective optimization where the human decision maker (or designer) is in the loop, delivering feedback about preliminary solutions which is used by the software to fine tune the search for improved designs.
Date:Friday Apr 9, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Anthony Roux, CTR Postdoctoral Fellow, Stanford University
Title:Numerical simulation to ease understanding the complex unsteady combustion features of ramjet burners
 
Abstract:Recent numerical predictions obtained by Large Eddy Simulations (LES) for turbulent reacting flows underline the power of the approach for laboratory and industry like configurations. Most recent LES have focused on gas turbine configurations because these systems exhibit a variety of difficult problems that are well addressed through that fully unsteady approach: ignition, quenching, thermo-acoustic instabilities... Only a few studies have been devoted specifically to LES of ramjets. This type of combustor exhibits significant differences compared to gas turbine flows: the velocities are much higher, combustion is not stabilized by swirl, a chocked nozzle terminates the chamber... The objective of this seminar is to present a methodology based on LES to contribute to the understanding of combustion in these devices. We present here the application of LES in the context of side-dump ramjet for which experimental data provided by ONERA are available for different non-reacting and reacting conditions. A particular attention is paid to the chemical scheme. It is shown that reproducing the evolution of the adiabatic flame speed for a wide range of equivalence ratio is critical because of the partially premixed combustion regime involved in this configuration: two different simulations are compared, both based on a one step chemistry, the only difference being the adjustment of the pre-exponential of the rate of heat release to match the laminar flame speed given by a detailed mechanism. Two distinct limit cycles are reached depending on the chemical scheme used in LES and strong flow variations, especially the upstream position of the flame, are noted. Results obtained with one step simple chemistry exhibits self-sustained oscillations of the combustor. The flame is stabilized in the vicinity of the shear layer developing at the exit of the air inlet. Use of the PEA scheme yields comparisons against the experiment that show very good agreement contrarily to the first case. For the second LES, the flame is detached from the air inlet. Temporal analysis of the instantaneous LES results exhibits all main experimental low frequencies including the first longitudinal acoustic mode whereas the high frequencies excited in the first simulation are damped. Three different cases are then simulated and excellent agreement is found with experimental data. The phenomenology and the different mechanisms governing combustion are studied for these three cases. This example gives an example of stratified combustion where reactants are neither perfectly premixed nor completely segregated. If this combustion scenario is usual and of foremost interest, only a few experimental test-rigs have been designed to create databases to validate combustion models for numerical simulation. In this presentation, we will finally present preliminary results for stratified combustion using a combustion model coupling a levelset function with a transported progress variable.
Date:Friday Mar 12, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Manuel Lombardini, Graduate Aeronautical Laboratories, California Institute of Technology
Title:Investigation of shock-driven mixing in Richtmyer-Meshkov flows under reshock conditions
 
Abstract:The Richtmyer-Meshkov instability (RMI) arises from the baroclinic generation of vorticity at a perturbed density interface when impacted by a shock wave, and is often thought of as the impulsive limit of the Rayleigh-Taylor instability. Baroclinic instabilities play a fundamental role in the context of different physical environments ranging from astrophysics to supersonic combustion. In such applications, the interface is often processed by multiple shock waves of different traveling directions, resulting in successive depositions of vorticity at the interface. This leads to a large dynamical range of scales across a resulting mixing region between the fluids of different initial densities. While the effect of the incident shock strength and initial interface perturbation shape have been the subject of extensive research in the planar RMI, we focus here on two other important aspects of the problem that have been much less covered: the geometry (planar vs. curved) and the initial density gradient (in amplitude and sign). The investigation relies on the use of large-eddy simulations using a hybrid numerical scheme that provides low numerical dissipation in the regions away from shocks but reverts to a shock-capturing stencil otherwise. This work gives a better understanding of some of the main features of shock-driven mixing that are not easily accessible experimentally.
Date:Friday Feb 26, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Andrea Lani, CTR Postdoctoral Scholar, Stanford University
Title:An Object Oriented and High Performance Platform for Aerothermodynamics Simulation
 
Abstract:This seminar presents some key aspects of the design and implementation of COOLFluiD, an object-oriented collaborative software environment for high-performance computing which has been developed at the Von Karman Institute during the last eight years. The platform is targeted towards the simulation of multi-physics phenomena on unstructured grids by means of multiple numerical discretizations. To this end, we introduce a number of design techniques that we have developed in order to provide scientific computing frameworks with extreme flexibility and reusability, allowing developers to dynamically integrate new functionalities such as arbitrary mesh-based data structures, numerical algorithms (space and time discretizations, linear system solvers, etc.) and physical models. We describe some parallel algorithms (including I/O) designed for generic unstructured meshes, showing their suitability for large scale computing. In this context, the driving goal has been to provide a reliable tool for handling high speed aerothermodynamic applications. Multiple systems of partial differential equations, characterizing gases in conditions of thermodynamic and chemical equilibrium (with fixed and variable elemental fractions) and, particularly, non-equilibrium (multi-temperature models) will be briefly reviewed. Finally, we present two multi-dimensional parallel implicit CFD solvers for arbitrary systems of PDE's which have been implemented into COOLFluiD in order to simulate such complex flows: a steady/unsteady cell-centered Finite Volume solver for hybrid grids and a steady vertex-centered Residual Distribution solver for meshes with simplex elements. All the developments have been validated on real-life testcases of current interest in the aerospace community, showing comparison with experiments and/or literature whenever possible.
Date:Thursday Feb 11, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Roddam Narasima, Jawaharlal Nehru Centre, Bangalore
Title:Laboratory simulation of cumulus cloud flows
 
Abstract:The dynamics of clouds is a subject that has become increasingly important in atmospheric general circulation models and in climate change science. Attempts in the 1960s and 70s to model cumulus clouds as similarity plumes failed to agree with field observations. This failure is here traced to inadequate accounting of the profound effects of the release of latent heat on condensation of water vapor to liquid water in cumulus clouds. The seminar will describe the results of an experimental program at JNC / IISc that successfully simulates cumulus cloud flows, including in particular cloud shapes and (hence as well as otherwise) their entrainment characteristics. The experiments have been supplemented by numerical simulations that highlight the role of baroclinic torque.
Date:Friday Jan 29, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Abdellah HADJADJ, National Institute of Applied Sciences, INSA & CORIA - Rouen - France
Title:Some salient aspects of shock wave/boundary layer interaction
 
