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

Abstract

We present molecular-scale computational rotational-vibrational relaxation studies for [Formula: see text], [Formula: see text], and [Formula: see text]. 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 [Formula: see text], [Formula: see text], and [Formula: see text] 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 [Formula: see text] 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 [Formula: see text] vibrational relaxation times are noticeably shorter due to vibration-vibration transfer. For [Formula: see text] we observe minimal temperature dependence. Our [Formula: see text] and [Formula: see text] predictions follow the M&W temperature trend at values roughly one order of magnitude smaller. For [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] 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.