Radiation from turbulence near a composite flexible boundary

The effect of surface inhomogeneities on the sound radiated by nearby turbulence is examined in a particular case. The surface considered consists of two semi-infinite flexible planes which have different elastic and inertia constants, and a turbulent boundary layer is formed on this composite boundary. Lighthill’s wave equation is converted into a pair of singular integral equations for this problem, and the equations are solved to obtain the farfield radiation. The solution shows how the propagating components of the turbulent pressure field are reflected and diffracted by the surface, but these effects do not basically augment the radiation. It is also shown, however, that the discontinuity in surface properties acts as a ‘wavenumber converter’, scattering the turbulence near-field into propagating sound. The scattered acoustic power varies as U4 when fluid loading on the surface is negligible (aeronautical applications), and as U6 when fluid loading is appreciable (underwater applications), U being a typical flow velocity. These results are interpreted in terms of the radiation from a homogeneous surface under the action of a point force, and show that scattering by surface inhomogeneities should be a very important feature of the noise fields found in practice.