Abstract:Shock wave/boundary layer interaction (SWBLI) and shock/shock interference are two fascinating problems which are closely linked. They are present nearly in all high-speed flows. In particular, shock induced separation is at the origin of low-frequency unsteadiness, that can be highly detrimental for vehicle structure. The present talk will give an overview of some recent advances in LES modeling of SWBLI at M=2.3 over an adiabatic flat plate. Additionally, the study will address the question of the relevance of the remaining sub-grid terms appearing in the energy equation, terms that are often neglected under the assumption of weakly compressible small-scale turbulence. Also, the issue of filtering across the shock will be highlighted.
Date:Friday Jan 15, 2010
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. James B. Grotberg, Ph.D., M.D., Department of Biomedical Engineering, University of Michigan
Title:RESPIRATORY FLUID MECHANICS
 
Abstract:Fluid mechanics in the pulmonary system involves biphasic flow phenomena of air flow through liquid-lined tubes (airways), airway closure instabilities that couple capillarity with wall flexibility, as well as liquid plug propagation of instilled medications. The air-liquid interface is influenced by surfactants, and the liquid layer has Newtonian and non-Newtonian properties and in some regions is a bi-layer. This overview of research in our group will touch on surfactant delivery into the lung for purposes of treating hyaline membrane disease in prematurely-born infants, the shock wave surfactant spreading previously identified in our lab, the clearance of mucus plugs from airways, and airway closure dynamics for single and bi-layers. Animal and benchtop experiments will be discussed as well as related theory and computational fluid dynamical models. A number of diseases are related to these phenomena including asthma, emphysema, surfactant deficiency, respiratory distress syndrome, and cystic fibrosis.
Date:Friday Dec 11, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Riccardo Rossi, Laboratorio di Termofluidodinamica Computazionale, Universita' di Bologna, Italy
Title:Numerical simulation of scalar mixing from a point source over a wavy wall
 
Abstract:The release of a passive tracer from a point source over a wavy wall is analyzed using Direct Numerical Simulations (DNS) to obtain a detailed description of the scalar plume dynamics over a complex topography. Although the scalar source is located on top of one of the wave crests, thus representative of a ground release (GS), the comparison with available results for scalar mixing from elevated sources (ES) shows that the initial decay of mean concentration is affected by the flow separation occurring in the first-half of each wave. Nonetheless, in the far-field the concentration profiles are reasonably described by empirical correlations for flows over rough surfaces. The budget of scalar fluxes also shows that each component obeys a local balance between advection, production and the scalar-pressure-gradient correlation. Similar results have been previously reported for the vertical and lateral fluxes; however, it is interesting to note how the streamwise component is usually dominated by the production-dissipation balance under simple-shear conditions. Finally, algebraic flux models in the form of the Generalized Gradient-Diffusion Hypothesis (GGDH) and its High-Order extension (HOGGDH) are evaluated against the Standard-Gradient Diffusion Hypothesis (SGDH) to predict the decay of mean concentration downstream of the source. The results are found noticeably improved by the algebraic models, which also appear to be a reasonable approximation for the spanwise component of the scalar fluxes.
Date:Friday Dec 4, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Joseph W. Nichols, Center for Turbulence Research Postdoctoral Fellow, Stanford University
Title:Global mode decomposition of supersonic jet noise
 
Abstract:Instability wave mechanisms are considered for sound generation in supersonic jets from the perspective of global mode analysis. While instability waves traditionally are important to low Reynolds number transitional flows, laboratory experiments by Suzuki & Colonius (JFM, 2006) have confirmed their presence in turbulent jets at Reynolds numbers up to 10^6. Moreover, in a supersonic jet, the supersonic convection of instability waves couples directly to the acoustic field, causing Mach wave radiation (Tam, ARFM, 1995). In the present work, instability waves and their coupled acoustic radiation are treated as "global modes," which are three dimensional temporal perturbation eigenfunctions of the system. As fully 3D eigenfunctions, global modes capture directly effects of base flow non-parallelism. The least stable global modes are extracted from simulations governed by the fully compressible linearized Navier--Stokes equations using a shift-and-invert Arnoldi method and the massively parallel linear solver, SuperLU. To compare with results obtained from the Parabolized Stability Equations (PSE) technique (see Cheung et al., AIAA, 2007), a cold laminar supersonic jet at Mach number M=2.5 is first considered. Although globally stable, this jet still supports a maximum transient energy growth of 10^7 computed from an optimal superposition of the (convectively) non-normal global modes. While purely linear, this transient growth is composed of several frequencies, and as such may be key to predicting eventual nonlinear mode interactions responsible for low frequency sound production. Finally, to more fully demonstrate the power of global modes to resolve effects of base flow non-parallelism, preliminary results of global modes computed about mean flow fields measured from large eddy simulations of a turbulent jet at Mach number M=1.5 computed using the CDP-CF4 code (Khalighi & Ham, 2009).
Date:Friday Oct 30, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Gaurav Bansal, Center for Turbulence Research
Title:Computational Studies of Autoignition and Combustion Relevant to Modern Engines
 
Abstract:We present computational studies of autoignition and combustion behavior encountered in modern internal combustion engines in which combustion is achieved primarily via autoignition of the reactant mixture. Computational tools with varying levels of complexity are employed to systematically investigate the phenomena under consideration. Firstly, turbulence-autoignition interaction for nonpremixed n-heptane/air mixture is studied using a counterflow configuration in which a well defined unsteady scalar dissipation rate oscillation represents the effects of the turbulent flow field. A newly defined ignitability parameter is proposed which systematically accounts for all the unsteady effects. Next, we present the results of high-fidelity direct numerical simulations (DNS) of autoignition in thermally and compositionally stratified turbulent mixtures. Using non-reacting RANS engine simulations, different initial conditions to be studied using DNS are identified. Diagnostic techniques are developed to quantitatively identify the different heat release modes ranging from premixed deflagration to homogeneous autoignition which are present in stratified mixtures. Finally, to aid in subgrid scale modeling of these complex autoigniting systems, a novel methodology based on Principal Component Analysis (PCA) is used to identify the intrinsic low-dimensional manifolds in these systems.
Date:Friday Oct 16, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Ali Mani, Center for Turbulence Research
Title:Reflectivity analysis of sponges in compressible flow simulations
 
Abstract:In this talk, I will go through the common reflectivity mechanisms due to flow/sponge interactions. The work mostly involves derivation of back-of-envelop-type relations governing sponge reflectivity, and will be presented in a PDE course style. I will mostly use the white board for the analysis, but will present some numerical experiments to support the results. The following paragraph is an introduction to my talk: In finite-domain compressible flow simulations, one remedy to address lack of boundary information is to gradually relax the flow near the external boundary to a known consistent far-field solution of the Navier-Stokes equations. This treatment, called the sponge treatment, is adopted in many calculations owing to its simplicity, generality and robustness. In practical calculations however, interactions of the sponges with flow features can reflect unphysical signatures into the CFD domain. If the sponge is not carefully designed these reflections can overwhelm the physics of interest particularly when acoustics are concerned. In this work we examine the physics of sponge/flow interactions through analytical and semi-analytical approaches. The reflectivity due to non-linear terms, oblique waves intersecting, and sponge/vortex interactions are separately analyzed. The optimal sponge profiles and the reflection coefficients for asymptotically small or large sponges (compared to flow features) are investigated. These analyses provide estimates of the sponge requirements for CFD calculations in a relatively general framework.
Date:Friday Oct 2, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Guido Lodato, Center for Turbulence Research
Title:Three-dimensional Boundary Conditions for Direct and Large-Eddy Simulation of Turbulent Flows.
 
