Acoustic streaming model for an intense sound beam in free space

Abstract

Steady acoustic streaming in a barotropic liquid medium (water), excited by a plane circular emitter at frequencies in the mega-hertz range, is examined for large hydrodynamic Reynolds numbers. A region of the flow smaller than the diffraction length but including the zone of jump formation and nonlinear damping of the sawtooth finite-amplitude wave is investigated. Experiments show that under these conditions the direct flow hardly goes beyond the limits of the beam. Inflow into the main stream takes place at every point on the lateral surface of the beam [7], with the possible exception of the region near the emitter where the acoustic field is substantially nonuniform [8]. This region is not considered. It is also assumed that the acoustic streaming does not affect the parameters of the sound wave within the beam. The main effect of the constant flow on the acoustic field is the increase in the speed of sound in the moving medium [9]. For water it is less than 0.1%, i.e., negligibly small.