# CTR Seminars Archive 2015

**Particle based simulation of turbulent sediment transport processes**

Date and Time: Friday, December 11, 2015 - 16:00

Location: Bldg 500 “Conference Room 501A”

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Justin Finn, Research Associate, University of Liverpool (UK)

The transport of sediments due to turbulent wave, current, and tidal flows can have lasting social and environmental consequences. This makes the development of improved predictive capabilities for sediment motion an engineering priority, and motivates fundamental investigations of particle-particle and particle-turbulence interactions in coastal, fluvial, and estuarine boundary layers. In this talk, I will first present practical multiphase modeling guidelines for conducting such simulations within a DNS/LES framework by recasting the particle Reynolds and Stokes number scaling from [1] in terms of the Shields parameter (non-dimensional shear stress) and sediment Galileo number. The scaling results show that a LES point-particle approach is well suited to perform simulations of sub-aqueous quartz sands (specific density ≈ 2.5, size ≈ 0.1mm−2mm) over a broad range of range of Shields parameters. I will then discuss a model of this type that has been tailored specifically to capture the individual and collective dynamics of sand grains having natural size and shape variations. Its capability for simulating sand transport in wave driven, oscillatory boundary layer flows will be demonstrated for both the bed-form (dune, ripple) regime as well as the more energetic “sheet flow” regime, with detailed comparison made to the mobile-bed experimental measurements of [2] and [3]. Discussion of the results will focus on the intra-wave variation of the particle size distribution and the influence of three-dimensional vortical features on sand entrainment and suspension processes.

**References**

[1] S Balachandar. A scaling analysis for point–particle approaches to turbulent multiphase flows. International Journal of Multiphase Flow, 35(9):801–810, 2009.

[2] Tom O’Donoghue and Scott Wright. Flow tunnel measurements of velocities and sand flux in oscillatory sheet flow for well-sorted and graded sands. Coastal Engineering, 51(11):1163–1184, 2004.

[3] JJ Van der Werf, JS Doucette, T O’Donoghue, and JS Ribberink. Detailed measurements of velocities and suspended sand concentrations over full-scale ripples in irregular oscillatory flow. Journal of Geophysical Research, 112(F2):F02012, 2007.

**Speaker Bio: **

Justin Finn received degrees in Mechanical Engineering from Oregon State University (PhD, Msc) and the University of Massachusetts (BSc). His graduate work involved simulation of flow in porous media, studies of bubble-vortex interactions, and coherent structure extraction from time dependent flows. He is currently appointed as a Research Associate in the Centre for Engineering Sustainability at the University of Liverpool (UK), where he develops computational tools to study Lagrangian transport behavior in geophysical and industrial systems.

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**Quantification of Modeling Uncertainties in Turbulent Flow Simulations**

Date and Time: Friday, November 13, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Wouter Edeling, CTR Postdoctoral Fellow

Our goal is to make predictive simulations with Reynolds-Averaged Navier-Stokes (RANS) turbulence models, i.e. simulations with a systematic treatment of model and data uncertainties and their propagation through a computational model to produce predictions of quantities of interest with quantified uncertainty [1]. To do so, we make use of the robust Bayesian statistical framework, in which the uncertainty is represented by probability.

We search for estimates of the uncertainties in the space of model coefficients, for a large range of different calibration scenarios. To capture the coefficient variability over the calibrations scenarios we perform multiple Bayesian calibrations, resulting in a set of joint posterior probability distributions. For an unmeasured prediction scenario, we then combine these distributions into a single predictive quantity of interest using Bayesian Model-Scenario Averaging (BMSA). This framework combines multiple turbulence models and calibration scenarios, allowing one to make predictions with quantified uncertainty.

A full BMSA approach would require many samples from the RANS code, making it computationally expensive for many flow cases of interest. To apply BMSA to complex topologies, we investigated two options. First, one can replace the expensive RANS code with a cheaper surrogate model, e.g. the Simplex-Stochastic Collocation Method [2]. Alternatively, we can keep the full RANS model and instead replace the expensive posterior distributions with Dirac delta distributions centered at their Maximum A-Posteriori values.

**References**

[1] T. Oden, R. Moser, and O. Ghattas. Computer predictions with quantified uncertainty. Technical report, ICES-REPORT 10-39, The institute for Computational Engineering and Sciences, The University of Texas at Austin, 2010.

[2] J.A.S. Witteveen and G. Iaccarino. Simplex Stochastic Collocation with ENO-type stencil selection for robust uncertainty quantification. Journal of Computational Physics, 239:1–21, 2013.

**Speaker Bio: **

Wouter Edeling received his PhD from Arts et Metiers ParisTech and Delft University of Technology. The subject was the quantification of modeling uncertainties in turbulent flow simulations. His MSc thesis, also performed in Delft, was on reduced-order modeling of unresolved scales.

