Mechanisms of size segregation in granular flows with different ambient fluids

¹ Key Laboratory of Mountain Hazards and Earth Surface Process / Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610000, China
² University of Chinese Academy of Sciences, Beijing 100000, China
³ China-Pakistan Joint Research Center on Earth Sciences, CAS-HEC
⁴ Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60208, USA

^* Corresponding author: gordon@imde.ac.cn

Published online: 7 June 2021

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.