Effect of packing fraction on the jamming of granular flow through small apertures

TL;DR

This study examines how the preparation method of granular materials affects their flow and jamming behavior through small openings, revealing that packing fraction alone does not predict jamming, and arch structures play a complex role.

Contribution

It demonstrates that packing fraction is not a reliable predictor of jamming and explores the relationship between bulk and jamming arches in granular flow.

Findings

Packing fraction does not correlate well with avalanche size.

Arch size and orientation influence jamming more than packing fraction.

A lower cutoff on arch span is imposed by the aperture size.

Abstract

We investigate the flow and jamming through small apertures of a column of granular disks via a pseudo-dynamic model. We focus on the effect that the preparation of the granular assembly has on the size of the avalanches obtained. Ensembles of packings with different mean packing fractions are created by tapping the system at different intensities. Surprisingly, packing fraction is not a good indicator of the ability of the deposit to jam a given orifice. Different mean avalanche sizes are obtained for deposits with the same mean packing fraction that were prepared with very different tap intensities. It has been speculated that the number and size of arches in the bulk of the granular column should be correlated with the ability of the system to jam a small opening. We show that this correlation, if exists, is rather poor. A comparison between bulk arches and jamming arches (i.e., arches that block the opening) reveals that the aperture imposes a lower cut-off on the horizontal span of the arches which is greater than the actual size of the opening. This is related to the fact that blocking arches have to have the appropriate orientation to fit the gap between two piles of grains resting at each side of the aperture.