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**Interaction between the atmospheric boundary layer and wind turbines**

Date and Time: Friday, November 6, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Mahdi Abkar, CTR Postdoctoral Fellow

Wind turbines extract kinetic energy from the ambient flow in the turbulent atmospheric boundary layer (ABL). A profound understanding of the mutual interactions between ABL and wind turbines will indeed shed light on the path to the optimized wind farms. Apart from parameters like wind turbine sittings relative to the incoming wind, the wind farm performance is entangled with ABL flow whose structures and dynamics change by the variation of land-surface fluxes, geostrophic forcing and topography. Wind farms have also been reported to change land-surface fluxes (e.g. momentum, heat, moisture, pollution, etc.) which in turn can impact the local weather and climate [1-4]. All in all, this indicates a complex coupling between ABL and wind farms. An accurate and detailed prediction of ABL flow passing through a wind farm can provide insight into the dynamics of wind-farm-atmosphere interactions. Numerical simulation is a powerful and cost efficient approach that can provide invaluable information a) to maximize wind energy production; b) to minimize the fatigue load and noise inside a wind farm; and c) to quantify the magnitude and the spatial extent of a wind farm impact.

The speciﬁc objectives of Mahdi Abkar’s research include: a) improving our understanding about the effect of thermally-stratiﬁed ABL ﬂows on the evolution of wind-turbine wakes as well as the performance of wind turbines in wind farms, and b) proposing new models to parameterize the effect of wind farms in large-scale atmospheric models (e.g., weather models).

**References:**

[1] Keith, D., DeCarolis, J., Denkenberger, D., Lenschow, D., Malyshev, S., Pacala, S., Rasch, P. J., 2004. The influence of large-scale wind power on global climate. Proc. Natl. Acad. Sci. 101.46, 16115-16120.

[2] Baidya Roy, S., Traiteur, J., 2010. Impacts of wind farms on surface air temperatures. Proc. Natl. Acad. Sci. 107.42, 17899-17904.

[3] Calaf, M., Meneveau, C., Meyers, J., 2010. Large eddy simulation study of fully developed wind turbine array boundary layers. Phys. Fluids 22, 015110.

[4] Porté-Agel, F., Wu, Y. T., Lu, H., Conzemius, R. J., 2011. Large-eddy simulation of atmospheric boundary layer flow through wind turbines and wind farms. J. Wind Eng. Ind. Aerodyn. 99 (4), 154-168.

**Speaker Bio: **

Mahdi Abkar received his Ph.D. in Mechanical Engineering from École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, in 2014. His research interests lie on Computational fluid dynamics, Atmospheric sciences and turbulence, and Wind energy.

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**Lagrangian subgrid modelling for the relative separation of tracers in turbulence**

Date and Time: Friday, October 2, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Federico Toschi, Team Leader PIV, German Aerospace Center

Turbulence has an important influence on the transport of small particulate matter like dust in the atmosphere, fuel droplets in combustion chambers and small biological organisms in marine environments.

The relative separation of small tracer particles is strongly influenced by the multi-‐scale space and time correlations of the turbulent velocity field. While in applications it is often necessary to model the smallest turbulent scales, this may have an important impact on particles’ dynamics and in particular on their relative dispersion.

We present recent results on the dispersion of particle pairs studied by means of fully resolved high-‐ resolution and high-‐statistics direct numerical simulations [1,2]. We further discuss a recently proposed subgrid Lagrangian model capable of reproducing the effect of unresolved eddies on single particle dynamics (absolute dispersion) as well as on the relative separation of tracers. The model is simple, computationally efficient and is capable to reproduce Richardson separation dynamics between pairs in a cloud composed by an arbitrary number of tracers [3].

[1] Scatamacchia, R., Biferale, L. & Toschi, F. (2012). Extreme events in the dispersions of two neighboring particles under the influence of fluid turbulence. Physical Review Letters, 109(14):144501. [2] Biferale, L., Lanotte, A.S., Scatamacchia, R. & Toschi, F. (2014). Intermittency in the relative separations of tracers and of heavy particles in turbulent flows. Journal of Fluid Mechanics, 757, 550-‐ 572. [3] Mazzitelli, I.M., Toschi, F. & Lanotte, A.S. (2014). An accurate and efficient Lagrangian sub-‐grid model. Physics of Fluids, 26(9), 095101-‐1/17.

**Speaker Bio: **

Professor Federico Toschi holds the chair of Computational Physics of Multi-‐scale Transport Phenomena in the department of Physics and in the Department of Mathematics and Computer Science at Eindhoven University of Technology. The CPMTP section has recently been established within the 3TU Centre of Excellence for Multiscale Phenomena. His research interests include fluid dynamics turbulence, statistical physics, micro and nano-‐fluids, numerical methods for fluid dynamics, high-‐performance computing. The focus of his current research are multi-‐scale problems related to the physics of fluid studied by means of state-‐of-‐the-‐art computational models. He has been awarded large scale computational grants (DEISA Extreme Computing Initiative, PRACE) to study the dynamics of heavy particles in turbulence and hemodynamics. He is the founder and administrator of the International CFD Database hosted at CINECA (Bologna, Italy), http://cfd.cineca.it. He served as chairman of the International Collaboration for Turbulence Research (ICTR, http://www.ictr.eu). He acted as Chair of the COST Action MP0806 on "Particles in turbulence" and is currently chair of the COST Action MP1305 "Flowing matter”.

