High Reynolds number flow over a backward-facing step: structure of the mean separation bubble

In the present paper, the structure of the mean separation bubble downstream of the backward-facing step is studied at large Reynolds numbers. The flow over the step at these Reynolds numbers is turbulent with the presence of unsteady large-scale structures. There is however a well-defined time-averaged mean separation bubble. We study the effect of Reynolds number and expansion ratio on the structure of this mean separation bubble, the expansion ratio being the primary geometrical parameter in this case. Using PIV measurements within the separation bubble, parameters such as the reattachment length, mean velocity field, and the turbulent stresses are systematically mapped out. These measurements show that there exists a high Reynolds number separation bubble structure that is nearly independent of both Reynolds number and expansion ratio, as long as the Reynolds numbers are large (Re > 36,000 based on step height). Within this large Reynolds number separation bubble, the normalized mean velocity field and the normalized turbulent stresses are found to be similar for all expansion ratio cases studied. Using these measurements, the streamwise force balance of the mean separation bubble is studied. The analysis of the data shows that in this case, the contribution to the streamwise force from both the Reynolds normal and shear stress is significant, although the Reynolds shear stress contribution is larger. Differences in the force contributions from other geometries are highlighted.