Characterization of focused-ultrasound-induced acoustic streaming

This study aims at characterizing acoustic streaming produced by High Intensity Focused Ultrasound (HIFU) propagating in a liquid medium. Particle Image Velocimetry (PIV) technique was used to determine velocity fields in an infinite medium of water subjected to a focused ultrasonic field. The tests were carried out at a transducer resonance frequency of 550 kHz for different applied acoustic pressure at the focus (ranging from 2.6 to 18.4 bar). The experimental results allowed characterizing the mean streaming flow and its evolution with the wave amplitude: In particular, it is shown that a region where viscous effects are negligible, which leads to a linear behavior of the axial velocity with respect to the excitation pressure amplitude, does exist but is limited to the region upstream of the focal area of the transducer. A scaling analysis provides an estimate of the maximum velocity, and shows that it obeys a 4/3 power law with respect to the excitation pressure amplitude, as long as the position of this maximum velocity corresponds to the focal point, which is not the case for highest pressure amplitude, for which it is shifted downstream of the focus. In addition, the streaming flow is compared to a classical free jet flow. Similarities and differences existing between both flows are reviewed and a self-similar zone for the velocity transverse profile is identified in the expansion zone of the flow.