State-resolved coarse-grain cross sections for rovibrational excitation and dissociation of nitrogen based on ab initio data for the N2-N system

TL;DR

This paper introduces a method to generate compact, analytical state-resolved cross sections for nitrogen molecule collisions, significantly improving computational efficiency for high-temperature gas-dynamics simulations.

Contribution

A novel analytical inversion approach compresses detailed rovibrational data into a small parameter set, enabling efficient large-scale DSMC simulations of nitrogen gas dynamics.

Findings

Analytical inversion effectively reproduces detailed cross sections.

Method reduces memory and computational costs in simulations.

Enables more accurate modeling of nonequilibrium nitrogen dissociation.

Abstract

In this paper, we present a method to generate state-resolved reaction cross sections in analytical form for rovibrational energy excitation and dissociation of a molecular gas. The method is applied to an ab initio database for the N2-N system devel- oped at NASA Ames Research Center. The detailed information on N2 +N collisions contained in this database has been reduced by adapting a Uniform RoVibrational- Collisional bin model originally developed for rate coefficients. Using a 10-bin system as an example, a comparison is made between two sets of coarse-grain cross sections, obtained by analytical inversion and direct binning respectively. The analytical in- version approach is especially powerful, because it manages to compress the entire set of rovibrational-level-specific data from the Ames database into a much smaller set of numerical parameters, sufficient to reconstruct all coarse-grain cross sections for any particular N2 +N-collision pair. As a result of this approach, the computational cost in in large-scale Direct Simulation Monte Carlo (DSMC) flow simulations is sig- nificantly reduced, both in terms of memory requirements and execution time. The intended application is the simulation of high-temperature gas-dynamics phenomena in shock-heated flows via the DSMC method. Such conditions are typically encoun- tered in high-altitude, high-speed atmospheric entry, or in shock-tube experiments. Using this coarse-grain model together with ab initio rate data will enable more accu- rate modeling nonequilibrium phenomena, such as vibrationally-favored dissociation, an effect that is not well-captured by the conventional models prevalent in DSMC (i.e. Larsen-Borgnakke and Total Collision Energy).