Predictions of Sound from Rough Wall Boundary Layers

Abstract

The Unified Theory of Roughness Noise is examined for the first time by making predictions of far-field noise normalized on the single-point rough wall pressure spectrum for a 40 grit stochastic sandpaper roughness and a 1-mm hemispherical element deterministic roughness with conditions ranging from transitionally to fully rough. These predictions are compared to experimentally measured values of noise from these surfaces in a wall-jet flow. Predicted values are within approximately 6 dB of the measured noise from the stochastic and deterministic roughness showing that reliable predictions of roughness noise can be completed using the Unified Theory for even fully rough conditions, as long as the roughness height to boundary-layer thicknesses are relatively moderate. The spectral shape of normalized roughness noise was found to be a predictable function of the relationship between the roughness geometry and the wall pressure wavenumber-frequency spectrum. The effect of the form of the wall pressure wavenumber-frequency spectrum on the predicted noise spectra was investigated finding that the convective velocity is the most significant parameter affecting the quality of predictions.