Flow noise reduction techniques for a planar array of hydrophones.

Abstract

This paper presents techniques for reducing turbulent boundary layer pressure fluctuations received by an array of hydrophones embedded within a plane layer of elastomer. Reduction of flow noise can be achieved by spatial filtering through the use of large finite hydrophones or an array of hydrophones, or by filtering high wave-number components through the use of an elastomer layer. The theoretical model used is a plane elastomer layer backed by an infinite elastic plate with finite thickness; the other side of the layer is exposed to turbulent flow. The baseline turbulent wall pressure spectrum that represents the flow excitation function is based on the Corcos model modified to fit a realistic low wave-number estimate. The occurrence of the convective ridge wave number is estimated by using a convective flow speed that is a function of frequency. Results presented are numerically calculated noise reductions, relative to the calculated noise level for a flush-mounted point hydrophone. Noise reductions that can be achieved by various techniques and their sensitivity to the flow noise model chosen, as well as the combined wave-number filtering mechanism, are presented. Some design approaches for achieving specific noise reductions using various combinations of hydrophone size, elastomer layer thickness, and array characteristics for flow over a plane surface will be discussed.