Cross-Wire Measurements in the Wake of an Airfoil at Low Reynolds Numbers With and Without Acoustic Excitation

The wake structure and vortex shedding characteristics of a NACA 0025 airfoil were studied experimentally. Wind tunnel experiments were carried out for three Reynolds numbers and three angles of attack by means of cross-wire measurements, spectral analysis and complementary surface flow visualization. Evidence of wake vortex shedding and flow separation was obtained for most of the cases examined, and dependence of these phenomena on Reynolds number and angle of attack was found. External acoustic excitation at a particular frequency was found to eliminate or reduce the separation region and decrease the airfoil wake. Moreover, the results show that the acoustic excitation has a significant effect on vortex shedding and can improve airfoil performance, i.e. produce an increase of the lift and/or decrease of the drag.