Head-on collisions of dense granular jets

TL;DR

This study uses simulations to analyze head-on collisions of dense granular jets, revealing a steady-state flow and challenging traditional scattering principles by showing impact dynamics depend mainly on energy dissipation.

Contribution

It introduces a novel impact regime with a steady translating flow and demonstrates that impact speed depends solely on energy dissipation rate, regardless of material details.

Findings

Discovery of a quasi-steady impact state with a translating free surface.

Impact speed scales with energy dissipation rate to the power 1.5.

Challenges the principle of scattering based on ejecta analysis.

Abstract

When a dense stream of dry, non-cohesive grains hits a fixed target, a collimated sheet is ejected from the impact region, very similar to what happens for a stream of water. In this study, as a continuation of the investigation why such remarkably different incident fluids produce such similar ejecta, we use discrete particle simulations to collide two unequal-width granular jets head-on in two dimensions. In addition to the familiar coherent ejecta, we observe that the impact produces a far less familiar quasi-steady-state corresponding to a uniformly translating free surface and flow field. Upon repeating such impacts with multiple continuum fluid simulations, we show that this translational speed is controlled only by the total energy dissipation rate to the power $1.5$, and is independent of the details of the jet composition. Our findings, together with those from impacts against fixed targets, challenge the principle of scattering in which material composition is inferred from observing the ejecta produced during impact.