Feedback Control of Globally Unstable Flows: Impinging Shear Flows

Abstract Global oscillations of impinging shear flows are excited by a fluid-dynamics feedback mechanism, by means of which the distortion of the vorticity field upon impingement is fed back upstream to the separation location. It is shown that these global oscillations can effectively be suppressed, without destabilizing other modes, by actively cancelling the effect of the fluid-dynamic feedback. This achieved with the aid of loudspeakers, focused on the upstream separation edge only and activated by a transducer measuring the flow response. With the aid of an adaptive digital controller, it was possible to eliminate the flow oscillation altogether, even at transitional test conditions at which the flow oscillations could occur at one or the other of the two possible modes.