Effect of Acoustic Excitation Amplitude on Airfoil Boundary Layer and Wake Development

The effect of acoustic excitation amplitude on boundary layer and wake development for a NACA 0025 airfoil was experimentally at low Reynolds numbers. Flow characteristics were investigated with hot-wire anemometry, pressure measurements, and flow visualization. A laminar boundary layer separation occurs on the upper of the airfoil, forming a separated shear layer, for all situations examined. When the flow is excited at the matching the frequency of the most amplified disturbance in the separated shear layer, natural shear layer lock onto the excitation frequency and transition is promoted. In the case when the separated shear fails to reattach, an increase of the excitation amplitude above a minimum threshold eventually results in shear reattachment. The results suggest that an increase of the excitation amplitude not only advances the location of but also delays boundary layer separation, thereby reducing the extent of the separation region. As a , excitation results in narrowing of the wake and diminishment of the length scales and coherence of the wake structures. However, this effect is eventually limited, and a maximum effective excitation amplitude be identified. It is also shown that an increase of the excitation amplitude has a broadly similar effect on flow to an increase of the chord Reynolds number.