Acoustic Phenomena in Relativistic Shear Flows

Abstract

Acoustic perturbations in a parallel relativistic flow of aninviscid fluid are considered. The general expression for thefrequency of the sound waves in a uniformly (with zero shear)moving medium is derived. It is shown that the perturbations cease tobe purely irrotational as they are in the nonrelativistic case. For anon-uniformly moving fluid it is shown that therecently found (1994 Phys. Rev. E 50 4283) velocity shearinduced acoustical effects (the excitation of vortical, transientlygrowing perturbations, and the excitation of sound-type perturbations,extracting the kinetic energy of the background flow) aresignificantly intensified by the relativistc character of the motion.The possible relevance of this remarkable circumstance to processes inastrophysical relativistic shear flows is pointed out.