Upstream open loop control of the recirculation area downstream of a backward-facing step

The flow downstream a backward-facing step is controlled using a pulsed jet placed upstream of the step edge. Experimental velocity fields are computed and used to quantify the recirculation area. The effects of jet amplitude, frequency and duty cycle on this recirculation area are investigated for two Reynolds numbers (Reh=2070Reh=2070 and Reh=2900Reh=2900). The results of this experimental study demonstrate that upstream actuation can be as efficient as actuation at the step edge when exciting the shear layer at its natural frequency. Moreover, it is shown that it is possible to minimize both jet amplitude and duty cycle and still achieve optimal efficiency. With minimal amplitude and a duty-cycle as low as 10%, the recirculation area is nearly canceled.