A unified description of gravity- and kinematics-induced segregation forces in dense granular flows

TL;DR

This paper presents a unified scaling law for particle segregation forces in dense granular flows, combining gravity and shear effects, validated across multiple flow configurations using simulations.

Contribution

It introduces a universal scaling relation for segregation forces that accounts for both gravity and shear rate effects, applicable across diverse flow setups.

Findings

The scaling law includes gravity and shear rate gradient terms.

Validation across various flow types confirms the law's universality.

Predictions of segregation direction match experimental and simulation data.

Abstract

Particle segregation is common in natural and industrial processes involving flowing granular materials. Complex, and seemingly contradictory, segregation phenomena have been observed for different boundary conditions and forcing. Using discrete element method simulations, we show that segregation of a single particle intruder can be described in a unified manner across different flow configurations. A scaling relation for the net segregation force is obtained by measuring forces on an intruder particle in controlled-velocity flows where gravity and flow kinematics are varied independently. The scaling law consists of two additive terms: a buoyancy-like gravity-induced pressure gradient term and a shear rate gradient term, both of which depend on the particle size ratio. The shear rate gradient term reflects a kinematics-driven mechanism whereby larger (smaller) intruders are pushed toward higher (lower) shear rate regions. The scaling is validated, without refitting, in wall-driven flows, inclined wall-driven flows, vertical silo flows, and free surface flows down inclines. Comparing the segregation force to the intruder weight results in predictions of the segregation direction that match experimental and computational results for various flow configurations.