Direct molecular simulation of oxygen dissociation across normal shocks

We present direct molecular simulations (DMSs) of rovibrational excitation and dissociation of oxygen across normal shock waves. These are the first atomistic simulations of normal shock waves to rely exclusively on ab initio potential energy surfaces to describe the full collision dynamics (accounting for elastic, inelastic and reactive processes) in a dilute gas mixture of molecular and atomic oxygen. The simulated setup aims to reproduce two of the experimental conditions in the shock tube tests of Ibraguimova et al. (J Chem Phys 139:034317, 2013). We compare mixture composition and vibrational temperatures extracted from our simulations with those inferred from the shock tube tests and observe good agreement. In addition to this, we report macroscopic moments due to dissipative transport across the shock, i.e., species diffusion fluxes, viscous stresses and heat flux. Furthermore, we examine the distributions of vibrational and total internal energy of the O-2 molecules at several locations across the shock wave. We are able to follow the gradual transition from pre-shock to post-shock population distributions and, in both cases studied, find depletion of the high-energy tail of the internal energy distributions due to preferential dissociation from states close to the dissociation energy D-0. Finally, we extract molecular velocity distributions functions (VDF) of O-2 and O at selected locations across the shock to delimit the region where continuum breakdown occurs.