Vibrational relaxation of diatomic molecules in rarefied gas flows

The direct simulation Monte Carlo (DSMC) method is widely used for simulations of rarefied gas flows. To make a vibrational relaxation model of diatomic molecules for the DSMC method, the collisions of diatomic molecules are investigated numerically. The collision cross section for vibrational relaxation can be described as a function of the relative translational energy, the rotational energy and the vibrational energy level of each colliding molecule. Thus the collision cross section could be evaluated with the Monte Carlo integration for these parameters. To achieve this idea, a lot of collisions with appropriate initial conditions should be calculated and analyzed statistically. A collision is simulated with the Semiclassical approach in which the vibrational energy is treated quantum mechanically and the rotational and the relative translational energies are treated classically. For the comparison of methods, the Quasiclassical approach is applied. The simulation by this method is the same as that by the classical method, and the obtained vibrational energy is discretized to the quantum level. The intermolecular potential is also compared between two different types that are the site-to-site Lennard-Jones and the Billing-Fisher potential.