Controlling Dynamic Stall with an Electrically Actuated Flexible Wall: the Mysterious Zero Pressure Gradient Point on the Suction Side of a Pitching Airfoil

An investigation has been made on the control of unsteady separating boundary layers, and the resulting dynamic stalls over a pitching NACA 0012 airfoil model using a Flexible Wall Transducer mounted on the leading edge. The transducer consisted of an aluminized Mylar membrane stretched across an array of strip shaped electrodes. An AC excitation was given to two adjacent strips covering 1.5 mm in stream wise direction. The amplitudes of vibrations of the membrane were measured to be about 0.01 microns. The power consumption was 1W. The experiments were carried out in a low speed wind tunnel at a Reynolds number about 6 E 05 and Mach no. of 0.1. A four bar mechanism was used for sinusoidal pitching of the airfoil. The pressures were measured along the central chord axis using a pressure scanner. The pressures showed a 2° delay in dynamic stall at certain frequencies of excitation. Drastic changes in pressure around the point of zero pressure gradient were found. The excitation was effective over a narrow zone around this point. A case of extra-ordinary lift over an initially pitching but subsequently static airfoil has also been reported.