We present a detailed comparison of two high-fidelity approaches for simulating non-equilibrium chemical processes in gases: The state-to-state master equation (StS-ME) and the direct molecular simulation (DMS) methods. The former is a deterministic method, which relies on the pre-computed kinetic database for the N2-N system based on the NASA Ames ab initio potential energy surface (PES) to describe the evolution of the molecules' internal energy states through a system of master equations. The latter is a stochastic interpretation of molecular dynamics relying exclusively on the same ab initio PES. It directly tracks the microscopic gas state through a particle ensemble undergoing a sequence of collisions. We study a mixture of nitrogen molecules and atoms forced into strong thermochemical non-equilibrium by sudden exposure of rovibrationally cold gas to a high-temperature heat bath. We observe excellent agreement between the DMS and StS-ME predictions for the transfer rates of translational into rotational and vibrational energy, as well as of dissociation rates across a wide range of temperatures. Both methods agree down to the microscopic scale, where they predict the same non-Boltzmann population distributions during quasi-steady-state dissociation. Beyond establishing the equivalence of both methods, this cross-validation helped in reinterpreting the NASA Ames kinetic database and resolve discrepancies observed in prior studies. The close agreement found between the StS-ME and DMS methods, whose sole model inputs are the PESs, lends confidence to their use as benchmark tools for studying high-temperature air chemistry.
Bibliographical noteFunding Information:
Dr. M. Panesi was supported by the Air Force Office of Scientific Research (AFOSR) under grant no. FA9550-18-1-0388. Dr. R. L. Macdonald was supported by the President’s Postdoctoral Fellowship Program from the University of Minnesota. Dr. T. Schwartzentruber and Dr. E. Torres were supported by the AFOSR under grant nos. FA9550-19-1-0219 and FA9550-17-1-0250. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the AFOSR or the US government. We would like to thank Dr. R. L. Jaffe and Dr. D. W. Schwenke from NASA Ames Research Center for access to the PES and kinetic database used in this work. a
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