Coherent structures in turbulent boundary layers with adverse pressure gradients

Abstract

Coherent structures of turbulent boundary layers (TBLs) subjected to adverse pressure gradients (APGs) were scrutinized by analyzing the database of direct numerical simulations. Equilibrium adverse pressure gradient flows were established by using a power law free-stream distribution. A ZPG flow was also examined for comparison. Population trends of spanwise vortices in APG TBLs showed that hairpin-like vortices are densely populated in the outer region of APG TBLs. These vortical structures induce low-momentum regions in the middle of the boundary layer which lead to the outer peak of the Reynolds shear stress. Wall-normal spatial characteristics of hairpin packets were investigated in the spanwise-wall-normal plane to examine three-dimensional features of the hairpin packets. Conditionally averaged velocity fields showed that there are counter rotating v-w swirling motions which represent cross-sectional evidences of the packets. Moreover, two-point correlations and linear stochastic estimations were employed to provide statistical evidence of the hairpin packet motions in the cross-stream plane of APG TBLs.