Particle Size Segregation In Granular Mass Flows With Different Ambient Fluids

Particle size segregation is ubiquitous in granular flows composed of differently sized particles where large particles rise towards the free surface while the small particles settle to the base.Otherwise known as inverse grading,this phenomena is a prominent feature in debris flows deposits and also contributes to its overall dynamics.Size segregation has been widely studied in dry granular flows where the interstitial fluid is negligible and does not affect the segregation process.Debris flows on the other hand are fluid-laden wherein the interaction between the solid and the fluid phases are significant and may therefore give rise to complex segregation behaviors depending on the mixture constituents' material properties.This thesis is dedicated to investigate the particle size segregation which occurs in granular-fluid mixture flows,particularly in its micro-mechanical origin and the effects of its interplay with different ambient fluids on the local rheology of bidisperse granular flows.To this end extensive discretecontinuum,two-phase numerical simulations are conducted using the coupled CFDDEM.Simulations of gravity-driven bidisperse granular flows down rough inclines fullyimmersed in different ambient fluids reveal that fluids affect segregation in two major ways.Buoyancy reduces particle contact forces necessary to push large particles toward the surface.Fluid drag forces,normal to the base are negligible and do not directly hinder the large particles' upward rise even when the viscosity is high.Instead,fluid drag acting along the streamwise direction,opposite to the granular flow,induces the formation of inhomogeneous flow profiles,such as plug flow regions,wherein local shear rates are significantly diminished.This reduction in local shear in turn results to reduced segregation.The rheology of granular-fluid flows across a wide range of the ambient fluid viscosities(i.e.,from air to water and slurry)can be defined using a visco-inertial constitutive model which expresses the dependence of the granular motion on the additive effects of grain inertia and viscous dissipation.This visco-inertial model is generalized for size-bidisperse mixtures by taking into account the mixture diameter and the improved packing resulting from having two particle sizes.It is also found that the local effective friction and volume fraction of mixtures with different particle sizes can be approximated from the rheology of single-component flows.The effective bulk friction is independent of the state and progress of vertical segregation.Through the use of a plane shear flow geometry,where the shear rates and confining pressures are controlled combined with a feedback scheme for the streamwise velocity,the inherent effects of fluid viscosity and density,independent of the effects of inhomogeneous flow rheology,is evaluated in detail.Viscous effects on segregation can be categorized into two regimes separated by transition boundaries.At low viscosities segregation does not depend on viscosity and remains constant unless the relative density of the particles and fluid,or the inertial conditions are varied.When the flows are highly viscous,segregation decreases linearly with viscosity.As the density of the fluid approaches that of the particles,segregation decreases due to buoyancy.Viscous effects are primarily manifested through the reduction of particle kinetic motion – the viscous dampening of random particle motion inhibits their ability to percolate and migrate within the mixture.The said fluids effects are summarized into an empirical scaling relationship which predicts the segregation velocity in the presence of different types of fluids.