Abstract
Particle size segregation is ubiquitous in granular systems with differently sized constituents but is found to diminish in the presence of viscous ambient fluids. We study this inhibiting effect through coupled fluid-particle numerical simulations. It is found that size segregation is indeed slower in the presence of fluid and this effect becomes more significant as fluid viscosity is increased. Direct calculation of segregation forcing terms reveal that the ambient fluids affect segregation in two major ways: buoyant forces reduce contact pressures, while viscous dissipation diminish particle-fluctuation driven kinetic pressures, both of which are necessary in driving large particles up. Surprisingly, the fluid drag in the normal direction is negligible regardless of the fluid viscosity and does not directly affect segregation.
Highlights
Granular flows consisting of differently sized particles have a tendency to segregate into homogeneous layers where large particles rise to the free-surface and small particles settle to the base
Through the analysis of segregation forcing terms derived from mixture theory and from stress partition arguments, we determine that fluid affects segregation in two major ways
Buoyant effects cannot account for the weakening of segregation with viscosity since the decrease in particle contact interactions is complemented by the decrease in the bulk particle weight
Summary
Granular flows consisting of differently sized particles have a tendency to segregate into homogeneous layers where large particles rise to the free-surface and small particles settle to the base. Real-world granular flows, are either fluid saturated [6] or entirely submerged [7], in which cases solid-fluid interactions become significant and particle segregation may be effectively inhibited. It is interesting to know how fluid forces such as fluid drag and buoyancy affect this process. Evaluation of these fluid effects will enable better prediction of grading patterns in granular-fluid mixtures
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