Packing, alignment and flow of shape-anisotropic grains in a 3D silo experiment

TL;DR

This study uses X-ray tomography to analyze how shape-anisotropic grains pack, align, and flow in a 3D silo, revealing orientation preferences, shear effects, and clogging mechanisms related to particle shape.

Contribution

It provides detailed 3D insights into particle orientations, packing densities, and clogging structures in a granular flow with shape-anisotropic particles, using advanced imaging techniques.

Findings

Particles show preferred orientation and higher order in flow zones.

Elongated grains exhibit more shear-induced dilation than spherical ones.

Clogging involves a dome-shaped layer whose shape depends on particle shape.

Abstract

Granular material flowing through bottlenecks like the openings of silos tend to clog and to inhibit further flow. We study this phenomenon in a three-dimensional hopper for spherical and shape-anisotropic particles by means of X-ray tomography. The X-ray tomograms provide information on the bulk of the granular filling, and allows to determine the particle positions and orientations inside the silo. In addition, it allows to calculate local packing densities in different parts of the container. We find that in the flowing zone of the silo particles show a preferred orientation and thereby a higher order. Similarly to simple shear flows, the average orientation of the particles is not parallel to the streamlines but encloses a certain angle with it. In most parts of the hopper, the angular distribution of the particles did not reach the one corresponding to stationary shear flow, thus the average orientation angle in the hopper deviates more from the streamlines than in stationary shear flows. In the flowing parts of the silo shear induced dilation is observed, which is more pronounced for elongated grains than for nearly spherical particles. The clogged state is characterized by a dome, i. e. the geometry of the layer of grains blocking the outflow. The shape of the dome depends on the particle shape.