ACOUSTIC STREAMING - COMPARISON OF LOW-AMPLITUDE LINEAR-MODEL WITH STREAMING VELOCITIES MEASURED BY 32-MHZ DOPPLER

The pressure gradient along the ultrasonic beam results in medium stre aming, Following Nyborg's analysis of the Navier-Stokes equation, Wu a nd Du developed an approximate solution for the streaming velocity gen erated by flat and weakly focused transducers. We have modified their solution of the Poisson equation by directly deriving the Dirichlet bo undary conditions to be applied for this type of equation, Our numeric al results (for the linear case) were about one half smaller for hat a nd weakly focused Gaussian beam transducers compared to the results by Wu and Du, The theoretical calculations were verified using a purpose -designed 32-MHz pulsed Doppler unit, The applied average acoustic pow er was changed from 1 mu W to 6 mW, the burst width was 0.5 mu s and t he pulse repetition frequency was 32 kHz. The experiments were done on il-mm-diameter hat and focused (focal distance = 8 and 12 mm) transdu cers. The streaming was measured along the ultrasonic beam from 0-20 m m; at all positions, the maximum Doppler frequency was estimated from the recorded spectra, Streaming was induced in a solution of water and corn starch, The experimental results showed that, for a given acoust ic power, the streaming velocity was independent of the starch density in water changed from 0.3-40 g of starch in 1 l of distilled mater, F or applied acoustic powers, the streaming velocity changed linearly fr om 0,2-40 mm/s, Both the theoretical solutions for plane and focused w aves and the experimental results were in good agreement, (C) 1997 Wor ld Federation for Ultrasound in Medicine & Biology.