Abstract:Two main topics related to Direct and Large-Eddy Simulation of turbulence are discussed: (a) on one side, numerical aspects regarding the implementation of numerically transparent boundary conditions are addressed; (b) on the other side, a structural sub-grid scale (SGS) model for Large- Eddy Simulation of weakly compressible turbulent confined flows is presented. A three-dimensional procedure for characteristic boundary conditions is proposed. This very sensitive point of boundary conditions, which is seldom thoroughly discussed in literature, was found to be closely related to convection and pressure gradients developing in the directions parallel to boundary faces, also called transverse terms. A method involving the inclusion of these transverse effects in the computation of the incoming wave amplitude variations is developed and extended to chemically reacting flows. This method, which is based on the NSCBC approach, removes the original one-dimensional inviscid assumption-leading to the so called LODI system-that is, in general, too stringent to correctly deal with the very complex flow structure obtained from DNS and LES of turbulent flows. Additional problems of wave coupling at the edges and corners of three-dimensional structured computational domains are also discussed. Hence, based on the three-dimensional characteristic formulation for the Navier-Stokes equations, a systematic procedure to solve edges and corners is developed. With regards to LES of weakly compressible turbulent flows, a structural mixed model, based on the similarity assumption is developed and tested on the impinging round-jet at Reynolds numbers 23000 and 70000. The difficulties of purely dissipative functional models based on the eddy-viscosity hypothesis, when dealing with such a complex flow, are addressed and the necessity to improve the modeling strategy by better accounting for the peculiar interaction terms arising from the use of non Reynolds operators are analyzed. The eddy-viscosity term together with the modified Leonard tensor allows good representation of non-local interactions as well as local interactions near the cutoff length, these last being responsible for local events of reverse energy transfer. Furthermore, the unphysical alignment between the SGS stress tensor and the resolved strain tensor-a condition which is intrinsically enforced by any eddy-viscosity model-is automatically removed. The use of the WALE model, in particular, allows proper wall scaling of the wall shear stresses. The new model was developed with particular attention to the correct reproduction of the average theoretical scaling within the viscous sub-layer for each component of the SGS tensor.
Date:Friday Sep 18, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Jeroen Witteveen, Center for Turbulence Research
Title:Extremum diminishing uncertainty quantification with constant error in time for computational fluid dynamics and fluid-structure interaction
 
Abstract:Numerical errors in computer simulations have shown a tremendous decrease over the last decades due to the increased availability of computational resources and efficient algorithms. Intrinsic uncertainties in model parameter values, and initial and boundary conditions limit the current predictive capabilities of numerical simulations. The effect of these physical uncertainties cannot be quantified by a Monte Carlo simulation of performing a large number of random computations due to the already high computational costs involved in a single deterministic simulation. Stochastic Collocation has been developed as a more efficient uncertainty quantification method based on Gauss quadrature sampling and Lagrangian interpolation in probability space. Its recent utilization in computational fluid dynamics and fluid-structure interaction applications has, however, revealed a number of shortcomings of the approach. The accurate approximation of discontinuous responses and unsteady behavior are two of these central challenges addressed in this presentation. For treating discontinuous response surfaces an extremum diminishing uncertainty quantification method is presented based on Newton-Cotes quadrature sampling in an adaptive simplex elements discretization of probability space. The method also satisfies the total variation diminishing robustness concept, which assures that no non-zero probabilities for unphysical realizations are predicted due to overshoots at discontinuities. In addition a methodology for unsteady oscillatory problems is developed which maintains a constant accuracy in time with a constant number of samples. The method based on interpolation of scaled samples at constant phase results in a bounded error in time for periodic and non-periodic responses. Multi-frequency behavior of continuous structures is treated by employing a wavelet decomposition pre-processing step. Applications to transonic flow problems and aero-elastic simulations with randomness in the flow and the structure are considered. Results for transonic flows show that the local production of standard deviation in the shock region due to the sensitive shock wave location amplifies the input uncertainty also to output integral quantities of interest such as the lift force. In the fluid-structure interaction applications the randomness is found to trigger an earlier onset of unstable flutter behavior, which a deterministic simulation would have missed. This suggests that the presented uncertainty quantification approach forms a more reliable design practice than using safety margins in combination with deterministic simulation results.
Date:Friday Sep 4, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Aaron Brandis, Center for Turbulence Research
Title:Nonequilibrium Radiation Intensity Measurements and Modeling Relevant To Titan and Earth Entry
 