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**Shake-The-Box: A 4D-PTV method for turbulence characterization at high particle densities**

Date and Time: Friday, September 18, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Andreas Schröder, Team Leader PIV, German Aerospace Center

In order to increase the prediction capabilities of advanced numerical methods for turbulent (wall bounded) flows at relatively high Reynolds numbers, accurate experimental validation data-sets including the full Reynolds stress tensor at high spatial resolution are urgently required. In particular the influence of pressure gradients and wall curvatures up to flow separation and the development of related shear layers need to be investigated experimentally in order to provide reliable data for the validation process and also to validate scaling laws, sub-grid and turbulence-models. Furthermore, for advanced unsteady flow simulation methods (LES, DES, DNS etc.), the integration times and domains which are necessary for resolving flow features with very low spatial or temporal frequencies (such as superstructures in high Reynolds number turbulent boundary layers or recirculation regions in separated flows) are often not sufficient for a fully converged solution. Consequently, the used experimental methods have to be able to resolve a large range of spatial and temporal scales to be useful for code validation.

Recently, the Shake-The-Box (STB) technique [1][2] has been developed, which is a 4D-PTV evaluation method for densely seeded flows capable of coping with ill-posed 3D particle reconstruction problems based on few camera projections by seizing the temporal information with predictive steps and applying an iterative particle reconstruction and image matching scheme (see Iterative Particle Reconstruction [3]). Within the resulting dense Lagrangian tracks the STB technique uses temporal fitting functions based on optimal Wiener filtering along all found particle paths. The parameters of an optimal Wiener filter are determined from statistical properties of the Lagrangian position, velocity and acceleration fluctuations along the reconstructed 3D particle positions and tracks for all three components separately. This temporal filtering approach enables an accurate estimation of position, velocity and acceleration vectors and enhancing the DVR to values > 1:1000, when sufficient track lengths are provided. Therefore, STB is able to deliver accurate mean and Reynolds stress values (and higher order statistics) by bin-averaging (down to sub-pixel spatial resolution) and additionally provide the complete time-resolved velocity gradient tensor at a relatively high spatial resolution (comparable to a very well resolved tomographic PIV measurement) by using a proper interpolation scheme given with the “Flow-Fit” algorithm that was recently developed (brief description in [4]). The STB method has been applied to wall bounded turbulence in air and water and to an m³-scale thermal plume experiment using Helium-Filled-Soap-Bubbles (HFSB) as tracers (sees Figure 1). The results will demonstrate that with STB valuable data for turbulence characterization with outstanding temporal and spatial resolution especially in (wall bounded) shear flow can be obtained.

**Speaker Bio: **

Andreas Schroeder from DLR Goettingen, Team Leader PIV, German Aerospace Center, Gottingen, Germany. He is working on the development of PIV, PTV and some other image based measurement techniques for aerodynamic research since almost 20 years. His growing interest in (wall bounded) turbulence research is as well motivated by the growing role of advanced particle based 2D and 3D measurement methods contributing to its understanding and analysis.

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**Turbulent Flow over Bluff Bodies: From Sub-Critical to Super-Critical Flow Regimes**

Date and Time: Friday, September 11, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Oriol Lehmkuhl, Professor, Technical University of Catalonia, Spain

This seminar will focus on the numerical simulation of the turbulent flow past bluff bodies which is of importance due to its presence in many engineering applications (flows past an airplane, submarine, automobile, in turbomachines, etc.). The numerical simulation of turbulent flows is a powerful tool for investigating and understanding the turbulence phenomena, and sheds light on the physics of turbulent flows with massive separation. The advances in computational fluid dynamics (CFD) together with the increasing capacity of parallel computers have made possible to tackle such complex problems by using high performance numerical techniques such as direct numerical simulation (DNS) and large eddy simulation (LES). Examples of these DNSs like the flow over a circular cylinder and spheres at subcritical Reynolds numbers, DNSs of the flow past a NACA0012 airfoil at Reynolds numbers Re = 50000 at different angles of attack, high-fidelity LES of drag crisis in circular cylinders, among others high performance simulations are going to be presented. These activities form part of a comprehensive long-term research line focused on drag reduction, flow control and loads alleviation with application to the aeronautic and mechanical engineering industries. Contributions from these studies have helped in the understanding of the complex physics of these flows and have paved the way to continue research focused on the roughness effects on these bodies.

