Analysis of outer flow effects on wall pressure spectra through underwater velocity and pressure measurements.

One of the most important mechanism of self-noise generation is the turbulent flow inducing wall pressure fluctuations. This phenomenon can highly reduce the performance of passive sonar sensor arrays. The flow field about marine vehicles can be rather complex depending on the surface geometry and on the presence of the water free-surface. In view of a better understanding of some peculiarities of wall pressure spectra in non-equilibrium boundary layers, typical of naval surface and underwater applications, velocity and wall pressure measurements have been performed on a large submerged body in a towing tank. With this setup, free surface and floor effects are negligible, an almost ideal flow, free of background turbulence, is guaranteed and quite high Reynolds numbers (Re? values up to 24000) can be attained. The spatial characterization of the velocity and pressure fields have been performed using PIV (Particle Image Velocimetry) and an array of pressure sensors, respectively. Outer flow effects on the low-mid frequency range of pressure spectra and on the other pressure statistical features are discussed and appropriate scaling parameters for its characterization are suggested. Available towing tank and literature data relative to different geometries are used for further validation of the proposed scaling.