Effects of the upstream boundary layer thickness and state on the structure of reattaching flows

Flow experiments were conducted on a backward-facing step in a two-dimensional channel of expansion ratio, ER = 1.25. The Reynolds number based on step height, Re(H), upstream boundary layer thickness ratio, delta/H, and state (laminar or turbulent) were changed independently over wide ranges. Reattachment distance, X(r), increased sharply, by about one step-height, upon transition of the separating boundary layer to turbulent flow; delta/H and Re(H) affected X(r) only slightly. In the recirculation zone, transition in the separating boundary layer also was the dominant effect. However, in the recovery region, downstream of reattachment, the effects of upstream boundary layer thickness ratio, delta/H are important if delta/h is greater than 0.4.