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**The outer zone of turbulent boundary layers**

Date and Time: Monday, August 17, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Prof. Javier Jimenez, Professor of Fluid Mechanics, School of Aeronautics, Universidad Politecnica de Madrid, Spain

Using data from new simulations of the turbulent boundary layer, we describe several features of the outer zone of wall-bounded flow at relatively high Reynolds numbers (Reth=6600). For example, using two-point velocity correlations, we show that the size and inclination of the different velocity components are different, and interpret this as representing different moments in their evolution. Other features are not so well understood, E.g., the transverse components (v and w) are similar in boundary layers and channels, but the streamwise velocity is much longer in the latter, and the outer zone of boundary layers is characterized by a large-scale oblique organization of w, of unknown origin, but reminiscent of some of the transitional structures observed in extended Couette flow.

We next discuss the turbulent/nonturbulent interface, detailing with some care the effect of how the interface is defined, including the variable being thresholded, the thresholding details, and the threshold. We note that the definition of distance between a point and a surface has a profound effect on the definition of the interface and, particularly, on the conditional analysis of the flow in its neighbourhood. We introduce an appropriate distance and show that the interface undergoes a topological transition as the threshold is increased and the interface enters the turbulent core. This can be used to defin a 'natural' threshold, and we show that this definition clarifies, at least for this flow, some of the hypotheses that have been advanced about the interface structure. A simple structure is proposed which roughly agrees with some of the old proposals by Townsend and others.

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**Modeling The Pressure Strain Correlation Under Uncertainties**

Date and Time: Friday, August 7, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Aashwin Mishra, Dept. of Aerospace Engineering, Texas A&M University

The cardinal issues forestalling a better understanding of the turbulence phenomenon are the nonlinearity of the inertial cascade physics and the non-local nature of the action of pressure. In this talk, we focus on analyzing, understanding and modeling the latter, manifested in the pressure strain correlation.

The Reynolds stresses provide an insufficient basis to describe the internal structure of turbulent flows, leading to an inherent degree of uncertainty in predictions using classical turbulence models. We carry out a detailed Dynamical Systems analysis of modal ensembles and individual modes in Fourier space to identify the range of possible behavior and the underlying physics. Based on this insight, Different aspects of the dynamics of pressure are discussed, individually and sequentially, vis-a-vis their amenability to the single point modeling paradigm. Thereon, a set of pragmatic compromises is constituted within the form and the scope of the model to outline a modeling framework. The predictions of an illustrative model are compared to numerical and experimental data while being contrasted against established modeling paradigms.

We conclude by quantifying the uncertainty in the modeling framework. For a set of different states of the mean gradient and the Reynolds stress tensors, we establish the range of this uncertainty for rapid pressure strain closures, identify statistically most likely behavior and their evolution.

**Speaker Bio: **

Aashwin Mishra received his Ph.D. in Aerospace Engineering from the Texas A&M University in 2014. His research interests include Turbulence Theory & Modeling, Dynamical Systems & Chaos, Statistical Inference, and, Uncertainty Quantification. After the conclusion of his doctoral research, he is a Post-Doctoral Researcher and Lecturer at TAMU.

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**Numerical Modeling Of Separated Flows At Moderate Reynolds Numbers Appropriate For Turbine Blades And Unmanned Aero Vehicles**

Date and Time: Friday, July 31, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Giacomo Castiglioni, Aerospace Engineering, University of Southern California

Flows over airfoils and blades in rotating machinery, for unmanned and micro-aerial vehicles, wind turbines, and propellers consist of a laminar boundary layer near the leading edge that is often followed by a laminar separation bubble and transition to turbulence further downstream. Typical Reynolds averaged Navier-Stokes turbulence models are inadequate for such flows. Direct numerical simulation is the most reliable, but is also the most computationally expensive alternative. This work assesses the capability of immersed boundary methods and large eddy simulations to reduce the computational requirements for such flows and still provide high quality results.

Two-dimensional and three-dimensional simulations of a laminar separation bubble on a NACA-0012 airfoil at Re_c = 50,000 at 5 degree of incidence have been performed with an immersed boundary code and a commercial code using body fitted grids. Several sub-grid scale models have been implemented in both codes and their performance evaluated.

The numerical dissipation inhibits the predictive capabilities of large eddy simulations whenever it is of the same order of magnitude or larger than the sub-grid scale dissipation. A particular emphasis is given to the quantification of the numerical dissipation in the commercial code.

**Speaker Bio: **

Dr. Giacomo Castiglioni graduated with a Bachelor of Science in Mechanical Engineering from the Polytechnic University of Milan (Italy) in 2009, and received his Master of Science in Aerospace Engineering form the University of Southern California in 2011. In 2010 he joined the group of Professor Domaradzki at the University of Southern California where he graduated with a PhD in Aerospace Engineering in 2015. During 2011 and 2013 he attended the SFB-TRR40 summer program at the Technical University of Munich, Germany. His research interests include turbulence modeling and high-order unstructured solvers.

