| 1. |
- Montecchia, Matteo, 1990-, et al.
(författare)
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Large-eddy simulation of turbulent channel flow using the explicit algebraic subgrid-scale model
- 2015
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Ingår i: Proceedings - 15th European Turbulence Conference, ETC 2015. - : TU Delft.
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Konferensbidrag (refereegranskat)abstract
- Large-eddy simulation (LES) of turbulent channel flow are performed with a new subgrid-scale (SGS) stress model. The simulations show that with this model we can well predict turbulent wall flows at coarse resolutions and moderately high Reynolds numbers. The commonly used dynamic Smagorinsky model fails at coarser resolutions.
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| 2. |
- Rasam, Amin, et al.
(författare)
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A comparison between isotropic and anisotropy-resolving closures in large eddy simulation of separated flow
- 2014
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 3. |
- Rasam, Amin, et al.
(författare)
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A stochastic extension of the explicit algebraic subgrid-scales models
- 2014
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Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 26:5, s. 055113-
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Tidskriftsartikel (refereegranskat)abstract
- The explicit algebraic subgrid-scale (SGS) stress model (EASM) of Marstorp et al. ["Explicit algebraic subgrid stress models with application to rotating channel flow," J. Fluid Mech. 639, 403-432 (2009)] and explicit algebraic SGS scalar flux model (EASFM) of Rasam et al. ["An explicit algebraic model for the subgrid-scale passive scalar flux,"J. Fluid Mech. 721, 541-577 (2013)] are extended with stochastic terms based on the Langevin equation formalism for the subgrid-scales by Marstorp et al. ["A stochastic subgrid model with application to turbulent flow and scalar mixing," Phys. Fluids 19, 035107 (2007)]. The EASM and EASFM are nonlinear mixed and tensor eddy-diffusivity models, which improve large eddy simulation (LES) predictions of the mean flow, Reynolds stresses, and scalar fluxes of wall-bounded flows compared to isotropic eddy-viscosity and eddy-diffusivity SGS models, especially at coarse resolutions. The purpose of the stochastic extension of the explicit algebraic SGS models is to further improve the characteristics of the kinetic energy and scalar variance SGS dissipation, which are key quantities that govern the small-scale mixing and dispersion dynamics. LES of turbulent channel flow with passive scalar transport shows that the stochastic terms enhance SGS dissipation statistics such as length scale, variance, and probability density functions and introduce a significant amount of backscatter of energy from the subgrid to the resolved scales without causing numerical stability problems. The improvements in the SGS dissipation predictions in turn enhances the predicted resolved statistics such as the mean scalar, scalar fluxes, Reynolds stresses, and correlation lengths. Moreover, the nonalignment between the SGS stress and resolved strain-rate tensors predicted by the EASM with stochastic extension is in much closer agreement with direct numerical simulation data.
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| 4. |
- Rasam, Amin, et al.
(författare)
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Aerodynamic noise prediction for a wind turbine using numerical flow simulations and semi-empirical modelling approaches
- 2016
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Ingår i: 22nd AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, aerodynamic and aero-acoustic simulations are performed for a small horizontal axis wind turbine, suitable for the integration of wind energy in urban and peri-urban areas. Detached-eddy simulation (DES) of compressible flow is performed to compute the flow field over the wind turbine. The far-field noise is computed using the Ffowcs - Williams and Hawkings acoustic analogy. Furthermore, the blade element momentum theory is used with a semi-empirical acoustic modeling approach to predict the wind turbine noise. The acoustic modeling approach is based on a semi-empirical formulation for airfoil self noise and an analytic formulation for inflow noise.
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| 5. |
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| 6. |
- Rasam, Amin, et al.
(författare)
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An explicit algebraic model for the subgrid-scale passive scalar flux
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 721, s. 541-577
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Tidskriftsartikel (refereegranskat)abstract
- In Marstorp et al. (J. Fluid Mech., vol. 639, 2009, pp. 403-432), an explicit algebraic subgrid stress model (EASSM) for large-eddy simulation (LES) was proposed, which was shown to considerably improve LES predictions of rotating and non-rotating turbulent channel flow. In this paper, we extend that work and present a new explicit algebraic subgrid scalar flux model (EASSFM) for LES, based on the modelled transport equation of the subgrid-scale (SGS) scalar flux. The new model is derived using the same kind of methodology that leads to the explicit algebraic scalar flux model of Wikstrom et al. (Phys. Fluids, vol. 12, 2000, pp. 688-702). The algebraic form is based on a weak equilibrium assumption and leads to a model that depends on the resolved strain-rate and rotation-rate tensors, the resolved scalar-gradient vector and, importantly, the SGS stress tensor. An accurate prediction of the SGS scalar flux is consequently strongly dependent on an accurate description of the SGS stresses. The new EASSFM is therefore primarily used in connection with the EASSM, since this model can accurately predict SGS stresses. The resulting SGS scalar flux is not necessarily aligned with the resolved scalar gradient, and the inherent dependence on the resolved rotation-rate tensor makes the model suitable for LES of rotating flow applications. The new EASSFM (together with the EASSM) is validated for the case of passive scalar transport in a fully developed turbulent channel flow with and without system rotation. LES results with the new model show good agreement with direct numerical simulation data for both cases. The new model predictions are also compared to those of the dynamic eddy diffusivity model (DEDM) and improvements are observed in the prediction of the resolved and SGS scalar quantities. In the non-rotating case, the model performance is studied at all relevant resolutions, showing that its predictions of the Nusselt number are much less dependent on the grid resolution and are more accurate. In channel flow with wall-normal rotation, where all the SGS stresses and fluxes are non-zero, the new model shows significant improvements over the DEDM predictions of the resolved and SGS quantities.
