Effect of acoustic excitation on the flow over a low-Re airfoil

Wind-tunnel measurements of lift, drag, and wake velocity spectra were carried out under (tonal) acoustic excitation for a smooth airfoil in the chord-Reynolds-number Re(c) range of 40,000-140,000. The data were supported by smoke-wire flow-visualization pictures. Small-amplitude excitation in a wide, low-frequency range is found to eliminate laminar separation that otherwise degrades the airfoil performance at low Re(c) near the design angle of attack. Excitation at high frequencies eliminates a prestall, periodic shedding of large-scale vortices. Significant improvement in lift is also achieved during poststall, but with large-amplitude excitation. Wind-tunnel resonances strongly influence the results, especially in cases requiring large amplitudes.