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**Numerical study of ignition and light-around process in aeronautical gas turbines.**

Date and Time: Friday, July 24, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Lucas Esclapez, CTR Postdoctoral Fellow

For safety reasons, in-flight relight of the engine must be guaranteed over a wide range of operating conditions but the increasing stringency of pollutant emission constraints requires the development of new aero-engine combustor whose design might be detrimental to the ignition capability. To improve the knowledge of the ignition process in aeronautical gas turbines and better combine conflicting technological solutions, current research relies on both complex experimental investigations and high fidelity numerical simulations. Numerical study of the ignition process in gas turbines from the energy deposit to the light-around is performed with several objectives: increase the level of confidence of Large Eddy Simulations tool for the analysis of the ignition process, investigate the mechanisms controlling ignition in conditions representative of realistic aeronautical gas turbine flows and improve the low-order methodologies for the prediction of ignition performance. In a first part, LES of the single burner installed at CORIA (France) is carried out and allows to highlight the LES accuracy and to build a database on which the main mechanisms controlling the ignition success are identified. Based on these results, a methodology is developed to predict the ignition performance at a low computational cost using the non-reacting flow statistics only. In a second part, the light-around is studied and the LES is showed to predict accurately the ignition process. LES results are then used, jointly with experiments, to analyse the mechanisms driving the flame propagation.

**Bio: **

Dr. Lucas Esclapez just received his PhD. conducted at CERFACS from Institut National Polytechnique of Toulouse (France). His research focuses Large-Eddy Simulation (LES) of the ignition process in gas turbines in a massively parrallel environment and the develoment of a reduced model for the prediction of ignition performances. This work has led to an extensive experience in LES of turbulent reacting flow and development of scientific code (C++/Fortran/MPI) in supercomputing context.

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**DNS AND LES OF STABILIZATION AND INSTABILITIES IN ROCKET ENGINES**

Date and Time: Friday, July 10, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Thierry Poinsot, Research Director, CNRS, Toulouse and Head of CFD group CERFACS

Liquid fuel rocket engines using H2/O2 combustion are again a major research theme in Europe especially for the future Ariane 6 rocket. This talk will present recent progress in two critical mechanisms controlling combustion in cryogenic H2/O2 rocket engines. (1) the stabilization zone of the H2/O2 flame on the lips of a coaxial injector will be studied using DNS with full chemistry and a dual resolution of heat transfer within the splitter plate of the injector. (2) the first full LES of a complete rocket engine installed at DLR Lampoldshausen will be described. Using LES and a simplified chemical model, this simulation allows to capture and identify the two strongest modes (10 and 20 kHz) appearing in this 100 MW engine. Comparisons with experiments show that LES is able to discriminate between stable and unstable regimes.

**Bio: **

Dr Thierry Poinsot is a research director at CNRS Toulouse (IMFT lab), head of the CFD group at CERFACS and senior research fellow at Stanford University. He has authored more than 180 papers in refereed journals and 200 communications in the field of combustion. He is the author of the textbook "Theoretical and numerical combustion" with Dr D. Veynante and an associate editor of Combustion and Flame.

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**Phase-field modeling of multiphase flows using the lattice Boltzmann method with adaptive mesh refinement**

Date and Time: Friday, June 5, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Abbas Fakhari, CTR Postdoctoral Fellowship Candidate, Mechanical Engineering, City College of New York

Numerical modeling of multiphase flows at high Reynolds numbers is a challenging task. Traditional methods based on sharp-interface models might encounter numerical difficulty in handling rapid topological changes such as breakup and coalescence. Therefore, diffuse-interface models are widely used for numerical simulation of multiphase flows. As a diffuse-interface model, the lattice Boltzmann method (LBM) is a well-established mesoscopic scheme for numerical study of complex fluids. The most interesting feature of the LBM is that all nonlinearity is local and all nonlocality is linear, which is favorable for utilization on massively parallel machines. In this study, we increase the numerical stability of the LBM for multiphase flows at high Reynolds numbers, and investigate the Kelvin-Helmholtz instability of a shear flow. In order to save the computational resources, we propose an efficient adaptive mesh refinement (AMR) algorithm which does not need to maintain or modify a tree data structure. We then reformulate the LBM on nonuniform grids and propose an AMR-LBM for two-phase flows. Various case studies such as rising bubble and falling drop under buoyancy force, drop splashing on a wet surface, and droplet coalescence onto a fluid bath are conducted for validation and verification. The Kelvin-Helmholtz instability of a stratified shear-layer flow is also scrutinized to assess the accuracy of the proposed model, and satisfactory agreement with benchmark studies is shown.

**Bio: **

Abbas Fakhari graduated with a Bachelor of Science in Physics from Zanjan University in 2006, and received his Master of Science in Mechanical Engineering from the University of Tehran in 2008. In 2010, he joined the Computational Multiphase Fluid Dynamics Group at the City College of New York, where he graduated with a PhD in Mechanical Engineering in 2015. His research interests include high-Reynolds-number multiphase flows, binary and ternary fluids, lattice Boltzmann methods, and adaptive mesh refinement techniques.

