Gas viscosity from first principles: A case study on nitrogen

Gas viscosity is a critical parameter in aerodynamics. However, the underlying mechanisms have not been fully uncovered, especially for the bulk viscosity. Viscosity of typical diatomic gas nitrogen (N2) was investigated from a microscopic perspective. An equilibrium molecular dynamics (EMD) method was programmed, where an essential high-precision potential energy surface (PES) was constructed by fitting justified density functional theory calculations. The achieved PES was effectively validated by comparing with the CCSD(T) potential. The research was conducted at a medium temperature range from 100 to 1100 K, which minimized the influence from vibrational energy generation in molecular collisions. Based on the first principles calculation and successively the EMD study, Green–Kubo formula was applied to compute the shear and bulk viscosities. Results from the established micro-to-macro transcale model were compared with various experimental and theoretical values from the literature and were essentially validated. The bulk viscosity turns out to demonstrate a novel nonlinear pseudo-exponential trend with temperature, relative to traditional methods with different approximations or errors. Nonetheless, a linearized fitting to the achieved shear viscosity shows another highlight, compared to the generally applied Sutherland's equation.