Vortex sound generation in the presence of porous materials with an application to dissipative silencers and lined ducts

Abstract

The main objective of this thesis is to study in details the vortex sound generation under the influence of porous materials. The effects of vortex strengths, separation distance and initial position are examined. The effects of effective fluid density and flow resistance inside the lattice of the porous material on sound generation are also explored. Acoustic analogy is employed in the present study in order to derive the flow potential, and the matched asymptotic expansion method is used to evaluate the far-field sound pressure. The present study is relevant to the problem of self-generated noise as the major function of the porous material is to attenuate the noise inside the ductwork system, but additional noise can be generated in the presence of the porous material at the same time. Vortex sound generation under the influence of a porous half cylinder mounted on an otherwise rigid plane, a porous wedge, a piece-wise porous material on an otherwise rigid plane and a lined duct are investigated. In general, the far-field sound pressure is higher when the effective fluid density or the flow resistance is small. A smaller separation of the vortex from the porous material also increases the far-field sound pressure. The acoustical energy radiated can be higher than that in the rigid surface case when the flow resistance is very small, the separation distance of the vortices is large or the difference of the vortex strengths is large. The far-field sound pressure increases as the length or the thickness of the porous material increases. The far-field sound pressure does not decrease monotonically with increasing flow resistance when the length of the porous material increases due to the substantially large rate of change of the vortex velocity.