On the diffraction of sound by a screen with circular apertures in the presence of a low Mach number grazing flow

Abstract
Acoustic energy is dissipated when sound generates vorticity at a sharp edge. This paper re-examines the classical problem of the diffraction of sound by a thin screen with circular apertures when account is taken of vorticity production at the aperture rims. A uniform grazing flow is assumed to exist on both sides of the screen which, by convecting away the shed vorticity, greatly enhances the level of attenuation. Perforated screens have been used to suppress structural resonances which are coupled with the aerodynamic sound produced by unsteady vortex shedding from the tube-banks in a cross-flow heat exchanger, and the theory of this paper goes some way to predicting the attenuation available from a screen under prescribed mean flow conditions. In particular, one can predict how the relative magnitudes of the open-area ratio of the screen and the grazing flow velocity should be chosen in order to achieve optimal attenuation.

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