An experimental study was made of the flow over a backward-facing step. Excitations were given to the separated flow by means of a sinusoidally-oscillating jet issuing from a thin slit near the separation line. As a model of practical application of local actuator, the influence of spanwise local forcing on the separated flow was scrutinized by varying the gap distance(g) and the block width(ω). An optimal value of ω/g was obtained for a fixed ω which gives the minimum reduction of the reattachment length. The experiment were made at $Re_H=33000$ by altering the forcing amplitude ($0≤A_o≤0.07$) and forcing frequency ($0≤St_H ≤1.0$). The present experimental results were compared with those by a two-dimensional local forcing, i.e., ω=0. The effect of streamwise and spanwise vortices by the local spanwise forcing on the shear layer was investigated by looking at the spectral and correlation analyses.
