Feedback-controlled forcefully attached flow on a stalled airfoil

Active maintenance of attached flow at natural poststall conditions requires a small intervention, relative to the one needed to force a separated flow to reattach under the same conditions. Experiments with slot suction applied near the leading edge of a stalled airfoil revealed a hysteresis of lift and drag that depends on the level of suction. This offers an opportunity to keep the flow attached at minimum input levels while guaranteeing that flow separation will not be allowed to occur. A simple approach was adopted that uses a rapidly responding pressure sensor located near the leading edge or in the interior reservoir of the airfoil for feedback control. The proposed controller used a prescribed pressure coefficient to keep the flow attached. Since a dimensionless pressure coefficient is required for this purpose, two similar sensors were installed in the pitot-static tube that monitored the freestream velocity. The impact of the time delay on the stability of the controller was briefly discussed and accounted for. The robustness of the controller was demonstrated under varying freestream velocities.