Abstract:The predictions of nonequilibrium radiation for a Titan aerocapture aeroshell vary significantly amongst Computational Fluid Dynamics (CFD) analyses and are limited by the physical models of the non- equilibrium flow. Of particular interest are the non-equilibrium processes associated with the cyanogen (CN) molecule which is known to be a strong radiator. It is therefore important to have experimental data for these radiating shock layers which will allow for validation of CFD models. Furthermore, a more detailed understanding of the chemical processes that lead to the formation of CN above equilibrium concentration is required. This thesis describes the modelling of the radiation behind a shock using a collisional-radiative (CR) model and presents measurements of radiation intensity behind a shock in simulated Titan and Martian atmospheres. The uncertainties in radiation is more significant at lower speeds (around 5-8 km/s) with these atmospheres when compared to Earth entry. This is due to the formation of CN and because of the highly non-equilibrium nature of the flow. The motivation for this work began with the successful landing of the Huygens probe on the surface of Titan which led to the renewed interest in inter-planetary missions. Thus radiative heating during atmospheric entry to Titan and Mars was the subject of several experimental campaigns and extensive computational analyses. In order to better understand the formation of CN, and the nonequilibrium radiation emitted under such atmospheric conditions, NASA Ames Research Center conducted a series of experiments on their Electric Arc Shock Tube facility, EAST. Furthermore, several research groups in Europe and the United States independently developed CR models to predict the measured levels of radiation. The results from these simulations showed some major discrepancies and highlighted a lack of knowledge and understanding about the fundamental physics behind the formation and decay of the CN molecule and its associated excited states. Based on a comparison of the various simulations with the CR models and the EAST experimental data, it was concluded that the absolute level of peak radiation was well predicted, however, there was a significant discrepancy related to the decay rate of the radiation. Therefore, to add to the relatively small amount of experimental data for these highly non- equilibrium radiating flow conditions, experiments were performed on the X2 shock tube at The University of Queensland with the aim of producing a comprehensive set of benchmark data for Titan entry. The data obtained from these experiments have been used to validate the results from the NASA Ames testing, and due to the large parametric variation, as a source for code validation. In addition to the experimental component of this thesis, an investigation into the simulation of CN nonequilibrium radiation was conducted. It has been previously concluded that there was a significant discrepancy between the experimentally measured radiation decay rate and the predicted value from CR models. Therefore, the primary aim of the simulation work presented in this thesis is to explain the reason behind this discrepancy. Through a parametric study of important reactions combined with an analysis of the reaction set, it was concluded that the coupling between the dissociation of N2 and the formation of CN (through the reaction N2 + C = CN + N) controlled the radiation decay rate. The reason for the super equilibrium concentrations was identified to be a result of the N2 + C = CN + N reaction continuing to over-produce CN after nominal equilibrium values are reached. This is due to the slow build up of N to drive the reverse reaction. Thus it has been shown in this thesis that the behaviour of the CN concentration is controlled by the rate of N2 dissociation. This led to the implementation of a more thorough method for simulating the dissociation process of molecular nitrogen. Therefore, a mono-quantum vibration state specific model that includes excitation and de-excitation reactions for all the vibrational states of nitrogen was incorporated into the CR model developed by Magin et al. The nitrogen vibration state specific model that was implemented was developed by Pierott and is based on SSH theory. The model developed in this thesis is known as the ViSpeN CR model (Vibrationally Specific Nitrogen). The ViSpeN results show significantly better agreement with experimental data in terms of the decay rate, initial rise of the radiation and the overall trends in the data. However, the work in this thesis has shown there are still discrepancies in predicting the absolute level of radiation measured in shock tunnel experiments. This led to the development of a modification to the ViSpeN model (known as ViSpeN-L) which includes a proposed new value for the radiative lifetime of the CN violet transition. The agreement between the experimental data and the ViSpeN-L model is excellent for conditions relevant to Titan entry.
Date:Friday Aug 21, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Professor Javier Jimenez, U. Politecnica de Madrid and the Center for Turbulence Research
Title:Transitional structures in high-Reynolds number wall-bounded
 
Abstract: We will summarized recent work regarding what part, if any, of linearized theory is relevant to high-Reynolds number sheared turbulence. The best known example is the free-shear case, where energy production is dominated by the linearized instabilities of the mean profile, but the profiles of wall-bounded turbulence are known to be stable. The last decades have shown that stability is not equivalent to lack of growth, and that transient growth factors can be large enough to lead to nonlinear, self-sustained, dynamics. However, the linearized equations in the wall-bounded case depend on viscosity, and, except for the reasonably well-established structures of the buffer layer, even basic agreement with experiments requires an eddy viscosity model. In essence, the energy-containing eddies away from the wall can only be modelled in the sense of LES. Recent results, both linear and nonlinear, are reviewed, including the comparison between amplification factors and observed spectra, which is only fair. Possible reasons for the remaining disagreements are summarized, including recent solutions for some of them, and work in progress on others.
Date:Friday Aug 7, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Michael Frewer, Institute of Fluid Dynamics, Technische Universitat Darmstad
Title:A Consistent 4D Invariant Turbulence Modeling Approach
 
Abstract:A new turbulence modelling approach is presented. Geometrically reformulating the averaged Navier-Stokes equations on a 4-dimensional non-Riemannian manifold without changing the physical content of the theory, additional modelling restrictions naturally emerge which are absent in the usual Euclidean (3+1)-dimensional framework. The modelled equations show full form-invariance for all Newtonian reference frames in that all involved quantities transform as true 4-tensors. Frame accelerations or inertial forces of any kind are universally described by the underlying 4-dimensional geometry. By constructing a non-linear eddy viscosity model within the k-epsilon family for high turbulent Reynolds numbers the new invariant modelling approach demonstrates the essential advantages over current (3+1)-dimensional modelling techniques. In particular, new invariants are gained which allow for a universal and consistent treatment of non-stationary effects within a turbulent flow. Furthermore, by consistently introducing via a Lie-group symmetry analysis a new internal modelling variable, the mean form-invariant pressure Hessian, it will be shown that already a quadratic non-linearity is sufficient to capture secondary flow effects, for which in current non-linear eddy viscosity models a higher non-linearity is needed.
Date:Friday Jul 24, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Ivan Bermejo Moreno, Center for Turbulence Research
Title:On the non-local geometry of turbulence
 
Abstract: We present a methodology for the study of the non-local geometry of structures in turbulence. Starting from a three-dimensional field it consists of three main steps: extraction (through a multi-scale decomposition, based on the curvelet transform, followed by iso-contouring of each component field), characterization (based on differential-geometry properties and their area-based joint probability functions) and classification (using geometrical signatures and enhanced with clustering techniques) of individual structures. We apply this methodology to several fields - passive scalar fluctuation, enstrophy, dissipation obtained from turbulence numerical databases with different grid resolutions (256^3, 512^3 and 1024^3). A transition, with decreasing scale, from blob-like to tube-like to highly stretched sheet-like structures is found. The differences among fields are discussed, as well as the effect of the grid resolution on the educed geometries. Additionally, an assessment of the geometries educed by two existing local identification criteria in turbulence - Q, for tubes, and [A_ij]_+, for sheets - is performed. Finally we introduce a new methodology for the study of proximity issues among different sets of structures, based also on geometrical and non-local analyses. We apply it to four of the fields previously studied. Tube-like structures of Q are mainly surrounded by sheets of [A_ij]_+, which appear at close distances. For the enstrophy, tube-like structures at an intermediate scale are primarily surrounded by sheets of smaller scales of the enstrophy and structures of dissipation at the same and smaller scales. A secondary contribution results from tubes of enstrophy at smaller scales appearing at farther distances. Different configurations of composite structures are presented.
Date:Friday Jul 10, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Jan Nordstrom, Visiting Professor from Uppsala University
Title:Accurate and Stable Calculations Involving Shocks Using a New Hybrid Scheme
 
Abstract:We develop a hybrid scheme consisting of a combination of a second order MUSCL scheme and a high order scheme. The full hybrid scheme is constructed in such a way that we can prove that it is conservative and stable for linear problems. We show by numerical experiments that it is high order accurate in smooth domains and oscillatory free close to shocks.
Date:Friday Jul 10, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Ms. Sofia Eriksson, Visiting Graduate Student from Uppsala University
Title:Analysis of mesh and boundary effects on the accuracy of node-centered finite volume schemes
 
