Comment II on J.M. Salazar and L. Brenig's article: ``Computer Simulations and Kinetic Theory of an Inelastic Granular Gas''

TL;DR

This paper critically examines Salazar and Brenig's claims against kinetic theory for granular gases, exposing derivation errors and questioning the validity of their simulation results at unphysical densities.

Contribution

It provides a detailed critique of the derivation errors and the physical relevance of the simulation results presented by Salazar and Brenig.

Findings

Identification of errors in the derivation of the hierarchy of equations.

Demonstration that the DSMC results are at unphysical densities.

Clarification that Haff's law holds beyond the timescales claimed by Salazar and Brenig.

Abstract

In two recent articles Salazar and Brenig question the validity of kinetic theory for granular gases and fluids, on the based of a supposedly exact hierarchy of coupled equations for the velocity moments, which the authors derive from the BBGKY-hierarchy. Their derivation contains several errors, which are exposed. Moreover, they support their findings with results from direct monte carlo simulation (DSMC) of the Boltzmann equation, which supposedly show that Haff's homogeneous cooling law only holds for times shorter than one mean free time. However, their DSMC results have no physical significance as they are carried out at a density, twice close packing.