Granular fluid thermostatted by a bath of elastic hard spheres

TL;DR

This paper analyzes the steady state of a granular fluid immersed in an elastic bath using the Sonine approximation, revealing how temperature and distribution kurtosis depend on physical parameters and identifying a critical cooling rate beyond which no steady state exists.

Contribution

It provides analytical expressions for temperature ratio and kurtosis in a granular fluid immersed in a bath, validated against numerical solutions, and explores limiting cases including heavy particles and critical cooling rates.

Findings

Excellent agreement with numerical solutions.

Identification of a critical cooling rate for steady state existence.

Bath acts as white noise for much heavier particles.

Abstract

The homogeneous steady state of a fluid of inelastic hard spheres immersed in a bath of elastic hard spheres kept at equilibrium is analyzed by means of the first Sonine approximation to the (spatially homogeneous) Enskog--Boltzmann equation. The temperature of the granular fluid relative to the bath temperature and the kurtosis of the granular distribution function are obtained as functions of the coefficient of restitution, the mass ratio, and a dimensionless parameter $\beta$ measuring the cooling rate relative to the friction constant. Comparison with recent results obtained from an iterative numerical solution of the Enskog--Boltzmann equation [Biben et al., Physica A 310, 308 (202)] shows an excellent agreement. Several limiting cases are also considered. In particular, when the granular particles are much heavier than the bath particles (but have a comparable size and number density), it is shown that the bath acts as a white noise external driving. In the general case, the Sonine approximation predicts the lack of a steady state if the control parameter $\beta$ is larger than a certain critical value $\beta_c$ that depends on the coefficient of restitution and the mass ratio. However, this phenomenon appears outside the expected domain of applicability of the approximation.