Effect of viscosity on the avalanche dynamics and flow transition of wet granular matter

The dynamic behaviour of granular flows is important in geo-mechanics and industrial applications, yet poorly understood. We studied the effects of liquid viscosity and particle size on the dynamics of wet granular material flowing in a slowly rotating drum, in order to detect the transition from the avalanching to the continuous flow regime. A discrete element method (DEM) model, in which contact forces and cohesive forces were considered, was employed to simulate this flow behaviour. The model was validated experimentally, using glass beads in a wooden drum and water–glycerol mixtures to tune the liquid viscosity. The DEM simulations showed comparable results to the experiments in terms of average slope angle and avalanche amplitude. We observed that the avalanche amplitude, flow layer velocity and granular temperature decrease as the liquid viscosity increases. This effect is more pronounced for smaller sized particles. The increase in viscous forces causes the flowing particles to behave as a bulk, pushing the free surface towards a convex shape. In addition, avalanches become less pronounced and the granular flow transitions from the avalanching regime to the continuous regime. The avalanching flow regime is marked by intermittent rigid body movement of the particulate bed and near-zero drops in the granular temperature, while no rigid body movement of the bed occurs in the continuous flow regime. We identified the avalanching-continuous flow transition region as a function of a dimensionless granular Galileo number.