Viscous Effects on the Particle Size Segregation in Geophysical Mass Flows: Insights From Immersed Granular Shear Flow Simulations

Size segregation is a common feature in geophysical mass flow deposits and is an active process during motion. The presence of interstitial fluids in such flows affect the motion of constituent particles and result in complex segregation behaviors. Effects of the viscosity and density of various interstitial fluids on the rate of particle size segregation are investigated through coupled granular-fluid simulations of immersed plane-sheared flows. The segregation rate decreases as the fluid viscosity increases, but remains constant when the viscosity is below certain threshold values. In the low viscosity limit, segregation is affected only by the relative density between the particles and the fluid, and by flow inertial conditions. Analysis of segregation forcing terms based on the mixture theory of segregation reveals that the decrease of segregation rates with the viscosity is due to the increase of fluid drag forces, which effectively weaken the contact stress gradients responsible for driving the large particles to migrate upward. Viscous damping also diminishes velocity fluctuations and thereby inhibits the migration of particles throughout the mixture. The transition in the viscosity dependence is shown to correspond to the transition between granular-fluid flow regimes. An empirical scaling formula is developed which accounts for the effects of fluid viscosity and the relative density on size segregation immersed in different fluids.