Aerofoil tones produced by a plate with cavity: the effect of acoustic forcing

A streamlined flat plate (herein ‘aerofoil’) containing a small rectangular cavity on one side was experimentally found to produce aerofoil tones (rather than cavity tones) under certain flow conditions. To clarify the responsible mechanism, the effect of sinusoidal acoustic forcing on the boundary layer was investigated by placing a loudspeaker downstream of the aerofoil. Velocity fluctuations in the boundary layer were then measured using a hot-wire probe. It was found that the boundary layer downstream of the cavity trailing edge responded strongly at the natural aerofoil tonal frequencies. This is due to enhanced feedback – as the naturally-occurring feedback loop is not saturated. However the shear layer over the cavity does not respond to the aerofoil’s tonal frequencies. The findings suggest that an aeroacoustic feedback loop exists between the aerofoil trailing edge and cavity trailing edge.