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. 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.