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Calibri 83ffff̙̙3f3fff3f3f33333f33333.sTU Delft Repositoryg auuidrepository linktitleauthorcontributorpublication yearabstract
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departmentresearch group programmeprojectcoordinates)uuid:41e8615d5c37449faa366fdc43bb5774Dhttp://resolver.tudelft.nl/uuid:41e8615d5c37449faa366fdc43bb5774OSimulation of threedimensional separation with a zonal nearwall approximation'Tessicini, F.; Li, N.; Leschziner, M.A.rThe focus of the paper is on the performance of an approximate 'zonal' near wall treatment applied within a LES strategy to the simulation of flow separating from a threedimensional hill at high Reynolds numbers. In the zonal scheme, the state of the nearwall layer of the flow is described by parabolized NavierStokes equations solved on a subgrid embedded within a global LES mesh. The solution of the boundarylayer equations returns the wall shear stress to the LES domain as a wall boundary condition. Simulations are presented for grids containing between 1.5 and 9.6 million nodes, the one on the finest grid being a pure LES. The comparisons included demonstrate that the zonal scheme provides a satisfactory representation of most flow properties, even on the coarsest grid, whereas the pure LES on the coarsest grid completely fails to capture the separation process.3LES; nearwall modelling; zonal twolayer modellingenconference paper)uuid:5d23e2a65675450dbf3d1dd40d736caeDhttp://resolver.tudelft.nl/uuid:5d23e2a65675450dbf3d1dd40d736caeYEvaluation of the SSTSAS Model: Channel Flow, Asymmetric Diffuser and Axisymmetric HillDavidson, L.The SAS model (Scale Adapted Simulation) was invented by Menter and coworkers. The idea behind the SSTSAS komega model is to add an additional production term  the SAS term  in the omega equation, which is sensitive to resolved (i.e. unsteady) fluctuations. When the flow equations resolve turbulence, the length scale based on velocity gradients is much smaller than that based on timeaveraged velocity gradients. Hence the von Karman length scale, L_vK, is an appropriate quantity to use as a sensor for detecting unsteadiness. In regions where the flow is on the limit of going unsteady, the objective of the SAS term is to increase omega. The result is that k and nu_t are reduced so that the dissipating (damping) effect of the turbulent viscosity on the resolved fluctuations is reduced, thereby promoting the momentum equations to switch from steady to unsteady mode. The SSTSAS model and the standard SSTURANS are evaluated for three flows: developing channel flow, the flow in an asymmetric, plane diffuser and the flow around a threedimensional axisymmetric hill. Unsteady inlet boundary conditions are prescribed in all cases by superimposing turbulent fluctuations on a steady inlet boundary velocity profile.Avon Karman length scale; unsteady; URANS; DES; LES; scaleadapted)uuid:71dc43eb7cde4c4d9bd644b54173c354Dhttp://resolver.tudelft.nl/uuid:71dc43eb7cde4c4d9bd644b54173c354+Recent progress on fluxlimiting based ILESGrinstein, F.F.; Fureby, C.We review our recent progress in understanding the theoretical basis of Implicit Large Eddy Simulation (ILES) and fundamental features relating to its performance. We use the Modified Equation Analysis (MEA) to show that the leading order truncation error terms introduced by a particular class of hybrid methods provide implicit Subgrid Scale (SGS) models similar in form to those of conventional mixed SGS models. Major properties of the implicit SGS model are related to: (i) the choice of high and loworder schemes where the former is wellbehaved in smooth flow regions and the latter is wellbehaved near sharp gradients; (ii) the choice of fluxlimiter which determines how these schemes are blended locally, depending on the flow; (iii) the balance of the dissipation and dispersion contributions to the numerical solution, which depend on the design details of each numerical method. The possibility of achieving ILES perf< ormance enhancements through improved design of the SGS physics capturing capabilities of the high resolution methods is emphasized in this context. Results from recent tests of the performance of various hybrid algorithms suitable for ILES is then demonstrated in the case of the TaylorGreen vortex (TGV) problem. The results show robustness of ILES in capturing established theoretical findings for transition and turbulence decay, in terms of the characteristic evolution of the kinetic energy dissipation, energy spectra, and enstrophy.?LES; implicit LES; limiting; nonoscillatory; modified equation)uuid:253c88ac0567482dbac7a5e4c1870f6eDhttp://resolver.tudelft.nl/uuid:253c88ac0567482dbac7a5e4c1870f6e`Numerical