Coupling of state-resolved rovibrational coarse-grain model for nitrogen to stochastic particle method for simulating internal energy excitation and dissociation

Abstract

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. A coarse-grained model for a rovibrational reaction mechanism of an ab initio database developed at NASA Ames Research Center for the N 2 -N system is modified based on variably-spaced energy bins. Thermodynamic properties of the modified coarse-grained model allow us to closely match those obtained with the full set of rovibrational levels over a wide temperature range, while using a number of bins significantly smaller than the complete mechanism. The chemical-kinetic behavior of equally-and variably-spaced bin formulations is compared by simulating internal energy excitation and dissociation of nitrogen in an adiabatic, isochoric reactor. We find that the variablyspaced formulation is better suited for reproducing the dynamics of the full database at conditions of interest in Earth atmospheric entry. Also in this paper, we discuss details of our particle method implementation for the URVC bin model and describe changes to the Direct Simulation Monte Carlo (DSMC) collision algorithm, which become necessary to accommodate our state-resolved reaction mechanism for excitation and dissociation reactions. The DSMC code is then verified against equivalent master equation calculations. In these simulations, state-resolved cross sections are used in analytical form. These cross sections verify micro-reversibility relations for the rovibrational bins and allow for fast execution of the DSMC code. In our verification calculations, we obtain very close agreement for the concentrations profiles of N and N 2 , as well as the translational and rovibrational mode temperatures obtained independently through both methods. In addition to macroscopic moments, we compare discrete internal energy populations predicted at selected time steps via DSMC and the master equations. We observe good agreement between the two sets of results within the limits imposed by statistical scatter, which is inherent to particle-based DSMC solutions. As future work, the rovibrational coarse-grain model coupled to the particle method will allow us to study 3D reentry flow configurations.

Dynamics on Multiple Potential Energy Surfaces: Quantitative Studies of Elementary Processes Relevant to Hypersonics

“…One possibility consists of developing more coarse-grained models either by averaging over rotational energies, or by using energy-binning strategies, to reduce the number of simulated transitions. However, it has been found that depending on the way in which this coarse-graining is carried out, the internal energy distributions, relaxations, and dissociation rates can be markedly different. , As an alternative, the direct molecular simulation (DMS) method has been developed. , …”

“…However, it has been found that depending on the way in which this coarse-graining is carried out, the internal energy distributions, relaxations, and dissociation rates can be markedly different. 170,171 As an alternative, the direct molecular simulation (DMS) method has been developed. 172,173 One recently explored possibility that can be applied to reactive and nonreactive state-to-state cross sections is to train a machine learning model based on neural networks (NN) from explicit QCT data for state-to-state cross sections from which all necessary information can be determined.…”

State-to-State Master Equation and Direct Molecular Simulation Study of Energy Transfer and Dissociation for the N2–N System

“…This includes vibrational StS-ME studies of the O 2 –O , and O 2 –O 2 systems, combined oxygen systems, and N 2 –N, − N 2 –N 2 , , N–O 2 , and N 2 –O 2 systems. In addition, efforts have been made to construct reduced-order models based on ab initio PESs using a coarse-grain method on internal energy states. ,,− …”

Dynamics on Multiple Potential Energy Surfaces: Quantitative Studies of Elementary Processes Relevant to Hypersonics

“…However, it has been found that depending on the way how this coarse-graining is carried out, the internal energy distributions, relaxations and dissociation rates can be markedly different. 158,159 As an alternative, the direct molecular simulation (DMS) method has been developed. 160,161 One recently explored possibility is to train a machine learned model based on neural networks from explicit QCT data for state-to-state cross sections from which all necessary information can be determined.…”