Sound transmission from a source outside a nonisothermal boundary layer

Abstract

The transmission of sound from a source outside a nonisothermal high-speed boundary layer is considered. The sound source is assumed to lie in a uniform stream and matched to a zero velocity at the wall by a linear velocity profile. The unidirectional shear mean flow is assumed to be isentropic but nonhomentropic, so that the entropy, the sound speed, and the temperature can vary from one streamline to the other. The condition of homoenergetic flow, or constant enthalpy, is used to relate the sound speed to the mean flowvelocity and specify the temperature profile in the boundary layer. Compared with a homentropic boundary layer, for which sound refraction is due to the shear flow alone, a nonhomentropic boundary layer introduces additional refraction due to the nonuniform sound speed and the associated temperature gradients. It is shown that for a high speed, even in isentropic conditions, the nonhomentropic effects of temperature gradients and the nonuniform sound speed can cause significant sound attenuation (viz., for the same sound source outside the boundary layer, the acoustic pressure at the wall can be substantially reduced). This agrees qualitatively with the results from testing propfans at high subsonic speeds, which showed significant sound attenuation in the fuselage boundary layer. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc.