Experimental application of an active control loop on backward-facing step flow

Experimental results are presented on the active control of a backward-facing step flow revealing some features observed in combustion chambers. These experiments were performed under nonreactive conditions, and the effect of the pressure fluctuations induced by the unsteady heat release was simulated by an external acoustic excitation produced by two loudspeakers placed far downstream from the step., This excitation signal was delivered by a generator or by a hot-wire probe placed in the shear layer. The latter configuration reproduces some aspects of the actual coupling that have been observed with combustion instabilities. This preexcited flow was then controlled using six synthetic jets located on the vertical side of the step. The control loop uses an recursive least-mean-square autoadaptive algorithm applied to a signal from either a microphone or a hot-wire probe placed downstream of the step edge. The influence of the external acoustic excitationon the flow organization and the effect of the control loop are described. Without excitation, the control algorithm is not able to reduce the natural unsteady phenomena. However, it does damp the shear-layer pressure or velocity fluctuations intensified by an external excitation and bring the flow close to its natural feature. The efficiency of the control depends on the frequency of the external excitation, the signal used in the control loop, and the phase shift between the velocity and the pressure signals in the shear layer.