study of the unsteady flow structures around trainshaped body subjected to side windsHemida, H.; Krajnovic, S.sLargeeddy simulation (LES) is made to investigate the flow around a generic train under side wind conditions. Two different sidewind yaw angles are used in the investigation: 35 and 90 degrees. The Reynolds numbers based on the height of the train and the freestream velocity are 300,000 and 370,000 for the 90 and 35 degrees yaw angles, respectively. Two computations on two different meshes are made for each simulation in order to check the effect of the mesh resolution on the results. The fine and the coarse meshes give similar results for each simulation meaning that the results are mesh independent. The results are also verified against experimental data that have been collected on the numerical model at the same Reynolds number. Good agreement is obtained between the LES results and the experimental data. The LES results showed that the flow around the train at 90 degree sidewind yaw angle is dominated by unsteady vortex shedding. The largescale instability associated with the shedding of largescale vortices from the recirculation region to the far wake flow and the smallscale instability associated with the smallscale Kelvin Helmholtz instability are yielding highly unsteady flow. The LES results for the 35 degree sidewind yaw angle show that two flow regimes exist in the wake. The first flow regime is represented with the steady vortex lines in the upper part of the wake flow. It changes into unsteady shedding after some distance from their onset on the surface of the train. The second flow regime is the unsteady movement of the lower part of the wake vortices which attach and detach from the surface of the train in a regular fashion. The influence of side wind yaw angle and the wake structures on the aerodynamic coefficients is discussed in the paper. The paper gives a picture of the unsteady flow and its instabilities around trains at large and low yaw angles.Cfluid dynamics; wake structures; high speed trains; side winds; LES)uuid:9b7c07ae806a4fa184ea7bd14d1fcbfbDhttp://resolver.tudelft.nl/uuid:9b7c07ae806a4fa184ea7bd14d1fcbfb(Large scale urban simulations with MILES)Patnaik, G.; Boris, J.P.; Grinstein, F.F.:Airborne contaminant transport in cities presents challenging new requirements for CFD. The unsteady flow physics is complicated by very complex geometry, multiphase particle and droplet effects, radiation, latent, and sensible heating effects, and buoyancy effects. Turbulence is one of the most important of these phenomena and yet the overall problem is sufficiently difficult that the turbulence must be included efficiently with an absolute minimum of extra memory and computing time. This paper describes the Monotone Integrated Large Eddy Simulation (MILES) methodology used in NRL's FAST3DCT simulation model for urban contaminant transport (CT). We also describe important relevant extensions of the underlying FluxCorrected Transport (FCT) convection algorithm. Validation studies and issues raised are discussed.)urban CFD; MILES; LES; urban aerodynamics)uuid:21fc268991db4d8c99532a1153395694Dhttp://resolver.tudelft.nl/uuid:21fc268991db4d8c99532a1153395694^ILES of shock waves and turbulent mixing using highresolution Riemann solvers and TVD methodsThornber, B.; Drik< akis, D.This paper derives the implicit model for highresolution MUSCL reconstructionbased upwind Godunov schemes for turbulent flows. A Mach number expansion of the numerical scheme allows analysis of the theoretical behaviour of the implicit subgrid model as the Mach number becomes small. This shows that different limiting methods within the same reconstruction technique have significantly different behaviour at low Mach numbers. For some methods the dissipation due to the Riemann solver dominates at low Mach, whereas for others the dominant terms are due to the averaging of the two limited quantities. Additionally it is shown that there are several terms which are identical in the implicit subgrid model as in the leading order of the Taylor series expansion of the large eddy simulations (LES) equations. Finally, results gained for threedimensional shockinduced turbulent mixing of two different gases using the van Leer limiter is compared to the experimental results of Holder and Barton (IWPCTM 9, 2004).CLES; implicit; shock; turbulent mixing; MUSCL; theoretical analysis)uuid:ce4bd9ee4eb4456983a26bf444b7d0ddDhttp://resolver.tudelft.nl/uuid:ce4bd9ee4eb4456983a26bf444b7d0dd3ILES and LES of Complex Engineering Turbulent Flows
