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This study discusses the necessity of anisotropic subgrid-scale (SGS) stressin large-eddy simulations (LESs) of turbulent shear flows using a coarse gridresolution. We decompose the SGS stress into two parts to observe the role ofSGS stress in turbulent shear flows in addition to the energy transfer betweengrid-scale (GS or resolved scale) and SGS. One is the isotropic eddy-viscosityterm, which contributes to energy transfer, and the other is the residualanisotropic term, which is separated from the energy transfer. We investigatethe budget equation for GS Reynolds stress in turbulent channel flowsaccompanied by the SGS stress decomposition. In addition, we examine the mediumand coarse filter length cases; the conventional eddy-viscosity models canfairly predict the mean velocity profile for the medium filter case and failsfor the coarse filter case. The budget for GS turbulent kinetic energy showsthat the anisotropic SGS stress has a negligible contribution to energytransfer. In contrast, the anisotropic stress has a large and non-dissipativecontribution to the streamwise and spanwise components of GS Reynolds stresswhen the filter size is large. Even for the medium-size filter case, theanisotropic stress contributes positively to the budget for the spanwise GSReynolds stress. Spectral analysis of the budget reveals that the positivecontribution is prominent at a scale consistent with the spacing of streaks inthe near-wall region. Therefore, we infer that anisotropic stress contributesto the generation mechanism of coherent structures. Predicting the positivecontribution of the anisotropic stress to the budget is key to furtherimproving SGS models.

Analysis of anisotropic subgrid-scale stress for coarse large-eddy simulation