Characteristic Vibrational and Rotational Relaxation Times for Air Species from First-Principles Calculations

We present molecular-scale computational rotational-vibrational relaxation studies for N-2, O-2, and NO. Characteristic relaxation times for diatom-diatom and diatom-atom interactions are calculated using direct molecular simulation (DMS), with ab initio potential energy surfaces (PESs) as the sole model input. Below approximately 8000 K our N-2-N-2, O-2-O-2, and O-2-N-2 vibrational relaxation times agree well with the Millikan-White (M&W) correlation, but gradually diverge at higher temperatures. Park's high-temperature correction produces a relatively steeper temperature rise compared to our estimates. DMS further shows that, with increasing temperature, the gap between vibrational and rotational relaxation times shrinks for all species. At T>30,000K their magnitudes become comparable and a clear distinction between both energy modes becomes meaningless. For other interactions, our DMS results differ substantially from the M&W correlation, both in magnitude and temperature dependence. Our predicted N-2-O-2 vibrational relaxation times are noticeably shorter due to vibration-vibration transfer. For O-2-O we observe minimal temperature dependence. Our O-2-N and N-2-N predictions follow the M&W temperature trend at values roughly one order of magnitude smaller. For NO-NO, N-2-O, NO-N, and NO-O we generate partial data due to currently incomplete PES sets. These first-principles-derived relaxation times are useful for informing relaxation models in gas-kinetic and fluid-dynamics simulations of high-enthalpy flows.