Bulk Viscosity of Dilute Gases using Nonequilibrium Molecular Dynamics Simulations

Recent studies have reported that bulk viscosity, μ_b, may play an important role in several fluid mechanical phenomena including fluid instabilities, turbulence, and hypersonic flows. However, accurate estimation of μ_b is a challenging task and relies on indirect techniques like acoustic spectroscopy, Rayleigh-Brillouin scattering, and Green-Kubo method. These techniques have several limitations. In the present work, a new method is proposed for the estimation of the bulk viscosity of dilute gases using nonequilibrium molecular dynamics simulations. In this method, the fluid is expanded or compressed with a known expansion/compression rate (▽.v), where v is the velocity of the fluid. During this volume change process, mechanical (p_mech) and thermodynamic pressures (p_thermo) are estimated using instantaneous translational and total kinetic energy, respectively. The μ_b is then obtained by the relation: μ_b = (p_thermo-p_mech) / ▽.v. The proposed method is applied to estimate the μ_b of dilute nitrogen gas. The results are compared with available experimental data and a good agreement is observed. Effects of temperature, pressure, rate and direction (i.e., expansion or compression) of volume change, and humidity on μ_b are reported. (Sharma, B., & Kumar, R., Phys. Rev. E 100, 013309)