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Attenuation of self-excited roll oscillations of low-aspect-ratio wings using acoustic excitation was studied in a wind tunnel. For a rectangular flat plate wing with an aspect ratio of 2, roll oscillations can be completely suppressed and the onset of the roll oscillations can be delayed with external acoustic excitation. Similar results were also obtained for wings with two different airfoil profiles. Velocity measurements indicated that acoustic excitation could restore a symmetric vortex flow over the free-to-roll wings thus eliminating the selfexcited roll oscillations. The effect of excitation is most noticeable for the side of the wing that sees a larger effective angle of attack due to the rolling motion. Acoustic excitation energizes the shear layer instabilities and results in reattachment or smaller separated flow region closer to wing surface, thus in turn suppressing the roll oscillations.

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A passive flow control method, which uses bleed from a slot near the wing tip, has been shown to attenuate self-excited roll oscillations of a low aspect ratio (AR = 2) rectangular flat-plate wing. This method was found to be successful across the whole range of angle of attack, and better than previous active flow control methods. The effectiveness of the slot strongly depends on its location and width. For effective slot geometries, the tip vortex becomes less coherent, almost eliminating the roll oscillations. Nonlinear interactions between the shear layers shed from the tip and the slot, as well as between the shear layer and the counter-rotating vortex may act as excitation, which can modify the response of the self-sustained oscillator. When the slot is located too close to the tip, there is rapid merging of the shear layers and less interaction, and the slot loses its effectiveness. Also, when the slot is narrow, there is insufficient bleed, resulting in less effective attenuation. Force measurements revealed that this technique can be used as an effective method to suppress roll oscillations without sacrificing and possibly improving aerodynamic performance.