On the Absorption of Sound by Turbulence and Other Hydrodynamic Flows

Abstract

The dissipation of acoustic energy which occurs when sound generates vorticity at the boundaries of a fluid or propagates across a field of turbulence is examined. Vorticity generation occurs typically during scattering or diffraction by surfaces with corners or edges, and an understanding of the consequent attenuation is important in the design of mufflers and other devices used for suppressing acoustic and mechanical vibrations. The rate of dissipation is generally a non-linear function of the acoustic intensity, but becomes linear and significantly greater in the presence of a mean flow. When sound propagates through turbulence it is partly scattered and partly absorbed. Lighthill's (1953) theory of scattering is extended to include the possibility of acoustic absorption by a direct transfer of energy to the turbulent motions. A similar but greatly enhanced level of absorption takes place when sound propagates in turbulent pipe flow. The energy transfer occurs principally in the acoustic momentum and thermal boundary layers at the walls, whose properties are subject to the modifying influences of turbulence convection. This is discussed in relation to experimental data of Ronneberger & Ahrens (1977).