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.
Archives:
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2008
| 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 |
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| 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.
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| 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 |
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| 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. |
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| 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. |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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) |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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.
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| 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 |
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| 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. |
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| 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 |
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| 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.
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| 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 |
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| 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. |
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| 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 |
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| 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.
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| 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 |
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|
| 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.
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| 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 |
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| 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.
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| 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. |
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| Abstract: | See Abstract. |
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| 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 |
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| 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. |
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| Date: | Friday Nov 30, 2007 |
| Time: | 4:00pm |
| Location: | CTR Conference Room |
| Speaker: | |
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| Date: | Friday Nov 30, 2007 |
| Time: | 4:00pm |
| Location: | CTR Conference Room |
| Speaker: | |
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