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**Phase-field modeling of multiphase flows using the lattice Boltzmann method with adaptive mesh refinement**

Date and Time: Monday, June 1, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Abbas Fakhari, CTR Postdoctoral Fellowship Candidate, Mechanical Engineering, City College of New York

Numerical modeling of multiphase flows at high Reynolds numbers is a challenging task. Traditional methods based on sharp-interface models might encounter numerical difficulty in handling rapid topological changes such as breakup and coalescence. Therefore, diffuse-interface models are widely used for numerical simulation of multiphase flows. As a diffuse-interface model, the lattice Boltzmann method (LBM) is a well-established mesoscopic scheme for numerical study of complex fluids. The most interesting feature of the LBM is that all nonlinearity is local and all nonlocality is linear, which is favorable for utilization on massively parallel machines.

In this study, we increase the numerical stability of the LBM for multiphase flows at high Reynolds numbers, and investigate the Kelvin-Helmholtz instability of a shear flow. In order to save the computational resources, we propose an efficient adaptive mesh refinement (AMR) algorithm which does not need to maintain or modify a tree data structure. We then reformulate the LBM on nonuniform grids and propose an AMR-LBM for two-phase flows. Various case studies such as rising bubble and falling drop under buoyancy force, drop splashing on a wet surface, and droplet coalescence onto a fluid bath are conducted for validation and verification. The Kelvin-Helmholtz instability of a stratified shear-layer flow is also scrutinized to assess the accuracy of the proposed model, and satisfactory agreement with benchmark studies is shown.

**Bio: **

Abbas Fakhari graduated with a Bachelor of Science in Physics from Zanjan University in 2006, and received his Master of Science in Mechanical Engineering from the University of Tehran in 2008. In 2010, he joined the Computational Multiphase Fluid Dynamics Group at the City College of New York, where he graduated with a PhD in Mechanical Engineering in 2015. His research interests include high-Reynolds-number multiphase flows, binary and ternary fluids, lattice Boltzmann methods, and adaptive mesh refinement techniques.

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**Identifying coherent structures in fluid flow**

Date and Time: Friday, May 29, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Prof. Gary Froyland, School of Mathematics and Statistics, University of South Wales, Sydney, Australia

The future behavior and transport mechanisms of complicated (chaotic or "turbulent") fluid flow is hard to predict. However, in many situations there are regions of predictability embedded in the flow that exhibit approximately regular behavior. These regions of predictability are often bounded by strong transport barriers and evolve as coherent, slightly leaky parcels of fluid. Geophysical examples include polar vortices in the stratosphere and ocean gyres and eddies. I will describe probabilistic (ergodic-theoretic) techniques based on spectral properties of transfer operators to identify these coherent regions, and the application of these techniques to two geophysical examples.

**Bio: **

Gary Froyland is currently an Australian Research Council Future Fellow and Professor in the School of Mathematics and Statistics at UNSW Australia. He received his PhD in mathematics from the University of Western Australia, and held an Australian Academy of Science Postdoctoral Fellowship at the University of Tokyo. During a break in his academic career, he worked for BHP Billiton, one of the world’s largest resource companies, and was awarded the BHP Billiton Innovation Prize. His research interests cover dynamical systems and discrete optimization. His dynamical systems research focuses on the interplay of probability and geometry in nonlinear and chaotic flows, and uses tools from ergodic theory. In addition to fundamental mathematical research in dynamical systems, he has applied his research methods to analyse oceanic, atmospheric, and granular flows, using models and observational data. His optimization research is focused on decision making in large and complicated systems, sometimes in the presence of uncertain information. His research concerns new modelling approaches in mathematical programming, integer programming, and stochastic programming. These new approaches have been applied to strategic planning of open pit mines, scheduling of maritime crane fleets, and robust scheduling of aircraft and flight crews under uncertain disruption. Current work concerns optimisation of cancer radiotherapy treatments.

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**Assessment of a fluidic oscillator utilized for active flow control**

Date and Time: Friday, May 15, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Eran Arad, Head of CFD Group, RAFAEL, Advanced Defense System, Israel Institute of Technology

Active flow control devices and applications received a lot of attention in the recent three decades. While synthetic jets were the preferred technique during the early stages of this practice, fluidic oscillators, also called sweeping jets actuators, were recently in the focus of several research and engineering projects. These actuators bring in the advantages of no-moving-parts and high amplitude signal. However, their control is not straight forward and many times the exact nature of their output is not fully comprehended.

A combined experimental and numerical-simulation study, towards improved understanding of the operation principles, control and output of such a device is currently conducted by US Army AFDD, Tel-Aviv University and Rafael. The current talk focuses on the simulation aspects of this effort.

The internal flow mechanism of a suction and blowing (SOAB) device wasnvestigated using Large Eddy Simulation. The necessity of this level of simulation was queried, followed by comparison with measurements. The simulation results were then utilized to construct a set of functional-fit velocity profiles, to be used as time-dependent boundary conditions for the oscillatory blowing.

Towards implementation of a bank of actuators in a turbulent boundary layer, the separated effects of the actuators output, like suction alone, were investigated within a laminar and turbulent boundary layers. Large eddy simulation was the tool employed for this task also, coupled with wind-tunnels measurements. Interesting aspects of shear enhancement, interaction between suction holes wakes and onset of turbulence were observed.

