Controlling dynamic stall with an electrically actuated flexible wall

An investigation has been made on the control of unsteady separating boundary layers, and the resulting dynamic lift and moment stalls over a pitching NACA 0012 airfoil model (291 mm chord, 422 mm span) using a leading-edge mounted electrically actuated patented Active Flexible Wall (or AFW) Transducer. The AFW consisted of a thin flexible metalized Mylar membrane stretched across an array of 0.2–0.4 mm wide and 300-mm long strip shaped electrodes oriented spanwise. Wall-normal amplitudes of vibrations of the membrane over the actuated strips were measured to be about 0.01 μm using a laser vibrometer. The flow control experiments were then carried out in a low speed wind tunnel, at a chord-based Reynolds number about 6 E 05 and Mach number of 0.1. A four bar mechanism was used to sinusoidally pitch the airfoil about its ¼-chord axis (reduced frequencies between 0.05 and 0.15). The AC electrical excitation was between two adjacent strips covering 1.5 mm in the stream wise direction. The pressures were measured over the pitch cycle along the central chord axis using a pressure scanner. In spite of the spanwise variation due to the finite aspect ratio of the airfoil and the blockage of the wind tunnel, the pressures showed a 2° delay in dynamic stall, including a lowering of pre-stall lift and moment due to the AFW excitation. However, an improper excitation location advanced the dynamic stall.; Drastic changes in pressure at the leading edge, around the point of zero pressure gradient on the suction side, were found. This point moves with the angle of attack. AFW excitation was effective only over a narrow zone in time and space around this point. This validated a previously postulated pressure-gradient wall vibration coupling model for the pitching airfoil. The zero pressure gradient point on the suction side moved down stream for cases where dynamic stall was delayed, and vice versa. Even though the possibility of perturbing the pressure gradient through electromagnetic forcing of ions exists, the effects were shown to be negligible for the present non-ionized air flow. In the present study, the maximum AFW actuation power consumption was 1W.