Surface roughness effects on flow noise

Abstract

The Corcos wall pressure model was modified to incorporate surface roughness effects. The longitudinal cross-spectral density part of the Corcos model was modeled from rough wall data taken from the literature. The wall pressure frequency spectrum was nondimensionalized by the wall shear stress and boundary layer displacement thickness and shown to fit both smooth and rough wall data. The boundary layer parameters were obtained from a finite-difference boundary layer code developed by the author to account for surface roughness effects by the discrete roughness approach [G. H. Christoph and R. H. Pletcher, AIAA J. 21, 509–515 (1983)]. The blockage of the roughness height, shape and spacing is explicitly accounted for in this approach. Flow noise was calculated for a variety of roughness element heights and spacings in air and in water. The calculations clearly show that element spacing as well as height is important and that the viscous sublayer height, not the boundary layer thickness, determines the influence of roughness on flow noise. Thus roughness is typically more important in water than in air because of the thinner viscous sublayer. Also, the effect of transducer size and viscoelastic layer thickness are shown for rough surfaces.