CLOSED-LOOP AERODYNAMIC FLOW CONTROL OF A FREE PITCHING AIRFOIL

Closed-loop feedback control of the attitude of a free pitching airfoil is effected without moving control surfaces by alternate manipulation of nominally-symmetric trapped vorticity concentrations on the suction and pressure surfaces near the trailing edge. The pitching moment is varied with minimal lift and drag penalties over a broad range of angles of attack when the baseline flow is fully attached. Accumulation (trapping) and regulation of vorticity is managed by integrated hybrid actuators (each comprised of a miniature [O(0.01c)] obstruction and a synthetic jet actuator). In the present work, the model is trimmed using a position feedback loop and a servo motor actuator. Once the model is trimmed, the position feedback loop is opened and the servo motor acts like an inner loop control to alter the model’s dynamic characteristics. Position control of the model is achieved using a reference model-based outer loop controller. I. OVERVIEW The aerodynamic effectiveness of lifting surfaces can be substantially improved by fluidic modification of their “apparent” shape through controlled interactions between arrays of surface-mounted synthetic jet actuators (Smith and Glezer 1998, Glezer and Amitay 2002) and the local cross flow that are also accompanied by local changes in the streamwise pressure gradients. These interactions lead to the formation of trapped vorticity concentrations where the balance between the trapped and shed vorticity is continuously regulated by the actuator jets. When the interaction domains are formed upstream of flow separation, the alteration of the local pressure gradients can result in complete or partial bypass (or suppression) of separation (e.g., Amitay et al. 1998, 2001 and Amitay and Glezer 2002, Glezer et al. 2005). Moreover, flow control by trapped vorticity is effective not only when the baseline flow is separated but also when it is fully attached, namely at low angles of attack (i.e., at cruise conditions). This approach was exploited in the earlier works of Chatlynne et al. (2001) and Amitay et al. (2001) which showed that the formation of a stationary trapped vortex above an airfoil at low angles of attack leads to pressure drag reduction that is comparable to the magnitude of the pressure drag of the baseline configuration with minimal lift penalty. Actuation was accomplished using a hybrid actuator comprised of a synthetic jet downstream from a miniature surface-mounted passive obstruction of scale O(0.01c) and the extent and strength of the trapped vortex was varied by varying the actuation frequency. Leveraging the presence of the miniature passive obstruction at low angles of attack drastically reduces the required actuation power compared to the use of the jet alone. This approach was adopted by DeSalvo, Amitay, and Glezer (2002) and later by DeSalvo and Glezer (2004) to manipulate the Kutta condition of an airfoil using concentrations of trapped vorticity that are induced and controlled near the trailing edge by a hybrid actuator similar to a Gurney flap. From the standpoint of aerodynamic flow control, both L/Dp and the pitching moment, CM, can be continuously adjusted by varying the actuation momentum coefficient over a broad range of angles of attack. The ensuing changes in the global flow near the trailing edge also result in a substantial reduction in drag (and therefore an increase in L/Dp) compared to both the baseline airfoil and the airfoil with inactive actuators. DeSalvo and Glezer realized an even greater decrease in pressure drag with virtually no loss in lift or significant change in skin friction drag by creating and manipulating trapped vorticity near the leading edge (2005) and more recently (2006) showed that similar actuation near the leading and trailing edges can lead to a significant simultaneous increase in lift and reduction in drag compared to the baseline airfoil.