Abstract:The accuracy of the node-centered finite volume method in one-dimension is analyzed. Numerical simulations and analysis are performed for both a hyperbolic and a elliptic case, for various types of grids. The results from the simulations agree with the analysis. The boundary conditions are implemented weakly using penaly technique. For the hyperbolic case we see that the type of grid has large impact on the order of accuracy, whereas the choice of penaly parameter only affect the error constant. For the elliptic case the grid has less impact on the order of accuracy. For both the hyperbolic and elliptic problem we show that the error contribution from the primal and dual grid can be treated separately.
Date:Friday Jun 19, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Mr. Daniel Mitchell, Visiting Researcher at Stanford University
Title:Spatially resolved thermometry in shock tube environments by way of toluene planar laser induced fluorescence
 
Abstract:Planar laser induced fluorescence has long been recognized as a valuable research tool in the study of both fluid mechanical and combustion phenomena. The present research project details the development and application of a toluene based PLIF technique to the study of shock tube flows. Toluene offers unprecedented sensitivity to temperature variation, and very high fluorescent quantum yield, allowing for much more precise measurements of spatial temperature variation than were previously possible. Shock tubes present a difficult challenge for the experimentalist, with the phenomena under investigation being high temperature and pressure, as well as extremely transient. Toluene PLIF has provided the opportunity to obtain quantitative maps of temperature in a variety of gas dynamic flows. Many of these flows have, in the past, only been accessible to qualitative techniques such as schlieren. This ability to obtain quantitative measurements yields opportunities for a better understanding of the underlying physics of the flow structure, as well as a greatly improved ability to validate numerical codes.
Date:Friday Jun 12, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Pierre Wolfe, CERFACS, Toulouse, France
Title:Massively parallel LES of azimuthal thermo-acoustic instabilities in annular gas turbines
 
Abstract:Increasingly stringent regulations and the need to tackle rising fuel prices have placed great emphasis on new designs for aeronautical gas turbines. These are often prone to combustion instabilities. In the particular field of annular combustion chambers, these instabilities usually take the form of azimuthal modes. To predict these modes, one must compute the full combustion chamber, which remained out of reach until very recently and the development of massively parallel computers. Since one of the most limiting factors in performing Large Eddy Simulation (LES) of real combustors is the mesh size, the effects of mesh resolution are investigated by computing full annular LES of a realistic helicopter combustion chamber on two grids, respectively made of 38 and 93 million elements. Results are compared in terms of mean and fluctuating fields. Two versions of this helicopter combustor, which differ only on the swirlers' design, are also computed. In both computations, LES captures self-established rotating azimuthal modes. However, the two cases exhibit different thermo-acoustic responses and the resulting limit-cycles are different. With the first design, a self-excited strong instability develops, leading to pulsating flames and strong heat release fluctuations. In the second case, the flames are much less affected by the azimuthal acoustic mode and remain stable, allowing an acceptable operation. This study therefore highlights the potential of LES for discriminating injection system designs.
Date:Friday Jun 5, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Simon Mendez, CTR Postdoctoral Fellow, Stanford University
Title:Large-Eddy Simulations for Supersonic Jet Noise Predictions
 
Abstract:Supersonic jet noise has been the topic of numerous investigations from the first studies on aeroacoustics. Jet noise is one of the most important components of aircraft noise, at take-off for example. Currently, motivations for supersonic jet noise research are multifold. For example, reducing the noise generated by military aircrafts is of course an important motivation, but it becomes crucial on aircraft carriers, where sailors are very close to the aircrafts at take-off. The present research project is funded by NASA in the framework of the NASA Research Opportunities in Aeronautics. The aim is to determine if sufficient noise reduction can be achieved in order to use supersonic aircrafts for civil transport in the future. One way of achieving noise reduction for jets at the engine exhaust is to modify the nozzle geometry: the most famous example is chevrons. In the last ten years, numerical predictions of jet noise have developed. In particular, Large-Eddy Simulation seems a promising method. However, structured solvers are often used, so that LES are limited to relatively simple geometries. To be able to support the development of innovative noise reduction techniques, handling complex geometries is crucial. In this presentation, we will present results for supersonic jet noise computed using an unstructured solver developed at CTR. For the moment, an axisymmetric nozzle is considered to validate the approach. In the presentation, the difficulties of performing LES for supersonic jet noise predictions will be stressed and preliminary results will be shown and analyzed.
Date:Friday May 29, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Didier Lucor, Institut Jean Le Rond d'Alembert, Université Pierre et Marie Curie
Title:Spectral Stochastic Approaches for Uncertain Nonlinear Hyperbolic Systems
 
Abstract:Uncertainty quantification through stochastic spectral methods has been recently applied to several kinds of non-linear stochastic PDEs. However, strong physical non-linearities such as steep fronts and shocks are tricky as they easily translate to the parametric stochastic space. Several numerical approaches can be pursued depending on the solution discontinuity with respect to the parametric uncertainty and/or the growth of the stochastic dimensionality of the problem. We will present two different approaches through several applications. The first part of the talk will focus on numerical investigation of airfoil performance at stochastic transonic flow regimes. Studies will be presented where a deterministic RANS compressible solver is coupled to a non-intrusive stochastic collocation solver to propagate several aerodynamic uncertainties through a steady flow around a NACA0012 and a OAT15A airfoils. The stochastic model is based on the generalized Polynomial Chaos theory combining the advantage not to modify the existing deterministic code while remaining accurate in the computations of the statistical moments of the stochastic flow. The robustness and efficiency of the present methodology are evaluated for the propagation of random disturbances associated to the angle of attack and the free-stream Mach number. Different stochastic flow regimes are analyzed in details by means of various post-processing procedures, including error bars, probabilistic density function of the aerodynamic field Sobol's coefficients... Two kinds of non linearities seem to be critical with respect to the skin-friction uncertainties: on one hand, the leeward shock movement and on the other hand, the boundary-layer separation on the aft part of the airfoil downstream the shock. In this case, the sensitivity analysis shows that a strong coupling exists between the uncertain parameters. In the second part of the talk, we introduce an intrusive formalism to tackle uncertain hyperbolic systems of conservation laws with Polynomial Chaos (PC) methods. The idea is to introduce a new variable, the entropic variable, in bijection with our vector of unknowns, which we develop on the polynomial basis: by performing a Galerkin projection, we obtain a deterministic system of conservation laws. We state several properties of this deterministic system in the case of a general uncertain system of conservation laws. We then apply the method to the case of the inviscid Burgers’equation with random initial conditions and we present some preliminary results for the Euler system. We systematically compare results from our new approach to results from the stochastic Galerkin method. In the vicinity of discontinuities, the new method bounds the oscillations due to Gibbs phenomenon to a certain range through the entropy of the system without the use of any adaptative random space discretizations.
Date:Friday May 22, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Florian Kummer, TU Darmstadt, Department of Mechanical Engineering, Germany
Title:BoSSS: Bounded Support Spectral Solver - A generic discontinuous Galerkin framework
 
