Effect of subgrid-scale anisotropy on wall-modeled large-eddy simulation for separated turbulent flow*

Recent studies have highlighted the pivotal role of subgrid-scale (SGS) model in wall-modeled large-eddy simulation (WMLES) for predicting flow separation. In our investigations of flow over a Gaussian bump using WMLES, we observe that eddy-viscosity-based SGS models often show a nonmonotonic prediction of separation bubble size with grid refinement. In contrast, SGS models augmented with anisotropic stress terms yield more consistent outcomes across different mesh resolutions. To pinpoint where SGS anisotropy becomes crucial, we introduced a virtual interface that divides the computational domain into upstream and downstream sections, applying different SGS models to each. Our findings indicate that just upstream of the bump peak, where a favorable pressure gradient is pronounced, anisotropic SGS stress significantly impacts downstream flow separation. We further conducted detailed budget analyses of the mean momentum and resolved Reynolds stress transport within this region. These analyses reveal that SGS stress plays a crucial role in shaping the boundary layer, influencing the streamwise pressure gradient, and consequently, the formation of downstream separation bubble. Importantly, anisotropic SGS stress positively contributes to the resolved Reynolds stress budget, thereby enhancing the accuracy of predictions.