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| 7. |
- Rasam, Amin, 1979-, et al.
(författare)
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An explicit algebraic subgrid-scale scalar variance model
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Konferensbidrag (refereegranskat)abstract
- We present a subgrid-scale (SGS) scalar variance model based on the explicit algebraic subgrid scalar flux model, EASSFM (8). The EASSFM is a dynamic mixed nonlinear tensor eddy diffusivity model, which is derived from the modeled transport equation of the SGS scalar flux. The explicit algebraic form is obtained using the weak equilibrium assumption. The resulting model improves the direction of the predicted SGS flux vector and enables the prediction of shear-induced SGS fluxes, in contrast with the eddy diffusivity model. The EASSFM has been used for large eddy simulation (LES) of turbulent channel flow with and without system rotation (8; 9) and has been found to improve LES predictions over the dynamic eddy diffusivity model. A priori analysis of the EASSFM using the filtered DNS data from a reacting turbulent wall-jet has been performed recently (6; 7), which also showed favorable results. In this study, we evaluate our SGS scalar variance model using the filtered DNS database of a turbulent reacting wall-jet, which is an extension of our previous study on reactive turbulent wall-jet flows (5; 7) to a larger simulation domain. The results show a good agreement between the filtered DNS and our model predictions for the passive and active scalars. This indicates that acceptable predictions of the SGS scalar variance can be obtained using the EASSFM with the new SGS scalar variance model.
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| 8. |
- Rasam, Amin
(författare)
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Anisotropy-resolving subgrid-scale modelling using explicit algebraic closures for large eddy simulation
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 9. |
- Rasam, Amin, et al.
(författare)
-
Assessment of subgrid-scale stress statistics in non-premixed turbulent wall-jet flames
- 2016
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Ingår i: Journal of Turbulence. - : Taylor & Francis. - 1468-5248. ; 17:5, s. 471-490
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the heat-release effects on the characteristics of the subgrid-scale (SGS) stress tensor and SGS dissipation of kinetic energy and enstrophy. Direct numerical simulation data of a non-premixed reacting turbulent wall-jet flow with and without substantial heat release is employed for the analysis. This study comprises, among others, an analysis of the eigenvalues of the resolved strain rate and SGS stress tensors, to identify the heat-release effects on their topology. An assessment of the alignment between the eigenvectors corresponding to the largest eigenvalues of these two tensors is also given to provide further information for modelling of the SGS stress tensor. To find out the heat-release effects on the dynamics of the turbulent kinetic energy and enstrophy dissipation, probability density functions (PDFs) and mean values are analysed. The mean SGS shear stress and turbulent kinetic energy both slightly increase in the buffer layer and substantially decrease further away from the wall, due to the heat-release effects. Contrary to the kinetic energy, heat release decreases the mean SGS dissipation of enstrophy in the near-wall region. Moreover, differences in the shapes of the PDFs between the isothermal and exothermic cases indicate changes in the intermittency level of both SGS dissipations. Heat release also increases the SGS stress anisotropy in the near-wall region. Although, the structure of the mean resolved strain-rate tensor only marginally differs between the isothermal and exothermic cases in the near-wall region, substantial differences are observed in the jet area, where compressibility effects are important and heat-release effects are found to promote compression states. The differences in the relative alignment between the SGS stress and resolved strain-rate tensors in the isothermal and exothermic cases are discussed in connection with the differences in the SGS dissipation of kinetic energy.
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| 10. |
- Rasam, Amin, et al.
