The Utility of hysteresis for closed-loop control applications that maintain attached flow under natural post stall conditions on airfoils

Much higher control input is required to attach separated flow than to keep an attached flow from separating under natural, post stall conditions. The experiments with slot suction applied near the leading-edge, revealed a hysteresis of lift and drag coefficients that depend on the level of suction. This offers an opportunity to keep the flow attached at minimal input levels, while guarantying that flow separation will be not be allowed to occur. A simple approach was adopted that uses a fast response pressure sensor located near the leading-edge of the airfoil for feedback control. Since a pressure coefficient is required for this purpose, two additional quick responding sensors were installed in the Pitot tube that measures the free stream velocity. The proposed controller used a prescribed pressure coefficient to keep the flow attached. The impact of the time delay on the stability of the controller is briefly discussed and accounted for. The robustness of the controller was demonstrated under varying free stream velocities.