Buffeting and Lock in of an Airfoil at High Angle of Attack

Abstract

In subsonic flow, buffeting refers to a fluid instability caused by flow separation in the flow around a bluff body or an airfoil at high angle of attack. Typically, there is a dominant frequency of these flow oscillations called the (nondimensional) Strouhal or buffet frequency. The goal of this paper was to explore the flow around a static and an oscillating airfoil at high angle of attack, and obtain a better understanding of buffet response by correlation of experimental results with existing computational methods. The effects of several parameters on the lift coefficient and flow-response frequency were studied, including Reynolds number, angle of attack, and blockage ratio of the airfoil size to the wind-tunnel dimensions. Also, more detailed flowfield characteristics were determined. For a static airfoil, a universal Strouhal-number scaling has been found for angles of attack from 30 to 90 deg, in which the flow around the airfoil is fully separated. Wind-tunnel walls lead to an increase in lift coefficient and Strouhal number. For an oscillating airfoil, the conditions for lock in are discussed. The differences between the lock-in case and the unlock-in case were also studied. The lock-in region in the confined flow is over a small range of excitation frequencies than in the unconfined flow.