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A comparison between isotropic and anisotropy-resolving closures in large eddy simulation of separated flow
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.ORCID iD: 0000-0002-3173-7502
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.ORCID iD: 0000-0001-8692-0956
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.ORCID iD: 0000-0002-9819-2906
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.ORCID iD: 0000-0002-2711-4687
2014 (English)Report (Other academic)
Abstract [en]

This study compares the conventional isotropic dynamic eddy viscosity model(DEVM) and anisotropy-resolving nonlinear explicit algebraic subgrid-scale(SGS) stress model (EASSM) of Marstorp et al. (J. Fluid Mech., vol. 639,2009, pp. 403–432) in large-eddy simulations (LESs) of flow separation in achannel with streamwise periodic hill-shaped constrictions and spanwise homogeneity(periodic hill flow). The results are validated with well-resolved LESdata of Breuer et al (Computers & Fluids, vol. 38, 2009, pp. 433-457). Threedifferent resolutions ranging from moderate to very coarse are used. LESs arecarried out with the Code Saturne, an unstructured collocated finite volumesolver for incompressible flows with a second-order central difference schemein space and a second-order discretisation in time. It has inherent numericaldissipation due to the low-order of the numerical method. LESs with no SGSmodel (NSM) are also carried out to analyse the influence of the SGS modelsin the presence of discretisation errors. LESs with the NSM show that the inherentnumerical dissipation is sufficient to give a reasonable prediction of themean velocity profiles at the finest resolution. The LES predictions of the meanvelocity and Reynolds stresses with the EASSM are found to be much moreaccurate than the ones with the DEVM at all resolutions. Although the SGSdissipation produced by the EASSM is found to be considerably lower than bythe DEVM, the EASSM predictions show appreciable improvements over theNSM, indicating the importance of the nonlinear part of the model. At thecoarsest resolution, where the SGS anisotropy is large, LES with the EASSMshows a reasonable prediction of the mean separation and reattachment points,whereas LES with the isotropic DEVM predicts a considerably delayed separationand early flow reattachment with a small separation bubble and the LESwith NSM does not display flow separation. At finer resolutions, the DEVMand NSM predict a shorter separation bubble than the EASSM, which has agood agreement with the well-resolved reference LES data. Hence, a correctprediction of the separation and reattachment by LES requires resolving the SGS anisotropy either by a fine grid or by an anisotropy-resolving SGS modelsuch as the EASSM.

Place, publisher, year, edition, pages
2014.
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-142399OAI: oai:DiVA.org:kth-142399DiVA: diva2:700110
Funder
Swedish Research Council, 621-2010- 6965
Note

QC 20140304

Available from: 2014-03-03 Created: 2014-03-03 Last updated: 2014-03-04Bibliographically approved
In thesis
1. Anisotropy-resolving subgrid-scale modelling using explicit algebraic closures for large eddy simulation
Open this publication in new window or tab >>Anisotropy-resolving subgrid-scale modelling using explicit algebraic closures for large eddy simulation
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The present thesis deals with the development and performance analysis ofanisotropy-resolving models for the small, unresolved scales (”sub-grid scales”,SGS) in large eddy simulation (LES). The models are characterised by a descriptionof anisotropy by use of explicit algebraic models for both the subgridscale(SGS) stress tensor (EASSM) and SGS scalar flux vector (EASSFM). Extensiveanalysis of the performance of the explicit algebraic SGS stress model(EASSM) has been performed and comparisons made with the conventionalisotropic dynamic eddy viscosity model (DEVM). The studies include LES ofplane channel flow at relatively high Reynolds numbers and a wide range ofresolutions and LES of separated flow in a channel with streamwise periodichill-shaped constrictions (periodic hill flow) at coarse resolutions. The formersimulations were carried out with a pseudo-spectral Navier–Stokes solver, whilethe latter simulations were computed with a second-order, finite-volume basedsolver for unstructured grids. The LESs of channel flow demonstrate that theEASSM gives a good description of the SGS anisotropy, which in turn gives ahigh degree of resolution independence, contrary to the behaviour of LES predictionsusing the DEVM. LESs of periodic hill flow showed that the EASSMalso for this case gives significantly better flow predictions than the DEVM.In particular, the reattachment point was much better predicted with the EASSMand reasonably well predicted even at very coarse resolutions, where theDEVM is unable to predict a proper flow separation.The explicit algebraic SGS scalar flux model (EASSFM) is developed toimprove LES predictions of complex anisotropic flows with turbulent heat ormass transfer, and can be described as a nonlinear tensor eddy diffusivity model.It was tested in combination with the EASSM for the SGS stresses, and itsperformance was compared to the conventional dynamic eddy diffusivity model(DEDM) in channel flow with and without system rotation in the wall-normaldirection. EASSM and EASSFM gave predictions of high accuracy for meanvelocity and mean scalar fields, as well as stresses and scalar flux components.An extension of the EASSM and EASSFM, based on stochastic differentialequations of Langevin type, gave further improvements. In contrast to conventionalmodels, these extended models are able to describe intermittent transferof energy from the small, unresolved scales, to the resolved large ones.The present study shows that the EASSM/EASSFM gives a clear improvementof LES of wall-bounded flows in simple, as well as in complex geometriesin comparison with simpler SGS models. This is also shown to hold for a widerange of resolutions and is particularly accentuated for coarse resolution. The advantages are also demonstrated both for high-order numerical schemes andfor solvers using low-order finite volume methods. The models therefore havea clear potential for more applied computational fluid mechanics.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. vii, 48 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014:07 2014:07
Keyword
Turbulence, large eddy simulation, explicit algebraic subgridscale model, passive scalar, stochastic modelling, periodic hill flow
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-142401 (URN)978-91-7595-038-9 (ISBN)
Public defence
2014-03-14, F3, Lindstedsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Projects
Explicit algebraic sub-grid scale modelling for large-eddy simulations
Funder
Swedish Research Council, 621-2010- 6965
Note

QC 20140304

Available from: 2014-03-04 Created: 2014-03-03 Last updated: 2014-03-04Bibliographically approved

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Rasam, AminWallin, StefanBrethouwer, GeertJohansson, Arne

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