Direct molecular simulation of dissociating oxygen under adiabatic and normal shock wave conditions

We present direct molecular simulations (DMS) of rovibrational excitation and dissociation of oxygen in a constant-volume, adiabatic reactor and across a normal shock wave. This setup aims to reproduce the nonequilibrium conditions in the shock tube experiments of Ibraguimova et al. (JChemPhys 2013). We examine internal energy and vibrational population distributions of the O₂ molecules at several stages of the process and observe overpopulated and depleted populations in the high-energy tail at different stages of dissociation. In the adiabatic simulations we observe how the characteristic length scales for vibrational relaxation and dissociation are affected by lowering the total enthalpy of the upstream gas. We then compare vibrational temperatures from our simulations with those inferred from the shock tube tests and obtain reasonable agreement. Finally, we compare the adiabatic reactor DMS calculations with the equivalent normal shock simulations. Our assessment is that, while the adiabatic reactor does a reasonable job of reproducing the thermo-chemical state of the shocked gas, it actually generates more severe initial nonequilibrium conditions than what is observed in the normal shock.