The effect of finite container size on granular jet formation

TL;DR

This study investigates how the size of the container influences the formation and characteristics of granular jets when an object impacts a bed of sand, revealing boundary effects on jet behavior and proposing a new explanation for jet structure.

Contribution

The paper provides experimental insights into boundary effects on granular jet formation and introduces a new explanation for the jet's thick-thin structure based on impact parameters.

Findings

Container size significantly affects jet height and shape.

Boundary confinement alters void collapse dynamics.

Proposes a new model for jet structure based on impact conditions.

Abstract

When an object is dropped into a bed of fine, loosely packed sand, a surprisingly energetic jet shoots out of the bed. In this work we study the effect that boundaries have on the granular jet formation. We did this by (i) decreasing the depth of the sand bed and (ii) reducing the container diameter to only a few ball diameters. These confinements change the behavior of the ball inside the bed, the void collapse, and the resulting jet height and shape. We map the parameter space of impact with Froude number, ambient pressure, and container dimensions as parameters. From these results we propose a new explanation for the thick-thin structure of the jet reported by several groups [J.R.Royer \textit{et al}., Nature Phys. \textbf{1}, 164 (2005)], [G.Caballero \textit{et al}., Phys. Rev. Lett. \textbf{99}, 018001 (2007)] and [J.O. Marston, \textit{et al}., Physics of Fluids \textbf{20}, 023301 (2008)].