Thermal collapse of a granular gas under gravity

TL;DR

This paper investigates how gravity influences the cooling behavior of granular gases, revealing that gravity causes the gas to undergo thermal collapse and condense at the bottom in finite time, contrasting with homogeneous cooling.

Contribution

It introduces a combined approach of simulations, hydrodynamic modeling, and theory to demonstrate gravity-induced thermal collapse in granular gases.

Findings

Granular gas cools to zero temperature under gravity.

The gas condenses at the container's bottom in finite time.

Gravity induces inhomogeneity leading to thermal collapse.

Abstract

Free cooling of a gas of inelastically colliding hard spheres represents a central paradigm of kinetic theory of granular gases. At zero gravity the temperature of a freely cooling homogeneous granular gas follows a power law in time. How does gravity, which brings inhomogeneity, affect the cooling? We combine molecular dynamics simulations, a numerical solution of hydrodynamic equations and an analytic theory to show that a granular gas cooling under gravity undergoes thermal collapse: it cools down to zero temperature and condenses on the bottom of the container in a finite time.