Sound absorption properties of porous metals under grazing flow conditions

Abstract

Porous metals could potentially be used for acoustic liners that can operate in adverse conditions such as high temperature and high pressure. Recent developments in synthetic techniques for porous metals have made it possible to produce them for specific applications, which are intended to provide increased sound absorption. However, if a porous metal is employed in the presence of grazing flow, it is difficult to predict its sound absorption properties. In this study, the acoustic properties of two well-known types of stainless-steel porous materials with different porosities, that is, sintered fiber and cellular foam, were investigated using impedance tube and flow duct experiments. The impedance in flow duct experiments in the absence of flow is almost the same as that in impedance tube experiments, except for the cellular foam sample with high porosity, which may not be a locally reacting material. Furthermore, the effects of grazing flow on the impedance of the porous sample can be separated into those due to the average flow velocity and those due to the flow velocity gradient in the wall-normal direction. Both effects depend on the pore structure and porosity of the porous samples.