The Influence of Surface Compliance on the Production of Sound by a Turbulent Boundary Layer

Abstract

The theory of aerodynamic sound in the form developed by Ffowcs-Williams and Hawkings [15] is applied to investigate the production of sound by turbulent boundary layer flow over a thin, flexible plate. Conventional theories of bounday layer noise attribute the radiation to the boundary layer quadrupoles and their (passive) images in the plate, and neglect the interaction of turbulence with the finite amplitude motion of the plate caused by the wall pressure fluctuations. This interaction generates sound whose intensity is characteristic of aerodynamic sources of dipole type. In (underwater) situations involving the high fluid loading of steel plates, it is shown that the intensity of the dipole noise may be comparable to that produced by the quadrupoles. The dipoles dominate the radiation from more compliant surfaces, of the type commonly used in experiments on compliant wall drag reduction, and it is suggested that the injudicious deployment of compliant wall coatings may result in the production of unacceptably high levels of aerodynamic noise.