The Relationship Between Roughness Noise and the Near- Field Pressure Spectrum

Abstract

Extensive measurements have been made of the roughness noise and wall pressure fluctuations produced by a wall jet boundary layer. Short sandpaper roughness fetches, with grit sizes from 220 to 20 were immersed in a 15 mm thick boundary layer and studied as a function of flow speed. A three to one range of boundary layer edge velocities produced a wide range of flows from hydrodynamically smooth through the low end of the fully rough regime. Roughness noise levels are typically 60 to 70 dB below the level of wall pressure fluctuations, and the spectra have quite different forms. Nevertheless, for all the rough surfaces and conditions tested, the roughness noise spectrum Φ(ω) varies closely as the product of the single-point wall pressure spectrum Gpp(ω), the frequency ω squared, and the mean-square roughness height hrms 2 . The only exception to this conclusion occurs for the largest roughnesses (40 grit or greater) at high frequencies, where the ratio of the roughness noise to wall pressure spectral values falls below the ω 2 curve.