Acoustic streaming
When linear sound is considered, the particle vibration of the media con be considered as as pure harmonic (i. e. the period average velocity is zero). However, if we retain the nonlinearity of the medium and dissipation effects as in real acoustic media, the particles exhibit a period average velocity. This means when an acoustic beam propagates trough a medium, the medium responds with a continuous flow, also called acoustic streaming or "quartz wind" in case of air.
Acoustic streaming by focused sources
In the image below, a FDTD simulation result for a nonlinear ultrasound focused source, showing the mean flow created by the acoustic wave. In color, nonlinear pressure envelope field is plotted.
