Localized instability of a granular layer submitted to an ascending liquid flow

International audienceUsing a very simple experimental setup, we study the response of a thin layer of immersed granular material to an ascending liquid-flow; a pressure difference Delta P is imposed between the two horizontal free surfaces of a thin layer of glass beads, such that the liquid tends to flow upwards, and the resulting flow-rate v is measured. As generally observed in fluidized beds, the layer destabilizes when the pressure force exactly compensates the weight of the grains. At the free surface, one then observes the formation of a localized fountain of granular material the characteristic size of which is found to be proportional to the grain size and, surprizingly, independent of both the flow-rate and the thickness of the granular layer. Simple theoretical arguments account for the main experimental features