Abstract:In the past 1 1/2 years, the authors have been working on an object-oriented framework for the discontinuous Galerkin (spectral element, DG) method, with a strong aim on CFD applications. This library was programmed in C# for Microsoft .NET and Mono framework. Up to our knowledge, it's the first ambitious CFD code which was implemented using the .NET framework. The talk is split into two parts, the first one giving an introduction into the Discontinuous Galerkin method, especially into the problems that arise when one tries to solve the Poisson equation. We present our actual work on the p-multigrid - method, which enables us to overcome the slow convergence of the Poisson Solver, caused by the bad condition number of the matrix resulting from the DG discretization. The second part of the talk cares about the software engineering aspects. In our opinion, managed languages offer a new perspective to supercomputing software development. We demonstrate that the key issues for supercomputing, portability (to supercomputers/clusters) and performance are ensured and demonstrate the benefits that we gain from using such languages. These benefits are binary platform independence, rich debugging features, and a runtime that classical languages can't compete with.
Date:Friday May 8, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Abdellah HADJADJ, Visiting Scholar, National Institute of Applied Sciences, INSA & CORIA - Rouen - France
Title:High-fidelity numerical simulation of high-speed flows including shock/shock and shock/boundary-layer interactions
 
Abstract:The first part of the presentation deals with numerical simulation of supersonic turbulence when shock/turbulent boundary layer interaction occurs. Such flows reveal the existence of complex mechanisms, which have to be well understood for an efficient design of propulsion systems. In this study, both DNS and LES are used to investigate unsteady mechanisms. Since a shock-capturing scheme is used, a hybrid numerical scheme has been developed to reduce its dissipative properties. The obtained results are analysed and discussed in terms of mean and turbulent quantities. Excellent agreement between LES, DNS and experimental data is obtained. Some features relative to the organization of the large eddies are given and the importance of the low frequencies shock unsteadiness is discussed in relation to the SBLI. Also, the validity of the assumptions of the strong Reynolds analogy (SRA) in SBLI is addressed. The second part of the presentation relates to numerical flow visualizations in high-speed aerodynamics. Numerical schlieren pictures as well as computed interferogram techniques are used to visualize the major features of physical phenomena that can be mostly encountered in supersonic flows, such as supersonic turbulence including shock/shock and shock/boundary layer interactions, shear-layer instability and transient flows. Some of the numerical visualization results constructed from computed Navier-Stokes flow-fields are directly compared to experimental images. Most of the features observed in the experiment are accurately reproduced by the simulations. The results of this study provide in general better understanding of the main characteristics of complex supersonic flows that are not easily accessible experimentally, and may be useful for flow controlling and practical high-speed aerodynamics design and improvement.
Date:Friday Apr 24, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Sergei Chumakov, Stanford University
Title:Development of one- and two-equation models in Large-Eddy Simulation
 
Abstract:One-equation subgrid models for Large Eddy Simulation use auxiliary quantities to close the equation of motions, and these quantities are calculated by adding an extra transport equation to the problem. One- equation models tend to be more accurate and have wide application area than zero-equation models such as classical Smagorinsky model. I will discuss several different approaches and present a new model for the dissipation rate of subgrid-scale kinetic energy - a work in progress that leads to a two-equation model that combines characteristic features of two very different one-equation approaches developed recently.
Date:Friday Mar 20, 2009
Time:4:00pm
Location:Building 300, Room 300
Speaker:Prof. Julian C. R. Hunt, University College of London
Title:Thin shear layers –the key to turbulence structure (with collaborators, I.Eames, P. Davidson, J.Westerweel, J.Fernando,S.Voropayev, M.Braza)
 
Abstract:Sharply sheared interfaces determine the structure of turbulent motions both on large and small scales, as recent experiments and simulations have demonstrated. They form wherever there are very large gradients in turbulence intensity and gradients in the large scale velocity field ; they move through the flow as a result of local scale ‘nibbling’ by Kelvin-Helmholtz instabilities and mixing at the interface , and by larger scale ‘engulfing’ motions of the interface of low turbulence fluid ; shear layers become stronger through the combined action of the large scale shear and the blocking of the inhomogeneous turbulence by the interface ; with low curvature of the large scale shear profile (such as in free shear layers), and high levels of external or internal perturbations , the engulfing motions of the interface are more effective than ‘nibbling’. Where turbulence exists outside shear layers, idealised models and experiments show how smaller eddies are distorted, and then disappear which transfers their energy to larger scales .Because they are inhomogeneous these eddies stretch and distort the larger scale vorticity so as to counter the diffusive tendency of the interface to thicken. These concepts are applied to the structure of high Reynolds number turbulence by focusing on the shear layers observed between or on the boundaries of large eddies. The numerical simulations of Ishihara, Gotoh and Kaneda (2009) show that on the edges of the layers and in their interiors thin viscous layers form, on the Taylor microscale. Intermittent small-scale vortices within these viscous layers are amplified up to the limit set by viscous diffusion –their reduced thickness reduces to the Kolmogorov micro length scale. But estimates of rms vorticity and velocity have to take into account the two length scales of this process, and the degree to which the thin layers are sufficiently convoluted to be ‘space-filling’ By blocking the external scale eddies impinging onto the interfaces ,a wide range of inertial range upscale and down scale inertial range motions scales are generated outside the layers with a characteristic power law spectrum, and where the skewness of the velocity derivatives is negative . The upscale motions may be significant for amplifying the shear layers. One of the most significant implications of this ‘interface dynamics ’ mechanism, is that small scale turbulence is produced significantly faster –as is observed- than by the more gradual ‘cascade’ mechanism proposed by L.F. Richardson., or by equivalent models based on statistical physics .There are possibilities for improved turbulence modeling based on this inhomogeneous-zonal analysis where processes on different ranges of scales occur in different parts of the flow field.
Date:Friday Feb 27, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Qiqi Wang, Stanford University
Title:A high order multivariate approximation scheme on arbitrary grids with potential applications in uncertainty quantification and numerical methods
 