**Bio: **

Dr Arad received his PhD from Aero-Space Engineering, Technion, Israel Institute of Technology on 1994. His thesis focused on turbulence modeling. Dr Arad spent 1994 to 1997 as a Research Fellow at Princeton University, in the computational fluid dynamic group headed by prof A Jameson. His research focused on high efficiency parallel CFD and applications to Large Eddy Simulation. Dr Arad is currently the head of the CFD group of RAFAEL, Advanced Defense Systems, Israel. His research activities include turbulence simulation and modeling, combustion simulation, large scale computing, industrial CFD and active flow control simulation.

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**Particulate flow across multi-scales: numerical strategies for momentum, heat and mass transfer**

Date and Time: Friday, April 24, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Anthony Wachs, Fluid Mechanics Department,IFP Energies nouvelles, Solaize, France

Particulate flows are ubiquitous in environmental, geophysical and engineering processes. The intricate dynamics of these two-phase flows is governed by the momentum transfer between the continuous fluid phase and the dispersed particulate phase. When significant temperature differences exist between the fluid and particles and/or chemical reactions take place at the fluid/particle interfaces, the phases also exchange heat and/or mass, respectively. While some multi-phase processes may be successfully modelled at the continuum scale through closure approximations, an increasing number of applications require resolution across scales, e.g. dense suspensions, fluidized beds. Within a multi-scale micro/meso/macro-framework, we develop robust numerical models at the micro and meso-scales, based on a Distributed Lagrange Multiplier/Fictitious Domain method and a two-way Euler/Lagrange method, respectively. Particles, assumed to be of finite size, potentially collide with each other and these collisions are modeled with a Discrete Element Method. We discuss the mathematical issues related to modeling this type of flows and present the main numerical and computational features of our simulation methods. We also illustrate what can be gained from massively parallel computations performed with our numerical code PeliGRIFF, in terms of physical insight into both fundamental questions and applications from the chemical engineering and process industry. Finally, we explain how knowledge gained at the micro scale can cascade upwards and contribute to the development of enhanced meso and macro-scale

**Bio: **

Dr Anthony Wachs received BS & MS degrees from University Louis Pasteur, Strasbourg and his PhD from the Institut National Polytechnique of Grenoble in 2000. He joined Institut Français du Pétrole in 2001 (now IFP Energies Nouvelles), passed his HDR (French Habilitation to Supervise Research) in 2010 and was later promoted Scientific Advisor in Multiphase Flows and Scientific Computing. He leads a team of researchers that develop both mathematical models and robust computational algorithms for the resolution of multi-phase flows, with an emphasis on reactive particulate flows (www.peligriff.com). His main research interests are non-Newtonian Flows, Multiphase Flows and High Performance Computing. He collaborates extensively with academic groups in Canada, Brazil, France and Germany.

**Numerical simulation of multiphase immiscible flow on unstructured meshes**

Date and Time: Friday, February 20, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Lluis Jofre-Cruanyes, Postdoctoral Fellow, Center for Turbulence Research

The present talk tries to summarize the work performed by Lluis Jofre-Cruanyes during his PhD.The research aims at developing a basis for the numerical simulation of multiphase flows of immiscible fluids. The formulation is based on a finite-volume approach and is suitable for three-dimensional (3-D) unstructured meshes, as well for Cartesian grids. The software implemented is part of an in-house high performance computing platform able to simulate multiphysics problems. This platform is based on object-oriented programming, using C++ language, and parallelization is accomplished via MPI. Hence, rather than focusing on the study of the physics associated to these flows, most of the work is focused on the numerical discretization of the equations that govern them.

The work can be separated into two large blocks. The first part consists in developing numerical models able to embed fluid interfaces on static grids. This is accomplished by proposing a Volume-of-Fluid (VOF) method, and its parallelization strategy, for capturing interfaces on 3-D Cartesian and unstructured meshes. The method reconstructs interfaces as first- and second-order piecewise planar approximations (PLIC), and advects volumes in a single unsplit Lagrangian-Eulerian (LE) geometrical algorithm based on constructing flux polyhedrons by tracing back the Lagrangian trajectories of the cell-vertex velocities.

The second part focuses on the development of a finite volume based discretization of the Navier-Stokes equations for multiphase immiscible flow on 3-D unstructured meshes that properly preserves mass, momentum and kinetic energy. In order to gain experience, the single-phase flow case is first studied to later extend it to the case of multiphase immiscible flow. Two main mesh discretizations have been analyzed: collocated and staggered schemes. Aside from accuracy, the focus has been placed on their capacity to discretely conserve kinetic energy, specially when solving turbulent flows.

Finally, the Richtmyer-Meshkov (RM) instability of two incompressible immiscible liquids has been numerically simulated. Rather than being a detailed study of the physical phenomena of RM instabilities, the tests performed are a general assessment of the numerical methods developed.