(författare)
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Assessment of subgrid-scale stress statistics in non-premixed turbulent wall-jet flames
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We investigate the properties of the subgrid-scale (SGS) stress tensor and SGS dissipation of kinetic energy and enstrophy,using the direct numerical simulation (DNS) data of a non-premixed reacting turbulent wall-jet flow, with and without heat release.The separation of scales, to obtain the SGS quantities, is achieved by application of a box filter.This study comprises an analysis on the topology of the resolved strain-rate andSGS stress tensors, through an assessment of their eigenvectors and their relative alignment. To find out the heat-release effects on the dynamics of the turbulent energy dissipation, SGS dissipation of kinetic energy andenstrophy are evaluated using length-scale, probability density functions (PDFs) and mean value analysis.It is found that the mean SGS shear stress and turbulent kinetic energy are suppressed by the heat release, whilethe SGS anisotropy is substantially increased.Although, the topology of the resolved strain-rate tensor only marginally differs between the isothermal and exothermic cases in the near-wall wall region,substantial differences are observed in the shear layer in the jet area, where the compressibility effects are large andthe exothermic effects are found to promote compression states.The relative alignment between the SGS stress and resolved strain-rate tensors is also affected by the heat release.The mean SGS dissipation of kinetic energy is increased, while the SGS dissipation of enstrophy is decreased by the heat release.Interesting differences in the shape of the PDFs of the SGS dissipation are observed between the isothermal and exothermic cases, such as thechange in the intermittency of both SGS dissipation terms.
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| 11. |
- Rasam, Amin, et al.
(författare)
-
Detached-eddy simulation of a horizontal-axis wind turbine
- 2016
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Ingår i: 6th Symposium on Hybrid RANS-LES Methods.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Aerodynamic simulations of a small horizontal-axis wind turbine, suit- able for integration of wind energy in urban and peri-urban areas, are performed. Im- proved delayed detached-eddy simulation is used in the computations. Simulations are carried out for three rotation rates and inlet conditions. Aerodynamic charac- teristics of the wind turbine such as forces, power production, pressure distribution as well as flow topology are presented. The effect of different rotation rates on the turbine aerodynamics is discussed.
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| 12. |
- Rasam, Amin, 1979-, et al.
(författare)
-
Detached-eddy simulation of a horizontal axis wind turbine
- 2018
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Ingår i: Progress in Hybrid RANS-LES Modelling. - Cham : Springer. ; , s. 357-367
-
Bokkapitel (refereegranskat)abstract
- Aerodynamic simulations of a small horizontal-axis wind turbine, suitable for integration of wind energy in urban and peri-urban areas, are performed using the improved delayed detached-eddy simulation method. Simulations are carried out for three rotation rates and inlet conditions. Aerodynamic characteristics of the wind turbine such as forces, power production, pressure distribution as well as flow topologies are presented. The effect of different rotation rates as well as the effect of free stream turbulence on the turbine aerodynamics are discussed.
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| 13. |
- Rasam, Amin, et al.
(författare)
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Effects of modelling, resolution and anisotropy of subgrid-scales on large eddy simulations of channel flow
- 2011
-
Ingår i: Journal of Turbulence. - : Institute of Physics Publishing (IOPP). - 1468-5248. ; 12:10, s. 1-20
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 14. |
- Rasam, Amin, 1979-
(författare)
-
Explicit algebraic subgrid-scale stress and passive scalar flux modeling in large eddy simulation
- 2011
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 15. |
- Rasam, Amin, et al.
(författare)
-
Large eddy simulation of channel flow with and without periodic constrictions using the explicit algebraic subgrid-scale model
- 2014
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Ingår i: Journal of Turbulence. - : Taylor & Francis. - 1468-5248. ; 15:11, s. 752-775
-
Tidskriftsartikel (refereegranskat)abstract
- We analyse the performance of the explicit algebraic subgrid-scale (SGS) stress model (EASSM) in large eddy simulation (LES) of plane channel flow and the flow in a channel with streamwise periodic hill-shaped constrictions (periodic hill flow) which induce separation. The LESs are performed with the Code_Saturne which is an unstructured collocated finite volume solver with a second-order spatial discretisation suitable for LES of incompressible flow in complex geometries. At first, performance of the EASSM in LES of plane channel flow at two different resolutions using the Code_Saturne and a pseudo-spectral method is analysed. It is observed that the EASSM predictions of the mean velocity and Reynolds stresses are more accurate than the conventional dynamic Smagorinsky model (DSM). The results with the pseudo-spectral method were, in general, more accurate. In the second step, LES with the EASSM of flow separation in the periodic hill flow is compared to LES with the DSM, no SGS model and a highly resolved LES data using the DSM. Results show that the mean velocity profiles, the friction and pressure coefficients, the length and shape of the recirculation bubble, as well as the Reynolds stresses are considerably better predicted by the EASSM than the DSM and the no SGS model simulations. It was also observed that in some parts of the domain, the resolved strain-rate and SGS shear stress have the same sign. The DSM cannot produce a correct SGS stress in this case, in contrast to the EASSM.
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| 16. |
- Rasam, Amin, et al.
