The noise from flow over rough surfaces with small and large roughness elements.

Abstract

This paper will review theoretical and experimental results for the sound radiation from turbulent boundary layer flows over rough surfaces. Two distinctly different regimes will be considered. The first is hydrodynamically smooth surfaces for which the surface roughness does not impact the boundary layer turbulence. These surfaces have been shown experimentally and theoretically to radiate enhanced sound levels due to the scattering of the hydrodynamic pressure fluctuations by the roughness. The second regime considers large roughness elements that directly impact the boundary layer flow. Numerical calculations show that the flow separates around the large roughness elements and theoretically this implies that the roughness radiates sound in proportion to the unsteady force on each element. It is still an unresolved issue as to whether the unsteady loading is caused by the distortion of the turbulent flow around the element or the unsteady loading associated with the flow separation. This paper will describe the theoretical approaches which have been used to predict roughness noise levels in each of these two regimes and the transition from one regime to the other.