Acoustic streaming generated by sharp edges: the coupled influences of liquid viscosity and acoustic frequency

TL;DR

This study investigates how liquid viscosity and acoustic frequency affect sharp-edge acoustic streaming, revealing that higher viscosity weakens flow velocity while frequency influences the maximum streaming velocity, with empirical laws provided.

Contribution

The paper provides new empirical scaling laws quantifying the effects of viscosity and frequency on acoustic streaming generated by sharp edges.

Findings

Streaming velocity increases with acoustic velocity squared.

Higher viscosity significantly weakens streaming velocity.

Flow pattern remains similar despite changes in viscosity.

Abstract

Acoustic streaming can be generated around sharp structures, even when the acoustic wavelength is much larger than the vessel size. This sharp-edge streaming can be relatively intense, \textcolor{blue}{owing to the strongly focused inertial effect experienced by the acoustic flow near the tip.} We conducted experiments with Particle Image Velocimetry to quantify this streaming flow through the influence of liquid viscosity $\nu$, from 1 mm$^2$/s to 30 mm$^2$/s, and acoustic frequency $f$ from 500 Hz to 3500 Hz. Both quantities supposedly influence the thickness of the viscous boundary layer $\delta = \left(\frac{\nu}{\pi f}\right)^{1/2}$. For all situations, the streaming flow appears as a main central jet from the tip, generating two lateral vortices beside the tip and outside the boundary layer. As a characteristic streaming velocity, the maximal velocity is located at a distance of $\delta$ from the tip, and it increases as the square of the acoustic velocity. We then provide empirical scaling laws to quantify the influence of $\nu$ and $f$ on the streaming velocity. Globally, the streaming velocity is dramatically weakened by a higher viscosity, whereas the flow pattern and the disturbance distance remain similar regardless of viscosity. Besides viscosity, the frequency also strongly influences the maximal streaming velocity.