Investigation of rotational relaxation in nitrogen via molecular dynamics simulation

The objective of this study is to use Molecular Dynamics simulation to evaluate the rotational relaxation process in nitrogen from a first-principles perspective. The intermolecular model used in the MD simulations is shown to (i) reproduce very well the shear viscosity of nitrogen over a wide range of temperatures, (ii) predict the near-equilibrium rotational collision number in good agreement with published trajectory calculations done on ab initio Potential Energy Surfaces, and (iii) produce shock wave profiles in excellent accordance with the experimental measurements. The rotational relaxation process is found to be dependent not only on the near-equilibrium temperature (i.e., when systems relax to equilibrium after a small perturbation), but more importantly on both the magnitude and direction of the initial deviation from the equilibrium state. Although this dependence has been previously recognized, it is here investigated systematically. The comparison between MD and DSMC, based on the Larsen-Borgnakke model, for shock waves (both at low and high temperatures) and one-dimensional expansions shows that a judicious choice of a constant Z_rot can produce DSMC results which are in relatively good agreement with MD. However, the selection of the rotational collision number is case-specific, depending not only on the temperature range, but more importantly on the type of flow (compression or expansion), with significant limitations for more complex simulations characterized both by expansion and compression zones. Parker's model, parametrized for nitrogen, overpredicts the magnitude of Z_rot for temperatures above about 300 K. It is also unable to describe the dependence of the relaxation process on the direction to equilibrium. Finally, based on the MD data, a preliminary formulation for a novel rotational relaxation model, which includes a dependence on both the rotational and the translational state of the gas, is presented." 2012 by P. Valentini, C. Zhang, and T. E. Schwartzentruber.