  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.
