Effect of confinement on dense packings of rigid frictionless spheres and polyhedra

TL;DR

This study numerically investigates how confinement affects the solid fraction and structure of dense, random close-packed granular materials, considering factors like grain shape, polydispersity, and wall friction.

Contribution

It introduces a simple geometrical model to describe how confinement linearly reduces solid fraction and coordination number across different grain shapes and polydispersities.

Findings

Solid fraction decreases linearly with confinement.

Polyhedral and spherical packings show similar confinement effects.

Polydispersity influences the gradient of solid fraction reduction.

Abstract

We study numerically the influence of confinement on the solid fraction and on the structure of three-dimensional random close packed (RCP) granular materials subject to gravity. The effects of grain shape (spherical or polyhedral), material polydispersity and confining wall friction 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. Furthermore, 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 confinement effect on the coordination number is also captured by an extension of the aforementioned model.