**Bio: **

Dr. Lluis Jofre-Cruanyes is a new postdoctoral fellow athe Center for Turbulence Research (CTR). He is interested in numerical methods for computational fluid dynamics and multiphysics. His research is mainly focused on multiphase turbulent flows, and includes raids into compressible and low-mach number flows, combustion processes, adaptive mesh refinement and high performance computing. His experience has been acquired, first, completing a Mechanical Engineering degree at the Technical University of Catalonia (2003-2008, Spain) with a Master's thesis at the Royal Institute of Technology (2008, Sweden). Second, in the context of a PhD stay at the University of Groningen (2012, The Netherlands) developing numerical models for fuid mechanics. Third, while conducting a PhD at the Technical University of Catalonia (2009-2014, Spain) on multiphase flows entitled "Numerical Simulation of Multiphase Immiscible Flow on Unstructured Meshes", and at present, by working as a postdoctoral researcher on fluid dynamics, acting as an adviser for different junior PhD students.

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**Rethinking tsunami preparedness - What is the role of vegetation?**

Date and Time: Friday, February 6, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Professor Jenny Suckale, Geophysics Department, Stanford University

The role of coastal forests in the mitigation of tsunami disasters has become a hotly debated topic in the aftermath of the devastating tsunami in the Indian Ocean in 2004 and Japan in 2011. Unfortunately, our knowledge of the interactions between tsunamis and vegetation is limited and the associated danger of ineffective or even potentially harmful policies is concerning. Through this project, we hope to contribute to bridging the gap between science and policy and provide new insights on whether and how coastal trees or forests may be promising for protective purposes. Since our research on this topic is only just starting up, we focus on outlining some of the outstanding questions in the field and the proposed research directions to address these.

**Bio: **

Dr. Jenny Suckale is an Assistant Professor in the Department of Geophysics at Stanford University. She is also an affiliated faculty member of Stanford’s Institute for Computational and Mathematical Engineering. She received her PhD in geophysics from MIT and also holds a Master in Public Administration from the Harvard Kennedy School. The goal of her research is to advance our basic understanding and predictive capabilities of complex multi-phase flow problems in Earth science. She pursues this goal by developing original computational methods customized for the geophysical system at hand. Prior to joining graduate school, Dr. Suckale worked as a scientific consultant for different international organizations aiming to reduce the impact of natural and environmental disasters in vulnerable communities. She has published on induced seismicity and seismic hazard.

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**Streak Instabilities in Pressure Gradient Boundary Layers**

Date and Time: Friday, January 23, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Philipp Hack, Imperial College, London

Linear stability analysis is applied to pre-transitional boundary layer flow fields extracted from direct numerical simulations (DNS) of bypass transition (Hack & Zaki, J. Fluid Mech., 2014a). The presence of broadband free-stream forcing in the DNS leads to the formation of a spectrum of high-amplitude streaks inside the boundary layer. The streaky base flow becomes susceptible to high-frequency modal instabilities which are localized on single streaks and which eventually induce breakdown to turbulence. It is shown that linear stability theory can capture the attributes of the instability modes and predict which particular streaks will cause breakdown. Representative examples of outer and inner modes of streak instabilities are examined. The consideration of a large number of instabilities provides statistically relevant results for the properties of the instabilities. Favorable pressure gradient is established to support the outer mode whereas adverse pressure gradient favors the inner instability.

The last part of the presentation focuses on a novel approach to the prediction of streak breakdown based on machine learning. The method is shown to be highly accurate at identifying unstable streaks while the associated computational effort is several orders of magnitude lower than that of the solution of the eigenvalue problem intrinsic to classical stability analysis.

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**Streak Instabilities in Pressure Gradient Boundary Layers**

Date and Time: Friday, January 9, 2015 - 16:00

Location: CTR Conference Room 103

Event Sponsor: Parviz Moin, Director of Center for Turbulence Research

Speaker(s): Dr. Babak Hejazi, Cascade Technologies

Linear stability analysis is applied to pre-transitional boundary layer flow fields extracted from direct numerical simulations (DNS) of bypass transition (Hack & Zaki, J. Fluid Mech., 2014a). The presence of broadband free-stream forcing in the DNS leads to the formation of a spectrum of high-amplitude streaks inside the boundary layer. The streaky base flow becomes susceptible to high-frequency modal instabilities which are localized on single streaks and which eventually induce breakdown to turbulence. It is shown that linear stability theory can capture the attributes of the instability modes and predict which particular streaks will cause breakdown. Representative examples of outer and inner modes of streak instabilities are examined. The consideration of a large number of instabilities provides statistically relevant results for the properties of the instabilities. Favorable pressure gradient is established to support the outer mode whereas adverse pressure gradient favors the inner instability.

The last part of the presentation focuses on a novel approach to the prediction of streak breakdown based on machine learning. The method is shown to be highly accurate at identifying unstable streaks while the associated computational effort is several orders of magnitude lower than that of the solution of the eigenvalue problem intrinsic to classical stability analysis.