Abstract:We construct a high order multi-variate interpolation scheme for arbitrary scattered data sets. The estimated approximation error is minimized by solving a equality constrained least squares. The approximation function is an interpolation when the data points are exact or a regression function when there are measurement errors. Using this formulation, the gradient information on each datapoint can be used to significantly reduce the interpolation error. The approximation converges exponentially on smooth functions for a variety of grids, including randomly scattered nodes. The output of the approximation scheme includes the estimated approximation error. Therefore, it is a natural method of estimating the uncertainties generated by the interpolation, as well as propagating uncertainties from the data points to the interpolated value. We also present methods of solving differential equations with collocation and tau methods using this interpolation scheme.
Date:Friday Feb 20, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Jan Nordstrom, Uppsala University
Title:Well Posed and Weakly Coupled Fluid Structure Interaction Problems
 
Abstract:We investigate model problems of fluid structure interaction type and aim for a formulation that leads to a well-posed problem and a stable numerical procedure. Our first objective is to investigate if the generally accepted formulations of the FSI problems are well posed and the only possible ones. Our second objective is to prove that the numerical coupling is truly stable. To accomplish that we will use a weak coupling summation-by-parts operators and penalty terms. In multiple dimensions this is a formidable task and we start by investigating the simplest possible model problem available. As a flow model we use the linearized Euler equations in one dimension and as the structure model we consider a spring.
Date:Friday Jan 16, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Chi-Wang Shu, Brown University
Title:High Order Well Balanced Schemes and Applications to Non- Equilibrium Flow with Stiff Source Terms
 
Abstract:The modeling of unsteady flow problems containing finite-rate chemistry or combustion poses additional numerical difficulties over and above the different scales associated with turbulence flows. One of the main difficulties stems from the appearance of the source terms, which are sometimes stiff. A well-balanced scheme, which can preserve certain non-trivial steady state solutions exactly, may help to resolve some of these difficulties. In this talk, we will first describe the general strategy to design high order well balanced finite difference schemes. We will then move to the discussion of a few schemes, including the high order WENO finite difference scheme based on the Roe building block, the high order WENO finite difference scheme based on the Lax-Friedrichs building block, and three well known second order TVD schemes, in terms of their well-balanced properties for a simple 1D model with one temperature and three species. We show through numerical experiments that, for the stationary steady state solutions of the reactive flow, the well balanced schemes will give machine round-off errors regardless of the mesh sizes, while the non-well balanced schemes give truncation errors consistent with the formal order of accuracy for the schemes. For a small perturbation of such steady state solutions, the well balanced schemes can resolve them well with very coarse meshes, while the non-well balanced schemes would give spurious structures in the numerical solutions, which will decrease and eventually disappear with a mesh refinement. Our work indicates the advantage of well balanced schemes: they can be used to resolve small perturbations of the steady state solutions using much coarser meshes than that for the non-well balanced schemes, thereby saving a lot of CPU time. This is a joint work with Wang, Yee and Sjogreen.
Date:Thursday Jan 15, 2009
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Joseph W. Nichols, Laboratoire d’Hydrodynamique (LadHyX), Ecole Polytechnique
Title:Simulation and global stability analysis of round fuel jets
 
Abstract:Direct numerical simulation (DNS) suggests that a light round fuel jet transitions to turbulence through a sequence of primary and secondary global instabilities. First, an axisymmetric primary global instability originates from a pocket of absolute instability near to the nozzle. The pocket of absolute instability acts as a “wavemaker” supporting self-sustaining oscillations which impart their frequency to the rest of the flow. This mechanism for global instability is verified by considering the effect of a lifted flame which forms on the fuel jet when ignited. For sufficiently low liftoff heights, the flame enters the pocket of absolute instability, destroys the wavemaker, and stabilizes the entire flow. For larger liftoff heights, a Krylov-subspace method is used to extract the least stable linear global perturbation modes, revealing their quenched spatial structure. Further downstream, axially elongated structures known as side jets form at regular intervals around the perimeter of the jet core. The same Krylov-subspace method, applied to the monodromy operator rather than the Jacobian, results in secondary global modes, one of which is found to be unstable. Furthermore, the superposition of these modes suggests that side-jet formation owes to a competition between a slow but exponentially growing global mode and a highly non-normal transient response. Finally, we propose to apply the same global stability methodology to study the phenomenon of combustion instability, a problem often arising in lean-burning gas turbine systems. We hypothesize that purely hydrodynamic modes owing to absolute instability may interrupt the coupling between heat-release and acoustic modes necessary to sustain combustion instability.
Date:Friday Dec 12, 2008
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Riccardo Rossi, Laboratorio di Termofluidodinamica Computazionale, Università di Bologna
Title:Progress in the numerical simulation of scalar dispersion in complex flows
 
Abstract:The seminar addresses problems arising in the numerical simulation of passive scalar dispersion in complex geometries using RANS and DNS techniques. In the first part of the talk, a review of the theoretical background for gradient-transport modeling of turbulent transport is presented. In spite of the prevalent use of the standard gradient-diffusion hypothesis (SGDH) in the framework of RANS simulations, that is, a simple similarity with random molecular motion where the eddy diffusivity and the turbulent Schmidt number are introduced, the analysis shows that the SGDH model is inadequate for modeling turbulent scalar fluxes even in the case of simple shear flows, leading to the failure of predicting scalar dispersion under strongly inhomogeneous and spatially developing flow conditions. A significant improvement in the modeling of turbulent transport can be obtained through the generalized gradient-diffusion hypothesis (GGDH) and its high-order extension (HOGGDH), where an algebraic closure for turbulent scalar fluxes is adopted. If the basic constraints for the applicability of gradient-transport type models are satisfied and the Reynolds stresses anisotropy is reasonably captured, numerical experiments on scalar dispersion downstream of a square obstacle show that the use of algebraic models leads to reliable predictions of turbulent scalar fluxes even in the presence of the counter-diffusion process. The second part of the talk is concerned to the use of random-forcing techniques for the generation of incoming fully developed turbulence for the scalar dispersion problem downstream of a two-dimensional obstacle. A preliminary analysis of results will be presented, where turbulence statistics obtained from the forcing technique will be compared to the synthetic turbulent-inflow specification method previously adopted in the computations.
Date:Friday Nov 14, 2008
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Olaf Marxen, Stanford University, Postdoctoral Fellow
Title:Progress towards a better understanding of disturbance evolution in a laminar hypersonic boundary-layer with roughness elements
 
