Effect of Acoustic Excitation on Airfoil Performance at Low Reynolds Numbers

Abstract

The wake structure, vortex shedding characteristics and boundary-layer separation of a NACA 0025 airfoil and the effect of external acoustic excitations on airfoil performance were studied experimentally. Wind tunnel experiments were carried out for three Reynolds numbers and three angles of attack. Velocities were measured with hot-wires. Spectral analysis of these data was used in conjunction with complementary surface flow visualization to diagnose the performance of the airfoil at low Reynolds numbers. Evidence of wake vortex shedding and flow separation was obtained for most cases examined, and dependence of these phenomena on Reynolds number and angle of attack was found. The results establish that external acoustic excitation at a particular frequency and appropriate amplitude eliminates or reduces the separation region and decreases the airfoil wake, i.e., produces an increase of the lift and/or decrease of the drag. The acoustic excitation also alters the vortex shedding characteristics, decreasing the vortex length scale and the coherency of the vortex structure.