Control of turbulent separated-reattaching flow over a backward-facing step by local forcing

Abstract

Two experiments were made of the flow over a backward-facing step by a local forcing. One was performed in a wind tunnel and the other was in a water channel. The local perturbation in a wind tunnel was made by means of a sinusoidal oscillation jet issued from a spanwise thin slit near the separation line by woofer speaker. The excitation in a water channel was given by a scotch-yoke system. The Reynolds number based on the step height (H) was varied $13000 \leg Re_H \leg 33000$ for air and $Re_H=1200$ for water. The effect of this local forcing on the flow structures was scrutinized by altering the forcing amplitude ($0 \leg A_o \leg 0.07$) for air and $A_o=0.3$ for water. The forcing frequency was controlled in a range of $0 \leg St_H \leg 5.0$. A small localized forcing near the separation edge enhances the shear layer growth rate and produces a large roll-up vortex at a separation edge. A large vortex in the shear layer gives rise to a higher rate of entrainment, and this leads to a reduction in reattachment length as compared to the unforced flow. The normalized minimum reattachment length $(x_r)_{min}/x_{ro}$ is obtained at $St_\theta＼cong0.01$. The most effective forcing frequency is found to be comparable to the shedding frequency of the separated shear layer. A similar trends in the time-mean flow structure and the reattachment variation for water flow was also detected at the air flow. A change of vortex structure in the separated and reattaching shear layer is demonstrated by the flow visualization. The large-scale vortices are produced by the amalgamation of initial rolling up vortices with counter rotation due to the local shear layer of periodic forcing. This large-scale vortices propagates downstream, which causes large increased entrainment and increased turbulent level. Then, it leads to the reduction of reattachment length at the effective forcing frequency. However, any pairing or any merging process of vortices is not observed at the high...