VISUALIZATION OF A LOCALLY-FORCED SEPARATED FLOW OVER A BACKWARD-FACING STEP

A laboratory water channel experiment was made of the separated flow o ver a backward-facing step. The flow was excited by a sinusoidally osc illating jet issuing from a separation line. The slit was connected to a cavity in which water was forced through a rigid pipe by a scotch-y oke system. The Reynolds number based on the step height (H) was fixed at Re-H = 1200. The forcing frequency was varied in the range 0.305 l ess than or equal to St(H) less than or equal to 0.955 at the forcing amplitude A(o) = 0.3. Time-averaged flow measurements were made by a L DV system, especially in the recirculating region behind the backward- facing step. To characterize the large-scale vortex evolution due to t he local forcing, flow visualizations were performed by a dye tracer m ethod with fluorescent ink. The vortex amalgamation process was captur ed at the effective forcing frequency (St(H) = 0.477) for laminar sepa ration. This vortex merging process enhances flow mixing, which leads to the shortening of the reattachment length.