Amin Rasam

Effects of modelling, resolution and anisotropy of subgrid-scales on large eddy simulations of channel flow

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 τ=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.

Large eddy simulation of channel flow with and without periodic constrictions using the explicit algebraic subgrid-scale model

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.

Assessment of subgrid-scale stress statistics in non-premixed turbulent wall-jet flames

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.

Detached-Eddy Simulation of a Horizontal Axis Wind Turbine

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.

Subgrid-Scale Model and Resolution Influences in Large Eddy Simulations of Channel Flow

Subgrid-scale (SGS) modeling and resolution are two important issues that can affect the quality of large eddy simulation (LES) to a large extent. Many SGS models are based on an isotropic description of the SGS motions. However, SGS turbulence is not always isotropic. Near-wall SGS motions in wall-bounded turbulent flows is an example of such. In that case, the quality of LES largely depend on the grid resolution and the SGS modeling used. On the other hand, one could speculate that dependence of LES results on the resolution would be less if the SGS model is able to properly take into account the anisotropy of SGS stresses. In this paper, the influence of resolution on LES is investigated. Three SGS models are used, namely: the standard dynamic Smagorinsky model (DS) based on (Germano et al., 1991) and modifications of (Lilly, 1992) which is an isotropic eddy viscosity model, the high-pass filtered dynamic Smagorinsky (HPF) model of (Stolz et al., 2005) which is based on the variational multiscale method and the recent explicit algebraic (EA) model of (Marstorp et al., 2009) which is capable of properly modeling the anisotropy of the SGS stresses. LES of channel flow is carried out using these SGS models at Re τ =934, based on friction velocity and channel half width, and the results are compared to DNS data.