  We propose to couple a state-resolved rovibrational coarse-grain model to a
stochastic particle method for simulating internal energy excitation and
dissociation of a molecular gas. An existing coarse-grain model based on the
NASA Ames ab initio database for the N2-N system is modified using
variably-spaced energy bins. Thermodynamic properties of the new coarse-grained
model closely match those of the full set of rovibrational levels over a wide
temperature range, using a number of bins much smaller than the complete
mechanism. The chemical-kinetic behavior of the original equally -- and new
variably -- spaced bin formulations is compared by simulating excitation and
dissociation of N2 in an adiabatic, isochoric reactor. The variably-spaced
formulation is better suited for reproducing the dynamics of the full database
at conditions of interest in Earth reentry. Furthermore, we discuss details of
our Direct Simulation Monte Carlo (DSMC) implementation for the coarse-grain
model and describe changes to the collision algorithm necessary to accommodate
our state-resolved reaction mechanism. The DSMC code is then verified against
equivalent master equation (ME) calculations. In these simulations,
state-resolved cross sections are used in analytical form. They verify
micro-reversibility for the bins and allow for faster execution of the code. In
our verification, we obtain very close agreement for the N and N2
concentrations, as well as the translational and rovibrational mode
temperatures obtained independently using both methods. In addition to
macroscopic moments, we compare internal energy populations predicted at
selected time steps via DSMC and ME. We observe good agreement between both
methods within the statistical scatter limits imposed by DSMC. In future work,
the rovibrational coarse-grain model coupled to the particle method will allow
us to study 3D reentry flow configurations.
