Bulk viscosity of dilute monatomic gases

TL;DR

This study uses molecular dynamics simulations to precisely estimate the bulk viscosity of dilute argon gas, revealing it is very small but non-zero, and examining its dependence on temperature and pressure.

Contribution

First detailed Green-Kubo calculations of bulk viscosity in dilute monatomic gases, providing insights into its magnitude and behavior under various conditions.

Findings

Bulk viscosity of dilute argon is around 10^{-10} Pa s.

Bulk viscosity varies quadratically with pressure at constant temperature.

At very low densities, bulk viscosity becomes temperature independent.

Abstract

Extensive research has been carried out in the past to estimate the bulk viscosity of dense monatomic fluids; however, little attention has been paid to estimate the same in the dilute gas regime. In this work, we perform precise Green-Kubo calculations in molecular dynamics simulations to estimate the bulk viscosity of dilute argon gas. The investigated temperature and pressure range is 300 to 750 K and 0.5 to 1.5 bar respectively. It is observed that the estimated bulk viscosity is $O(10^{-10})$ Pa s, which is several orders of magnitude smaller than that of other diatomic and polyatomic gases, but nonetheless, not an absolute zero as typically assumed. It implies that Stokes' hypothesis is true for dilute monatomic gases only in an approximate rather than absolute sense. The variation of bulk viscosity with pressures at constant temperatures has also been studied and is found to be of quadratic nature. The obtained bulk viscosity values are also reported in the reduced Lennard-Jones units to enable extension of the present results to other noble gases as well. It has also been observed that the bulk viscosity of Lennard-Jones monatomic gases becomes temperature independent at very low densities.