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Explicit algebraic subgrid-scale stress and passive scalar flux modeling in large eddy simulation
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. (Turbulence)ORCID iD: 0000-0002-3173-7502
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The present thesis deals with a number of challenges in the field of large eddy simulation (LES). These include the performance of subgrid-scale (SGS) models at fairly high Reynolds numbers and coarse resolutions, passive scalar and stochastic modeling in LES. The fully-developed turbulent channel flow is used as the test case for these investigations. The advantage of this particular test case is that highly accurate pseudo-spectral methods can be used for the discretization of the governing equations. In the absence of discretization errors, a better understanding of the subgrid-scale model performance can be achieved. Moreover, the turbulent channel flow is a challenging test case for LES, since it shares some of the common important features of all wall-bounded turbulent flows. Most commonly used eddy-viscosity-type models are suitable for moderately to highly-resolved LES cases, where the unresolved scales are approximately isotropic. However, this makes simulations of high Reynolds number wall-bounded flows computationally expensive. In contrast, explicit algebraic (EA) model takes into account the anisotropy of SGS motions and performs well in predicting the flow statistics in coarse-grid LES cases. Therefore, LES of high Reynolds number wall-bounded flows can be performed at much lower number of grid points in comparison with other models. A demonstration of the resolution requirements for the EA model in comparison with the dynamic Smagorinsky and its high-pass filtered version for a fairly high Reynolds number is given in this thesis. One of the shortcomings of the commonly used eddy diffusivity model arises from its assumption of alignment of the SGS scalar flux vector with the resolved scalar gradients. However, better SGS scalar flux models that overcome this issue are very few. Using the same methodology that led to the EA SGS stress model, a new explicit algebraic SGS scalar flux model is developed, which allows the SGS scalar fluxes to be partially independent of the resolved scalar gradient. The model predictions are verified and found to improve the scalar statistics in comparison with the eddy diffusivity model. The intermittent nature of energy transfer between the large and small scales of turbulence is often not fully taken into account in the formulation of SGS models both for velocity and scalar. Using the Langevin stochastic differential equation, the EA models are extended to incorporate random variations in their predictions which lead to a reasonable amount of backscatter of energy from the SGS to the resolved scales. The stochastic EA models improve the predictions of the SGS dissipation by decreasing its length scale and improving the shape of its probability density function.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , 30 p.
Series
Trita-MEK, ISSN 0348-467X ; 2011:05
Keyword [en]
Turbulence, Large eddy simulation, explicit algebraic subgrid-scale model, passive scalar, stochastic modeling, backscatter
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-34453ISBN: 978-91-7501-016-8OAI: oai:DiVA.org:kth-34453DiVA: diva2:421504
Presentation
2011-05-30, H1, KTH, Teknikringen 33, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 621-2010-6965Swedish e‐Science Research Center
Note
QC 20110615Available from: 2011-06-15 Created: 2011-06-08 Last updated: 2012-05-24Bibliographically approved
List of papers
1. Effects of modelling, resolution and anisotropy of subgrid-scales on large eddy simulations of channel flow
Open this publication in new window or tab >>Effects of modelling, resolution and anisotropy of subgrid-scales on large eddy simulations of channel flow
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2011 (English)In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 12, no 10, 1-20 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, the effect of subgrid-scale (SGS) modelling, grid resolution and anisotropy of the subgrid-scales on large eddy simulation (LES) is investigated. LES of turbulent channel flow is performed at Re=934, based on friction velocity and channel half width, for a wide range of resolutions. The dynamic Smagorinsky model (DS), the high-pass filtered dynamic Smagorinsky model (HPF) based on the variational multiscale method and the recent explicit algebraic model (EA), which accounts for the anisotropy of the SGS stresses are considered. The first part of the paper is focused on the resolution effects on LES, where the performances of the three SGS models at different resolutions are compared to direct numerical simulation (DNS) results. The results show that LES using eddy viscosity SGS models is very sensitive to resolution. At coarse resolutions, LES with the DS and the HPF models deviate considerably from DNS, whereas LES with the EA model still gives reasonable results. Further analysis shows that the two former models do not accurately predict the SGS dissipation near the wall, while the latter does, even at coarse resolutions. In the second part, the effect of SGS modelling on LES predictions of near-wall and outer-layer turbulent structures is discussed. It is found that different models predict near-wall turbulent structures of different sizes. Analysis of the spectra shows that although near-wall scales are not resolved at coarse resolutions, large-scale motions can be reasonably captured in LES using all the tested models.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2011
Keyword
large eddy simulation, subgrid-scale modelling, resolution effects, subgrid-scale anisotropy, channel flow
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-31322 (URN)10.1080/14685248.2010.541920 (DOI)000287703700001 ()2-s2.0-83055170803 (ScopusID)
Funder
Swedish Research Council, 621-2010-6965Swedish Research Council, 621-2007-4232Swedish e‐Science Research Center
Note

QC 20110318

Available from: 2011-03-18 Created: 2011-03-14 Last updated: 2016-04-12Bibliographically approved
2. An explicit algebraic model for the subgrid-scale passive scalar flux
Open this publication in new window or tab >>An explicit algebraic model for the subgrid-scale passive scalar flux
2011 (English)Report (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-34764 (URN)
Available from: 2011-06-15 Created: 2011-06-15 Last updated: 2011-06-15Bibliographically approved
3. Stochastic explicit algebraic subgrid-scale stress and scalar flux models
Open this publication in new window or tab >>Stochastic explicit algebraic subgrid-scale stress and scalar flux models
2011 (English)Report (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-34765 (URN)
Available from: 2011-06-15 Created: 2011-06-15 Last updated: 2011-06-15Bibliographically approved

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