LAUSR

The stability of the interface between a low-viscosity fluid and a granular medium in a horizontal rotating cylinder is experimentally studied. Two cases are studied, namely, (i) plastic particles in a more dense fluid and (ii) plastic particles in a less dense fluid. We consider a moderate rotation when particles can form an axisymmetric core in the cylinder center or an annular layer near the cylindrical wall under the action of centrifugal force. The buoyancy force acting on the particles of the granular core fluidizes the granular bed and induces the rotation of the suspended particles with a velocity different from that of the rotating fluid. In the same way, the gravitational force tends to fluidize the surface of an annular layer of granular material in experiments with a fluid with a density less than that of grains. The observations revealed that the suspended particles arrange themselves into regular ripples at the interface. In the present study, the stability threshold and the size of regular ripples are studied in dependence on the rotation rate, the relative density of two media, and the volume of granular material. The instability is found to be of the same nature as the Kelvin–Helmholtz instability observed in classical fluids but with characteristics that can differ due to the specificity of the granular rheology.