Ultrasonic Streaming in incompressible fluids - modelling and measurements

Abstract

Acoustic streaming is a fundamental nonlinear phenomenon resulting from high frequency vibration in fluids. Known and investigated by Rayleigh and contemporary scientists, it attracts renewed interest due to the availability of powerful computational tools, advanced photography and precise laser velocimetry instrumentation, which can produce accurate experimental results. Its physical mechanism however is still not clearly understood. The analysis appears limited by the traditional premises of harmonic analysis, radiation force and wave propagation and reflection with the focus on nonlinear terms of the inertial frame formulations. Following our earlier analysis of nonlinear effects on rigid particles in a streaming fluid using time domain (TD) finite element (FE) analysis with a moving mesh via Comsol™, we present the modelling of ultrasonic streaming alone. We use state of the art laser velocimetry instrumentation to measure the average velocity of 0.5μm latex tracer particles in a 0.3-4 mms-1 streaming water insonified in the 1MHz frequency range. We use LabView™ virtual instrument to analyse light scattered by a swarm of particles in the moving fringes of crossed laser beams and find the ensemble particle motion from the frequency spectrum of the signal. In order to verify the FE modelling results with respect to the streaming velocity, the electric power is monitored at the transducer terminals. Our FE simulation, based on the Navier-Stokes (NS) equation for viscous incompressible fluids, does not involve wave propagation and radiation but is capable of representing the transient development of streaming, effects of boundaries and effects of the character of the ultrasonic source. Our investigation shows that streaming is neither implied by a time-varying topology nor associated with the asymmetry or even with the movement of the source or the fluid surface. Surprisingly, the streaming velocity is increased by making the enclosure fully symmetrical