Experimental investigation of surface roughness generated flow noise

An experimental investigation has been undertaken to investigate the importance of surface roughness as a source of aerodynamically generated noise. The inside, initially smooth, surface of a circular pipe was roughened systematically by coating the pipe interior with uniformly distributed different sized sandpaper and wire mesh specimens. The far-field sound generated by the interaction between the pipe turbulent boundary-layer and the roughness was measured in a semi-anechoic room. There are three principal findings of this study. First, surface roughness generated flow noise is dipole in character. Second, measured power spectral density of the roughness generated sound energy has been correlated in terms of the wall shear stress friction velocity and the specimen equivalent sand paper roughness. Analysis of the data indicates that the equivalent sandpaper roughness height is much larger than the maximum size of the specimen. Third, for a given roughness, the efficiency of the production of sound increases as the boundary layer thickness decreases.