Clustering instability in a freely falling granular jet

TL;DR

This study examines how granular jets of glass spheres break into clusters during free fall, analyzing the influence of air pressure and flow fluctuations on the clustering process.

Contribution

It provides experimental insights into clustering instability in granular jets and clarifies the role of air and initial fluctuations in cluster formation.

Findings

Clustering occurs even at very low air pressures.

Air presence results in larger clusters but does not cause clustering.

Drop size is determined by initial particle fluctuations at the nozzle.

Abstract

This paper investigates a clustering instability of a freely falling granular jet composed of 100 micron glass spheres. The granular flow out of a circular nozzle starts out spatially uniform and then, further downstream, breaks up into well defined clusters. The role of air is investigated in this phenomenon by changing the ambient air pressure down to 1/5000th atm. An optical method is used that measures inhomogeneities in the flow in order to quantify the growth of the clusters. Clustering is observed down to the lowest pressure and the presence of air leads to larger drops but does not initiate the drop formation. The analysis shows that the drop size is set by fluctuations on the order of the size of the particles at the nozzle.