(författare)
-
Large eddy simulation of channel flow with andwithout periodic constrictions using the explicit algebraic subgrid-scale model
-
Ingår i: Journal of Turbulence. - 1468-5248.
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We analyse the performance of the explicit algebraic subgrid-scale stress model(EASSM) of Marstorp et al. (J. Fluid Mech., vol. 639, 2009, pp. 403–432) inlarge eddy simulation (LES) of plane channel and the flow in a channel withstreamwise periodic hill-shaped constrictions (periodic hill flow) which induceseparation. The LESs are performed with Code Saturne which is an unstructuredcollocated finite volume solver with a second-order spatial discretisationsuitable for LES of incompressible flow in complex geometries. At first, performanceof the EASSM in LES of plane channel flow at two different resolutionsusing the Code Saturne and a pseudo-spectral method is analyzed. It is observedthat EASSM predictions of the mean velocity and Reynolds stresses aremore accurate than with the conventional dynamic Smagorinsky model (DSM).The results with the pseudo-spectral method were in general more accurate.In the second step, LES with the EASSM of flow separation in the periodichill flow is compared to LES with the DSM, no subgrid-scale model and thehighly resolved LES data of Breuer et al. (Computers & Fluids, vol. 38, 2009,pp. 433–457) using the DSM. Results show that the mean velocity profiles,the friction and pressure coefficients, the length and shape of the recirculationbubble, as well as the Reynolds stresses are considerably better predicted bythe EASSM than the DSM and the no subgrid-scale model simulations. It wasalso observed that in some parts of the domain the resolved strain-rate andsubgrid-scale shear stress have the same sign. The DSM cannot produce acorrect subgrid-scale stress in this case, in contrast to the EASSM.
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| 17. |
- Rasam, Amin, 1979-, et al.
(författare)
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Large-eddy simulation using the explicit algebraic subgrid model in complex geometries
- 2013
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Ingår i: International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2013. - : TSFP-8. - 9780000000002
-
Konferensbidrag (refereegranskat)abstract
- In Rasam et al. (2011) we compared the performance of the explicit algebraic subgrid-scale (SGS) model (EASSM) (Marstorp et al., 2009) with that of the conventional dynamic Smagorinsky model (DSM) in large eddy simulation (LES) of channel flow using a pseudo-spectral Navier-Stokes solver. We showed that, due to the better prediction of the individual SGS stresses in a wide range of grid resolutions and due to the nonlinear SGS stress contribution by the model, the EASSM predictions were less resolution dependent and more accurate than those of the DSM. As the first step in this study towards LES in complex geometries, we extend our previous study and perform LES of turbulent channel flow at Ret = 590 using the EASSM and the code Saturne, which is an unstructured finite volume solver suitable for LES in complex geometries. The results are compared to those of the DSM and show that the EASSM predictions of the wall shear and the Reynolds stresses are more accurate. LES results using the EASSM obtained from the code Saturne are also compared to those obtained using the pseudo-spectral solver obtained in our previous study (Rasam et al., 2013). As the next step, we are performing LES of flow over periodic hill using the EASSM and the code Saturne. The results will be compared with those of the DSM and the reference LES and will be presented at the conference.
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| 18. |
- Rasam, Amin, 1979-, et al.
(författare)
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Stochastic and non-stochastic explicit algebraic models for les
- 2011
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Ingår i: 7th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2011.
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Konferensbidrag (refereegranskat)abstract
- This paper consists of three parts. In the first part, we demonstrate the performance of the explicit algebraic (EA) subgrid-scale (SGS) stress model at Reτ = 934 and Reτ = 2003, based on friction velocity and channel half-width, for the case of large eddy simulation (LES) of turbulent channel flow. Performance of the EA model is compared to that of the dynamic Smagorinsky (DS) model for four different coarse resolutions and statistics are compared to the DNS of del Álamo & Jiménez (2003) and Hoyas & Jiménez (2008). Mean velocity profiles and Reynolds stresses are presented for the different cases. The EA model predictions are found to be reasonably close to the DNS profiles at all resolutions, while the DS model predictions are only in agreement at the finest resolution. The EA model predictions are found to be less resolution dependent than those with the DS model at both Reynolds numbers. In the second and third parts, we use Langevin stochastic differential equations to extend the EA model with stochastic contributions for SGS stresses and scalar fluxes. LES of turbulent channel flow at Reτ = 590, including a passive scalar, is carried out using the stochastic EA (SEA) models and the results are compared to the EA model predictions as well as DNS data. Investigations, show that the SEA model provides for a reasonable amount of backscatter of energy both for velocity and scalar, while the EA models do not provide for backscatter. The SEA model also improves the variance and length-scale of the SGS dissipation for velocity and scalar. However, the resolved statistics like the mean velocity, temperature, Reynolds stresses and scalar fluxes are hardly affected by the inclusion of the stochastic terms.
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