Tip vortex sound

Abstract

The aerodynamic sound generated by the convection of a tip vortex past a trailing edge is examined. For angles of attack greater than ± 5 deg, a vortex is generated that produces a high-frequency fluctuating pressure field several hundred hertz in bandwidth, and as much as 5 dB above the boundary layer wall pressure field, centered at roughly twice the frequency of vortex shedding due to Helmholtz wake instabilities. Sound is generated as the vortex-induced pressure field is scattered by the trailing edge. The increase in the acoustic source level, quantified by fluctuating boundary layer surface pressure statistics both near the trailing edge of the foil and under the tip vortex, corresponds in level and frequency to the increase in measured radiated noise. The wall pressure field due to tip vortex flow is superimposed on the boundary layer pressure field and apparently does not alter the wall pressure field at other frequencies. The directivity of the radiated tip vortex sound shows a reduction in the sound in the plane of the foil around the tip. This suggests that a quarter plane Green's function is required to describe the radiated noise. [Work supported by O.N.R.]