Simulating rotationally inelastic collisions using a Direct Simulation Monte Carlo method

TL;DR

This paper introduces a DSMC-based simulation method for rotational cooling in gas mixtures, utilizing ab initio cross sections to accurately model rotational state changes during collisions, validated against experimental data.

Contribution

It presents a novel DSMC approach incorporating ab initio rotational cross sections to simulate rotational cooling in gas mixtures, extending the applicability of DSMC methods.

Findings

Simulation results agree with experimental measurements of rotational temperature.

The method accurately predicts on-axis density, temperature, and velocity profiles.

Applicable to various gas mixture dynamics under non-uniform conditions.

Abstract

A new approach to simulating rotational cooling using a direct simulation Monte Carlo (DSMC) method is described and applied to the rotational cooling of ammonia seeded into a helium supersonic jet. The method makes use of ab initio rotational state changing cross sections calculated as a function of collision energy. Each particle in the DSMC simulations is labelled with a vector of rotational populations that evolves with time. Transfer of energy into translation is calculated from the mean energy transfer for this population at the specified collision energy. The simulations are compared with a continuum model for the on-axis density, temperature and velocity; rotational temperature as a function of distance from the nozzle is in accord with expectations from experimental measurements. The method could be applied to other types of gas mixture dynamics under non-uniform conditions, such as buffer gas cooling of NH$_3$ by He.