Dynamic Characteristics in a Control System of Backward Facing Step Flow by Vortex Generator Jets

Abstract

*† The present study is conducted in an attempt to construct a closed loop control system of the reattachment process for a backward facing step flow by using a row of vortex generator jets. Two-dimensional reattachment is found to occur when the spanwise pitch of the injected jets is 10d to 20d, where d is the jet diameter, because measurement of the forward flow fraction indicates that the reattachment line is approximately parallel to the spanwise direction. It is necessary to examine the dynamic characteristics of the injection jet on the reattachment process in the design of the control system. The coherence and the frequency transfer characteristics between the injection jet and the near-wall flow in the reattachment region are examined in an open-loop system. The experimental result shows that the coherence of the jet velocity with the surface static pressure is higher than that with the surface friction velocity. The surface static pressure has better frequency transfer gain characteristics near the reattachment point. A step response of the near-wall flow is also conducted in order to investigate the transient characteristics. Damped oscillation with respect to the surface static pressure is found to occur immediately after a step change in the jet velocity.