Fureby, C.5The present study concerns the application of Large Eddy Simulation (LES) and Implicit LES (ILES) to engineering flow problems. Such applications are often very complicated, involving both complex geometries and complex physics, such as turbulence, chemical reactions, phase changes and compressibility. The aim of the study is to illustrate what problems occur when attempting to perform such engineering flow calculations using LES and ILES, and put these in relation to the issues originally motivating the calculations. The issues of subgrid modeling are discussed with particular emphasis on the complex physics that needs to be incorporated into the LES models. Results from representative calculations, involving incompressible flows around complex geometries, aerodynamic noise, compressible flows, combustion and cavitation, are presented, discussed and compared with experimental data whenever possible. In addition, we also compare predictions from LES and ILES with conventional Reynolds Averaged NavierStokes (RANS) models and Detatched Eddy Simulations (DES) for academically challenging flows, such as the flow around a cylinder and around a surface mounted 3D hill. It is found that both LES and ILES predict these flows more accurately than RANS and DES, and include more information about the dynamics of the flow.LES; ILES; engineering applications; incompressible flow; compressible flow; flow noise; turbulent premixed combustion; cavitation)uuid:ee8643817d0f446ebe98248f688e81b9Dhttp://resolver.tudelft.nl/uuid:ee8643817d0f446ebe98248f688e81b9VUse of explicit filtering, secondorder scheme and SGS models in LES of turbulent flowBrandt, T.T.In large eddy simulations (LES) using loworder finitedifferencetype methods, the numerical error can be large in comparison to the effect of the subgrid scale (SGS) model. In this paper, we study two approaches to explicit filtering which can reduce the numerical error involved. In the first approach, the nonlinear convection term of the NavierStokes equations is filtered explicitly, and in the second one, the smallscale shear stress is devided into subfilter (SFS) and subgrid (SGS) components, and filtering is provided via the model. The aims are to clarify the effect of SGS modelling in the first approach and to compare the two approaches. For SGS and SFS modelling, the standard and dynamic Smagorinsky models and the scalesimilarity model are applied. When the convection term is filtered explicitly, the effect of modelling on the simulation results reduces and differences between the models are rather small. Filtering with a smooth threedimensional filter has a strong effect on the flow statistics, and none of the models is able to compensate for this. Although the high frequencies are damped efficiently, the total simulation err< or does not decrease. When filtering is provided only via modelling, improved results are obtained if both SGS and SFS modelling are applied. However in this approach, all terms in the equations do not have the same frequency content, and the high frequencies that are badly described by the discrete gird are not damped as efficiently as when the convection term is explicitly filtered.ALES; explicit filtering; SGS modeling; SFS modeling; channel flow)uuid:cfaab9be35e849b295ecc0cfcb0421a1Dhttp://resolver.tudelft.nl/uuid:cfaab9be35e849b295ecc0cfcb0421a1OA generalized patch AMR platform that uses cell centered or cell vertex solvers Borrel, M.; Ryan, J.; Billet, G.A patch adaptive mesh refinement (AMR) platform is presented. Presently two NavierStokes solvers are available within this platform : a MUSCL and DG solver (FLUX AMR) and a multispecies MUSCL solver (MAJIC) for reacting flows. The first solver is based on cell centered approaches of finite volume type, the second solver is based on a cell vertex and a time splitting method. The modifications of AMR treatments, especially those concerning the interpolation at finecoarse boundaries are detailed. This platform is first tested on the subsonic flow over a deep cavity and secondly on the interaction of a steady planar shock with a H2air circular diffusion flame.iAMR platform; fluid dynamics; reacting gas flow; combustion; MUSCL; DG; LES; cavity flow; diffusion flame)uuid:ed4bd9dbf34741738ccb65634ac5fd25Dhttp://resolver.tudelft.nl/uuid:ed4bd9dbf34741738ccb65634ac5fd25BLargeeddy Simulation of Isotropic Homogeneous Decaying TurbulenceSimulations of three dimensional freely decaying homogeneous turbulence in a periodic cube have been used to examine in a detailed and quantitative manner the behaviour of a Large Eddy Simulation (LES) using implicit subgrid modelling. This paper details the form and behaviour of the implicit subgrid models for the Minmod and third order limiting methods at several mesh resolutions. It is shown that for simulations above 32^3 the decay of kinetic energy follows a power law with a decay exponent between 1:2 and 1:4, except in the case of turbulence with a constrained length scale for which the decay exponent is 2:1. This is in very good agreement with experimental data and theoretical analysis where the exponent ? 