Clustering and Velocity Distributions in Granular Gases Cooling by Solid Friction

TL;DR

This study uses large-scale molecular dynamics simulations to analyze how granular gases cool via solid friction, revealing clustering, velocity field ordering, and a transition from Maxwell-Boltzmann to non-MB velocity distributions over time.

Contribution

The paper demonstrates the evolution of clustering and velocity distributions in granular gases cooling by solid friction, highlighting the power-law growth of cluster size and the transition in velocity distribution behavior.

Findings

Cluster size grows as t^{1/3}

Velocity distribution transitions from MB to non-MB

Density remains homogeneous during early evolution

Abstract

We present large-scale molecular dynamics simulations to study the free evolution of granular gases. Initially, the density of particles is homogeneous and the velocity follows a Maxwell-Boltzmann (MB) distribution. The system cools down due to solid friction between the granular particles. The density remains homogeneous, and the velocity distribution remains MB at early times, while the kinetic energy of the system decays with time. However, fluctuations in the density and velocity fields grow, and the system evolves via formation of clusters in the density field and the local ordering of velocity field, consistent with the onset of plug flow. This is accompanied by a transition of the velocity distribution function from MB to non-MB behaviour. We used equal-time correlation functions and structure factors of the density and velocity fields to study the morphology of clustering. From the correlation functions, we obtain the cluster size, $L$, as a function of time, $t$. We show that it exhibits power law growth with $L(t)\sim t^{1/3}$