Acoustic streaming

Acoustic streaming is a steady flow in a fluid driven by the absorption of high amplitude acoustic oscillations. This phenomenon can be observed near sound emitters, or in the standing waves within a Kundt's tube. Acoustic streaming was explained first by Lord Rayleigh in 1884.^[1] It is the less-known opposite of sound generation by a flow.

There are two situations where sound is absorbed in its medium of propagation:

• during propagation in bulk flow ('Eckart streaming').^[2] The attenuation coefficient is ${\displaystyle \alpha =2\eta \omega ^{2}/(3\rho c^{3})}$, following Stokes' law (sound attenuation). This effect is more intense at elevated frequencies and is much greater in air (where attenuation occurs on a characteristic distance ${\displaystyle \alpha ^{-1}}$~10 cm at 1 MHz) than in water (${\displaystyle \alpha ^{-1}}$~100 m at 1 MHz). In air it is known as the Quartz wind.
• near a boundary ('Rayleigh streaming'). Either when sound reaches a boundary, or when a boundary is vibrating in a still medium.^[3] A wall vibrating parallel to itself generates a shear wave, of attenuated amplitude within the Stokes oscillating boundary layer. This effect is localised on an attenuation length of characteristic size ${\displaystyle \delta =[\eta /(\rho \omega )]^{1/2}}$ whose order of magnitude is a few micrometres in both air and water at 1 MHz. The streaming flow generated due to the interaction of sound waves and microbubbles, elastic polymers,^[4] and even biological cells^[5] are examples of boundary driven acoustic streaming.