Acoustic streaming is a steady current 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. 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. The attenuation coefficient is, 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 ~10 cm at 1 MHz) than in water (~100 m at 1 MHz). In air it is known as the Quartz wind.
- near a boundary. Either when sound reaches a boundary, or when a boundary is vibrating in a still medium. 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 whose order of magnitude is a few micrometres in both air and water at 1 MHz.
Read more about Acoustic Streaming: Origin: A Body Force Due To Acoustic Absorption in The Fluid, Order of Magnitude of Acoustic Streaming Velocities
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