Shear induced alignment and dynamics of elongated granular particles

TL;DR

This study investigates how elongated granular particles align and rotate under shear flow, revealing that they develop a shear alignment angle independent of shear rate, which decreases with particle aspect ratio, and that particle rotation speeds vary based on orientation.

Contribution

The paper combines experiments and simulations to analyze shear-induced alignment and rotation of elongated particles, highlighting the independence of alignment angle from shear rate and its dependence on aspect ratio.

Findings

Particles develop a shear alignment angle that is independent of shear rate.

The shear alignment angle decreases as particle aspect ratio increases.

Particles exhibit different rotation speeds depending on their orientation relative to the shear flow.

Abstract

The alignment, ordering and rotation of elongated granular particles was studied in shear flow. The time evolution of the orientation of a large number of particles was monitored in laboratory experiments by particle tracking using optical imaging and x-ray computed tomography. The experiments were complemented by discrete element simulations. The particles develop an orientational order. In the steady state the time and ensemble averaged direction of the main axis of the particles encloses a small angle with the streamlines. This shear alignment angle is independent of the applied shear rate, and it decreases with increasing grain aspect ratio. At the grain level the steady state is characterized by a net rotation of the particles, as dictated by the shear flow. The distribution of particle rotational velocities was measured both in the steady state and also during the initial transients. The average rotation speed of particles with their long axis perpendicular to the shear alignment angle is larger, while shear aligned particles rotate slower. The ratio of this fast/slow rotation increases with particle aspect ratio. During the initial transient starting from an unaligned initial condition, particles having an orientation just beyond the shear alignment angle rotate opposite to the direction dictated by the shear flow.