Abstract:Prediction of heat load on the surface of vehicles (re-)entering a planetary atmosphere is important for heat-shield design. As turbulent flow induces a much higher heating than laminar flow, the prediction of laminar-turbulent transition is a key factor in defining the dimensions and materials used for the thermal protection system. Yet, fundamental physical processes related to laminar-turbulent transition in high-speed boundary layers are not well understood. Our understanding is even less comprehensive if two- or three-dimensional roughness elements are present inside the boundary layer. Examples of localized roughness elements are fences in 2-d and bolts in 3-d that may be present on modern heat shields. High-speed, compressible boundary layers often exhibit qualitatively different phenomena than low-speed, incompressible ones, such as shocks and multiple instability modes. Appropriate simulation tools are necessary to accurately capture these physical phenomena associated with compressible boundary layers. The presence of roughness-induced shocks, boundary-layer separation, and vortical structures may lead to strong growth of instability modes and the generation of additional disturbances. Recent results obtained from numerical simulations of a boundary layer with two- and three-dimensional roughness elements will be discussed.
Date:Friday Oct 31, 2008
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Mohammed Zamir Afsar, University of Cambridge, United Kingdom
Title:Jet noise modeling
 
Abstract:In this presentation we show very accurate jet noise predictions can be made using an acoustic analogy. The analogy is based on a form of the linearized Navier Stokes equations derived by Goldstein (2002), and we use it to analyze the sound pressure of a non-heated jet flow. We develop a unified approach to jet noise modeling and start by showing how the jet noise spectrum can be thought of as being composed of two terms, one that accounts for the high frequency noise, and another term that represents the peak sound pressure. In this case, the sound predictions we show are based upon a Reynolds averaged Navier Stokes (RANS) calculation of the Stromberg jet, which has a Reynolds number (Re) of 3600 and Mach number (M) of 0.9. Although the jet noise predictions we obtain are reasonable, they require some empirical tuning of the turbulence properties. We therefore extend the jet noise model and show that very accurate noise predictions can be made without having any empirical tuning involved. The turbulence properties are now found by directly post processing a Large Eddy Simulation (LES) of the jet flow and in this particular case we analyze a high Reynolds number jet, where Re = 106 and M = 0.75. We show the LES-based turbulence properties are in good agreement with the data from experiment, for the forth order longitudinal correlation function. The final optimized jet noise model gives very accurate predictions across the spectrum for various observation locations, at 90°, and closer to the jet axis where the peak noise occurs.
Date:Friday Oct 24, 2008
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Eric Johnsen, Stanford University, Postdoctoral Fellow
Title:Accurate simulations of slowly moving shocks
 
Abstract:Many commonly used shock-capturing schemes exhibit unsteady errors when they are applied to problems in which shock waves move slowly compared to the grid. Though this drawback has been known for several decades, the underlying causes are not well understood; current fixes introduce extra dissipation and perform well for only for specific flow conditions. In this talk, we analyze and characterize the causes for such errors carefully. By specifying appropriate bounds on the wave speeds used in the HLL approximate Riemann solver, we find that no spurious oscillations are generated for first-order accurate methods. We further discuss the extension of this fix to higher-order accurate schemes. The effect of these errors on the flow field is illustrated by considering the interaction between a shock and an acoustic wave.
Date:Wednesday Sep 24, 2008
Time:1:30pm
Location:CTR Conference Room
Speaker:Prof. James Glimm, Department of Applied Mathematics and Statistics, University at Stony Brook
Title:The Mathematics and Numerics of Chaotic Mixing Flows
 
Abstract:This talk will be divided into two parts. In the first part we will explain the operation and use of Front Tracking from the point of view of a potential user, who might be interested in adding a front tracking capability to an existing CFD code. Locations for additional documentation will be presented. The second part of this talk will illustrate the use of Front Tracking for the study of turbulent mixing. There is current interest in combining the separate capabilities of capturing codes, which are efficient for shock waves (and, as is important, steep gradients of concentration, or near contact discontinuities) with accurate capabilities to model turbulent transport. For this part of the talk, we start by identifying the goals: to establish convergence for locations of primary waves and mixing zone edges (macro variables) and the joint PDFs for concentration and temperature (micro variables). Front tracking can be viewed as an enhanced version of the capturing codes, in which effective numerical control is obtained over contact discontinuities and steep solution gradients. To model turbulent transport in this framework, we have added the dynamic subgrid scale models. But the use we make of them is original, in that we do not present these models with a smooth solution, but rather one with steep concentration, shear and thermal gradients. For convergence of the above mentioned observables, we achieve in this manner what appears to be the efficient calculation of resolved quantities, even with high Schmidt numbers.
Date:Friday Sep 12, 2008
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Hong Zhao, University of Illinois at Urbana-Champaign
Title:Simulating flow and flexible structure interactions at medium and low Reynolds number
 
Abstract:Flow and flexible structure interaction is important in many biological phenomena from insect flying to cellular interactions in the microcirculation. We discuss our efforts on simulating such systems in finite Reynolds number flows and in the special case of Stokes flows. The finite-Reynolds-number system consists of complex geometry elastic solid and its surrounding fluid, both of which are incompressible. The effect of the solid, under certain assumptions, is equivalent to a distribution of surface and body forces that are applied to an otherwise purely fluid system. The motion of system is hence governed by the Navier--Stokes equations with the additional forces due to the structure. These equations are discretized and efficiently solved by a fractional step method on the fixed Cartesian mesh, with the solid forces transferred to the Cartesian mesh via a momentum-conserving Galerkin projection. This algorithm is demonstrated by simulation results including the swimming of a model jellyfish. In the Stokes-flow limit, we consider the motion of closely packed red blood cells flowing in microcirculations. The Stokes flow system is solved by using a boundary integral equation method, which evaluates the boundary integrals with an overall computational cost of $O(N \log N)$ by using Ewald sums and subsequently smooth particle-mesh Ewald method. The cell structures are modeled as elastic membranes with finite bending modulus that enclose a more viscous hemoglobin solution relative to plasma. The surface geometries and variables are represented by spherical harmonic expansions, which result in high numerical accuracy and also enable robust stabilization through dealiasing. We present the simulation results for the relaxation time scale for deformed cells and the apparent viscosity of blood flow through narrow cylindrical tubes. These results agree well with the published experimental results.
Date:Friday Sep 5, 2008
Time:4:00pm
Location:CTR Conference Room
Speaker:Dr. Gregory C. Burton, Lawrence Livermore National Laboratory
Title:The Nonlinear LES (nLES) Method: A fundamental paradigm shift in turbulence modeling.
 
Abstract: See Abstract.
Date:Tuesday Aug 26, 2008
Time:4:00pm
Location:CTR Conference Room
Speaker:Prof. Jan Nordstrom, Uppsala University
Title:A Hybrid Methodology for Unsteady Compressible Flows
 
Abstract:Unsteady compressible flow problems exhibit many flow features such as vortices, shocks, turbulence, etc. It is highly unlikely that any single numerical method can be the "best one" for all cases. We discus the concept of hybrid methods and exemplify with problems that deal with complex geometry and shocks.