Acoustic streaming and temperature elevation in a high viscous fluid by irradiation of ultrasound beams

Abstract

Sound energy losses due to viscosity and heat conduction in a fluid cause the medium movement as acoustic streaming and cause temperature elevation in beams. In most fluids, the buoyancy force by heat expansion is generally weaker than the driving force of the streaming, so the temperature field is basically determined by the heat transfer equation with convective terms. As some physical parameters such as viscosity depend generally on temperature, the sound field is changed with the irradiation time of cw beams. In addition to the viscosity, two other parameters of sound absorption and sound speed are taken account of their temperature dependency in the present theory. The hydrodynamic flow, forced convection heat transfer, and wave equations are simultaneously solved for axisymmetric ultrasound beams by a finite difference scheme. Numerical examples of the streaming velocity and temperature rise are given. They demonstrate that sound pressure amplitude is intrinsically changed with time and sound self-action is generated. The present method of numerical calculation is easily extended to the problems of focusing and multi-layered beam systems.