External acoustic excitation applied at particular frequencies and suitable amplitudes can substantially reduce or suppress the separation region associated with an airfoil at low Reynolds numbers, so that an increase in lift and a decrease in drag result. The effect of the excitation strongly depends on the excitation frequency and amplitude. In particular, the effective frequency range decreases with a decrease of the excitation amplitude. For a constant amplitude excitation, this range narrows with a decrease of the Reynolds number or increase of the angle of attack. The acoustic excitation also alters wake structure, decreasing the vortex length scale and the coherency of the vortices.
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