UNDERWATER FLOW NOISE

Lighthill's theory of aerodynamic sound provides an effective way of i nvestigating underwater flow noise. When combined with a model of the coherent vortical structures in a turbulent boundary layer, it predict s the wave-number frequency pressure spectrum on a rigid surface and, in particular, highlights the role of surface viscous stresses as a so urce of low wave-number pressure fluctuations on a plane surface. The inclusion of surface curvature and flexibility enables the theory to b e applied to acoustic streamers (sometimes known as towed arrays). The effect of the interior mechanical structure of the streamers on the f low noise is investigated. Simple algebraic forms are derived for the comparative performance of liquid and visco-elastic-filled streamers. The introduction of porous foam into a liquid streamer is found to be a particularly effective way of attenuating low wave-number disturbanc es, and theoretical predictions are compared with experiment.