Experimental Investigations of Sound From Flow Over Rough Surfaces

Turbulent boundary layer flows over rough surfaces are known to produce elevated far-field acoustic sound levels. The nature by which surface irregularities alter the near-field surface pressures and subsequently affect the sound generation to the scattering of high wavenumber convective pressures to low wavenumber acoustic pressures, which is typically interpreted as a dipole-like source. The focus of the current investigation is the experimental interrogation of both near- and far-field pressures due to the flow over roughened surfaces in order to identify the source mechanisms and to validate physical models of roughness sound. For rough surfaces composed of large geometrical elements (defined by large Reynolds numbers based on roughness height and friction velocity), such as hemispheres and cubes, the measured near-field surfaces pressures indicate that the local interstitial flows become important in determining the sound radiation characteristics. In order to describe the aeroacoustic source region, scaling laws are developed for surface pressures at locations around the roughness elements for various roughness configurations and flow speeds. Relationships between surface pressures amongst the rough surface elements and far-field pressures measured at several directional aspects were examined to identify roughness sound source mechanisms. Measurements of a dipole directivity pattern and dipole efficiency factors obtained when normalizing radiated sound by surface pressures offer support to the scattering theories for roughness sound. Using existing pressure scattering models as a basis, an empirical model for roughness sound is generated.