Experimental Aerodynamic Response for an Oscillating Airfoil in Buffeting Flow

Abstract

In prior work, several authors including the present ones have noted the analogy between the fluid oscillations induced by a bluff body in cross flow (e.g., the classic von Kármán vortex street behind a cylinder) and the flow oscillations often called buffet that occur for flow around wings and airfoils in transonic flow at sufficiently large angles of attack. In the present study, an airfoil in low-speed flow is placed at very high angles of attack and buffet and/or a von Kármán vortex street is indeed found. Of interest is the fact that the required angles of attack at these low speeds are well beyond the angle of attack for the onset of stall. The dependence of the characteristic Strouhal or reduced frequency on angle of attack and flow velocity (Reynolds number) is also determined, as are the peak amplitudes of the oscillating flow. At the large static angles of attack where buffet occurs, the airfoil is oscillated at various frequencies near the buffet frequency and with several amplitudes. Lock-in is found for certain combinations of airfoil oscillation frequencies and amplitudes.