Time evolution of acoustic streaming from a planar ultrasound source

Eckart-type acoustic streaming induced in confined sound beams from a piston source is examined in water theoretically and experimentally. Axisymmetric flow equations with a spatially distributed driving force in the beams are based on the continuity equation and the Navier–Stokes equation in a viscous, incompressible fluid. They are solved numerically by the stream-function vorticity method [T. Kamakura et al., J. Acoust. Soc. Am. 97, 2740–2746 (1995)]. Experiments are conducted using a 5-MHz planar transducer with a 9.5-mm radius aperture. All measurements of the streaming velocities are carried out by a laser Doppler velocimeter and are compared with the numerical computations including the enhancement of the force due to finite-amplitude sound distortion. These measurements agree well with the theoretical prediction. It is noted that diffraction of sound beams plays an important role in the generation of streaming, particularly in the early stage. Consistency between experiments and computations suggests that both acoustic and hydrodynamic nonlinearities should be taken into account in the present observation system.