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Anisotropy-resolving subgrid-scale modelling using explicit algebraic closures for large eddy simulation


Respondent	Huvudhandledare	Bihandledare	Datum
Amin Rasam	Arne Johansson	Geert Brethouwer	2014-03-14

Opponent
Bernard Geurts, University of Twente

Betygsnämd
Lars Davidson, Chalmers Gunilla Efraimsson, KTH Aeronautics and Vehicle Engineering Shia-Hui Peng, FOI

Abstract

The present thesis deals with the development and performance analysis of anisotropy-resolving models for the small, unresolved scales (”sub-grid scales”, SGS) in large eddy simulation (LES). The models are characterised by a description of anisotropy by use of explicit algebraic models for both the subgrid-scale (SGS) stress tensor (EASSM) and SGS scalar flux vector (EASSFM). Extensive analysis of the performance of the explicit algebraic SGS stress model (EASSM) has been performed and comparisons made with the conventional isotropic dynamic eddy viscosity model (DEVM). The studies include LES of plane channel flow at relatively high Reynolds numbers and a wide range of resolutions and LES of separated flow in a channel with streamwise periodic hill-shaped constrictions (periodic hill flow) at coarse resolutions. The former simulations were carried out with a pseudo-spectral Navier–Stokes solver, while the latter simulations were computed with a second-order, finite-volume based solver for unstructured grids. The LESs of channel flow demonstrate that the EASSM gives a good description of the SGS anisotropy, which in turn gives a high degree of resolution independence, contrary to the behaviour of LES predictions using the DEVM. LESs of periodic hill flow showed that the EASSM also for this case gives significantly better flow predictions than the DEVM. In particular, the reattachment point was much better predicted with the EASSM and reasonably well predicted even at very coarse resolutions, where the DEVM is unable to predict a proper flow separation. The explicit algebraic SGS scalar flux model (EASSFM) is developed to improve LES predictions of complex anisotropic flows with turbulent heat or mass 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 its performance was compared to the conventional dynamic eddy diffusivity model (DEDM) in channel flow with and without system rotation in the wall-normal direction. EASSM and EASSFM gave predictions of high accuracy for mean velocity and mean scalar fields, as well as stresses and scalar flux components. An extension of the EASSM and EASSFM, based on stochastic differential equations of Langevin type, gave further improvements. In contrast to conventional models, these extended models are able to describe intermittent transfer of energy from the small, unresolved scales, to the resolved large ones. The present study shows that the EASSM/EASSFM gives a clear improvement of LES of wall-bounded flows in simple, as well as in complex geometries in comparison with simpler SGS models. This is also shown to hold for a wide range of resolutions and is particularly accentuated for coarse resolution. The advantages are also demonstrated both for high-order numerical schemes and for solvers using low-order finite volume methods. The models therefore have a clear potential for more applied computational fluid mechanics.
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