Lift Force on a Moving Intruder in Granular Shear Flow

TL;DR

This study uses DEM simulations to measure the lift force on a spherical intruder in granular shear flow, revealing a non-monotonic behavior opposite to fluid Saffman lift, with implications for understanding granular flow dynamics.

Contribution

The paper provides the first detailed characterization of lift forces on intruders in granular shear flow across a range of slip velocities, highlighting differences from fluid flows.

Findings

Lift force acts opposite to fluid Saffman lift at low slip velocities.

Lift force exhibits a maximum and then reverses direction as slip velocity increases.

Flow field around the intruder is similar in granular and fluid cases, but shear stress differs significantly.

Abstract

Lift and drag forces on moving intruders in granular materials are of fundamental interest. While the drag force on an intruder in granular flow has been studied, the few studies characterizing the lift force explore a relatively limited range of parameters. Here we use discrete element method (DEM) simulations to measure the lift force, $F_\mathrm{L}$, on a spherical intruder in a uniformly sheared bed of smaller spheres for a range of intruder slip velocities, $u_\mathrm{s}$, relative to the unperturbed flow. In what at first appears as a puzzling result, $F_\mathrm{L}$ in granular shear flow acts in the opposite direction to the Saffman lift force on a sphere in a sheared fluid at low $u_\mathrm{s}$, reaches a maximum value, and then decreases, eventually reversing direction and becoming comparable to $F_\mathrm{L}$ for a fluid. This non-monotonic response holds over a range of flow conditions, and the $F_\mathrm{L}$ versus $u_\mathrm{s}$ data can be collapsed by scaling both quantities using the particle sizes, shear rate, and overburden pressure. Analogous fluid simulations demonstrate that the flow field around the intruder particle is similar in the granular and fluid cases. However, the shear stress acting on the intruder in a granular shear flow is much less than that in a fluid shear flow. This difference, combined with a void region behind the intruder in granular flow, which alters the pressure and shear stress on the trailing side of the intruder, significantly changes the lift-force inducing stresses acting on the intruder between the granular and fluid cases.