- Published on 30 June 2015
A recent EPJ E Colloquium by Jose Manuel Valverde looks at the fundamental physics that causes convection and fluidization in vibrated beds of particles with large inertia. The author examines the question of whether acoustic streaming arising from oscillatory viscous flows might play a role on the onset of convection and fluidization in vertically vibrated granular beds.
Acoustic streaming, first observed by Faraday in 1831, is an enigmatic phenomenon that has puzzled physicists for a long time. It occurs when a viscous fluid oscillates in the presence of a solid boundary. The dissipation of energy by viscous friction leads to a secondary steady circulation of fluid in a boundary layer near the surface of the solid, which enhances the gas-solid viscous interaction. Granular beds display, at sufficiently high vibrational intensities, surface patterns that bear a stunning resemblance to the surface ripples (Faraday waves) observed for low viscosity liquids. This suggests that the granular bed transits to a liquid-like regime, despite the large inertia of the particles.
The estimations in this Colloquium show that, thanks to acoustic streaming, the fluidization of beds of relatively large particles could be enhanced by oscillatory flows at not too large Reynolds numbers, giving rise to the observed liquid-like behavior. A similar mechanism could also be relevant to understanding geological events, such as fluidization of landslides and soil liquefaction by earthquakes and sound waves.
Jose Manuel Valverde (2015),
Colloquium: Convection and fluidization in oscillatory granular flows. The role of acoustic streaming,
European Physical Journal E, DOI: 10.1140/epje/i2015-15066-7