Airfoil Performance at Low Reynolds Numbers in the Presence of Periodic Disturbances

The boundary-layer separation and wake structure of a NACA 0025 airfoil and the effect of external excitations in presence of structural vibrations on airfoil performance were studied experimentally. Wind tunnel experiments were carried out for three Reynolds numbers and three angles of attack, involving hot-wire measurements and complementary surface flow visualization. The results establish that external acoustic excitation at a particular frequency and appropriate amplitude suppresses 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 characteristics of the vortical structures in the wake, decreasing the vortex length scale and coherency. Optimum excitation frequencies were found to correlate with the fundamental frequencies of the naturally amplified disturbances in the separated shear layer. The results suggest that acoustic waves play a dominant role in exciting the separated shear layer of the airfoil. Moreover, low-frequency structural vibrations are found to have a significant effect on airfoil performance, as they enhance the sound pressure levels within the test section.