Three-Dimensional Vortical Structures of a Backward-Facing Step Flow at Moderate Reynolds Numbers

The three-dimensional vortical structures created in the downstream of the moderate-Reynolds-number (Re ≤ 700) backward-facing step flow are studied based on the direct numerical simulation (DNS). The flow is assumed to be periodic in the spanwise direction. The vortical structure manifests itself in the form of zig-zag arrangement of alternating quasi-streamwise structures with positive and negative streamwise vorticity, which has been observed in the buffer region of the turbulent channel flow by DNS [J. Fluid Mech. 332 (1997) 185]. In order to excite the perturbation, we triggered the flow by giving discontinuity of the time derivative of the inlet flow initially. It is found that the condition whether the three-dimensional vortical structures appear depends on the rate of temporal increase of the perturbation amplitude as t > 0. The present study is closely related to the generation mechanism of the coherent vortical structures in the wall turbulence.