Effect of interstitial fluid on a granular flowing layer

Abstract

A dominant aspect of granular flows is flow in thin surface layers. While an understanding of the dynamics of dry granular surface flow has begun to emerge, the case of flow when air is completely replaced by a liquid is largely unexplored. Experiments were performed using particle tracking velocimetry (PTV) in a quasi-two-dimensional rotating tumbler to measure the velocity field within the flowing layer of monodisperse spherical particles fully submerged in liquids, a granular slurry, for a range of Froude numbers, bead sizes, fluid densities and fluid viscosities. The thickness of the flowing layer and the angle of repose with a liquid interstitial fluid are generally larger than for the dry system under similar conditions, although the shear rate is generally smaller. The experimental measurements of shear rate match the theoretical predictions (dependent on the particle size, dynamic angle of repose, and static angle of repose) independent of the interstitial fluid. Furthermore, the velocity profiles for larger beads collapse independent of the interstitial fluid, while for smaller beads these profiles collapse on two distinct curves when using a scaling based on mass balance. However, a normalization based on the velocity of beads at the surface causes a collapse to a nearly linear velocity profile except where the velocity approaches zero logarithmically near the fixed bed, regardless of interstitial fluid. Likewise, the scaled number density profiles collapse, regardless of the interstitial fluid. The similarity in the flows of dry granular materials and granular materials submerged in liquids indicates that the physics of the flow is not strongly altered by the interstitial fluid.