1:2 ? 1:4 unconstrained, and 2:0 when constrained. At a resolution of 32^3 the number of degrees of freedom are not sufficient to allow a turbulent flow, and velocity derivative statistics are Gaussian. The skewness of the velocity derivative is lower than existing explicit LES and Direct Numerical Simulation (DNS) simulations, but in good agreement with the most recent experimental results. There is a limited subinertial range with the threedimensional Kolmogorov constant ? 1:9, also in agreement with DNS. The less dissipative nature of the third order limiter gives better skewness at a lower grid resolutions, however both give good results in terms of energy dissipation and growth of the length scales.2LES; implicit; isotropic; turbulence; decay; MUSCL)uuid:4cc72b55a66d492780ebc3244fd38347Dhttp://resolver.tudelft.nl/uuid:4cc72b55a66d492780ebc3244fd38347SInfluence of Numerical Parameters for Large Eddy Simulations of Complex Flow Fields0Sternel, D.C.; Schfer, M.; Gauss, F.; Yigit, S.Large eddy simulation (LES) becomes more and more important for the simulation of practical turbulent flows. The application for technically relevant, complex configurations leads to systems from around 500,000 up to several million degrees of freedom. For a statistical analysis of the turbulent flow field a large number of time steps have to be computed. Efficient algorithms are required to ensure acceptable computing times. When using implicit schemes  allowing for relatively large time step sizes  the chosen convergence criteria for stopping the solution procedure within the individual time steps, turns out to be an important issue. For optimizing the computational effort it is beneficial to have a detailed knowledge abou<t the influence of the convergence criteria to the characteristic values of the flow field. The investigation of the corresponding effects is the major subject of the present paper. The flow solver employed is the fully parallel multigrid finitevolume code FASTEST with secondorder accurate discretizations in space and time. For the LES, the Smagorinsky model with the dynamic approach of Germano is implemented. The attainable convergence criteria for LES in complex geometries when using a standard geometric multigrid scheme is limited by the visible frequencies on the coarse grid . To overcome this, and to reach smaller residuals within acceptable computing times, an improved multigrid algorithm is introduced which exploits the good convergence rate of the multigrid algorithm in the first Vcycles and then employ the fine grid for the last iterations. As a complex application of practical interest we consider the large eddy simulation of the flow in a mixing chamber corresponding to the swirling flow in an aircraft combustion chamber. Comparisons to experimental results are presented and the influence of the convergence criteria is studied in detail by analyzing statistical values, energy spectra, and computational effort.%LES; quality assessment; complex flow)uuid:5e947fcfc25745c8b7500a942d5512d3Dhttp://resolver.tudelft.nl/uuid:5e947fcfc25745c8b7500a942d5512d3sLES and URANS Unsteady Boundary Layer Strategies for Pulsating and Oscillating Turbulent Channel Flows Applications%Panara, D.; Porta, M.; Schoenfeld, T.The use of wall functions has been investigated for LES and URANS numerical simulation in pulsating and oscillating channel flow applications. The results show that the wall function approach is accurate in the socalled quasisteady regime but there are discrepancies with the experimental results in the intermediate frequency range. A special attention is given to the wallshear stress prediction, and in particular on the wallshear stress phase shift with respect to the free stream velocity. In order to capture such unsteady flow effects, the boundary layer needs to be resolved. Different approaches such as Low Reynolds Number near wall turbulence modeling (URANS) or the proposed WallNormal Resolved strategy (LES) seem to be suited for this purpose. The drawback is unfortunately the increasing of computational points in the boundary layer and consequently the higher computational costs.ofluid dynamics; turbulence; URANS; LES; pulsating flow; oscillating flow; near wall modeling; wall shear stress
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