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

Abstract

We present molecular-scale computational rotational-vibrational relaxation studies for N2, O2, 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 N2−N2, O2−O2, and O2−N2 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 N2−O2 vibrational relaxation times are noticeably shorter due to vibration-vibration transfer. For O2−O we observe minimal temperature dependence. Our O2−N and N2−N predictions follow the M&W temperature trend at values roughly one order of magnitude smaller. For NO−NO, N2−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.