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

Abstract

An experimental study was made of the flow over a backward-facing step. Excitations were given to separated flow by means of a sinusoidally oscillating jet issuing from a thin slit near the separation line. The Reynolds number based on the step height (H) varied 13 000 less than or equal to Re-H less than or equal to 33 000. Effect of local forcing on the flow structure was scrutinized by altering the forcing amplitude (0 less than or equal to A(0) less than or equal to 0.07) and forcing frequency (0 less than or equal to St(H) less than or equal to 5.0). Small localized forcing near the separation edge enhanced the shear-layer growth rate and produced a large roll-up vortex at the separation edge. A large vortex in the shear layer gave rise to a higher rate of entrainment,which lead to a reduction in reattachment length as compared to the unforced flow. The normalized minimum reattachment length (X(r))(min)/x(r0) was obtained at St(theta) congruent to 0.01. The most effective forcing frequency was found to be comparable to the shedding frequency of the separated shear layer.