Bulk viscosity of dilute monatomic gases revisited

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. Here, the term ‘dilute’ is used in reference to kinetic theory of gases and refers to low density conditions where average inter-molecular spacing is much greater than the size of the molecules. In this work, we have performed molecular dynamics simulations and used the Green–Kubo method to make precise estimations for the bulk viscosity of argon gas at atmospheric conditions. 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.