Confined packings of frictionless spheres and polyhedra

TL;DR

This study uses numerical simulations to examine how confinement affects the packing density and coordination number of frictionless spheres and polyhedra, revealing a linear decrease in solid fraction with confinement regardless of shape.

Contribution

It introduces a geometrical model that predicts the linear decrease of solid fraction and extends it to include the effect on coordination number for different grain shapes and polydispersity.

Findings

Solid fraction decreases linearly with confinement.

The decrease is consistent across sphere and polyhedral packings.

Polydispersity reduces the gradient of solid fraction decrease.

Abstract

By means of numerical simulations, we study the influence of confinement on three-dimensional random close packed (RCP) granular materials subject to gravity. The effects of grain shape (spherical or polyhedral) and polydispersity on this dependence are investigated. In agreement with a simple geometrical model, the solid fraction is found to decrease linearly for increasing confinement no matter the grain shape. This decrease remains valid for bidisperse sphere packings although the gradient seems to reduce significantly when the proportion of small particles reaches 40% by volume. The aforementioned model is extended to capture the effect of the confinement on the coordination number.