Maninder S. Grover

Dissociation and Internal Excitation of Molecular Nitrogen Due to N + N2 Collisions Using Direct Molecular Simulation

populated as the system evolves. It is found that the non-equilibrium dissociation rate coe cients for the N + N2 process are larger than those for the N2 + N2 process. This is attributed to the non-equilibrium vibrational energy distributions for the N + N2 process being less depleted than that for the N2 +N2 process. For an isothermal simulation we find that the probability of dissociation goes as 1/Ttr for molecules with internal energy (✏int) less than ⇠ 9.9eV , while for molecules with ✏int > 9.9eV the dissociation probability was weakly dependent on translational temperature of the system. We compared non-equilibrium

Construction of a coarse-grain quasi-classical trajectory method. II. Comparison against the direct molecular simulation method

This work presents the analysis of non-equilibrium energy transfer and dissociation of nitrogen molecules (N2(Σg+1)) using two different approaches: the direct molecular simulation (DMS) method and the coarse-grain quasi-classical trajectory (CG-QCT) method. The two methods are used to study thermochemical relaxation in a zero-dimensional isochoric and isothermal reactor in which the nitrogen molecules are heated to several thousand degrees Kelvin, forcing the system into strong non-equilibrium. The analysis considers thermochemical relaxation for temperatures ranging from 10 000 to 25 000 K. Both methods make use of the same potential energy surface for the N2(Σg+1)-N2(Σg+1) system taken from the NASA Ames quantum chemistry database. Within the CG-QCT method, the rovibrational energy levels of the electronic ground state of the nitrogen molecule are lumped into a reduced number of bins. Two different grouping strategies are used: the more conventional vibrational-based grouping, widely used in the literature, and energy-based grouping. The analysis of both the internal state populations and concentration profiles show excellent agreement between the energy-based grouping and the DMS solutions. During the energy transfer process, discrepancies arise between the energy-based grouping and DMS solution due to the increased importance of mode separation for low energy states. By contrast, the vibrational grouping, traditionally considered state-of-the-art, captures well the behavior of the energy relaxation but fails to consistently predict the dissociation process. The deficiency of the vibrational grouping model is due to the assumption of strict mode separation and equilibrium of rotational energy states. These assumptions result in errors predicting the energy contribution to dissociation from the rotational and vibrational modes, with rotational energy actually contributing 30%-40% of the energy required to dissociate a molecule. This work confirms the findings discussed in Paper I [R. L. Macdonald et al., J. Chem. Phys. 148, 054309 (2018)], which underlines the importance of rotational energy to the dissociation process, and demonstrates that an accurate non-equilibrium chemistry model must accurately predict the deviation of rovibrational distribution from equilibrium.

Comparison of quantum mechanical and empirical potential energy surfaces and computed rate coefficients for N2 dissociation

Introduction Physics-based modeling of hypersonic flows is predicated on the availability of chemical reaction rate coefficients and cross sections for the collisional processes. This approach has been built around the use of quantum mechanical calculations to describe the interaction between the colliding particles. In this approach a potential energy surface (PES) is computed by solving the electronic Schrödinger equation and collision cross sections are determined for that PES using classical, semiclassical or quantum mechanical scattering methods. The rate coefficients are computed by integrating the thermally weighted cross sections. State-to-state rate coefficients are determined by only integrating over a thermal distribution of collisional energies. Finally, thermal rate

Coupled rotational-vibrational excitation in shock waves using trajectory-based direct simulation Monte Carlo

This paper describes the implementation of Classical Trajectory Calculation Direct Simulation Monte Carlo (CTC-DSMC) for one dimensional shock waves in molecular nitrogen including rotational-vibrational excitation. It is demonstrated that CTC-DSMC and Molecular Dynamics simulations agree exactly for the translational, rotational, and vibrational temperature profiles within a one dimensional shock. By comparing various shocks with the harmonic oscillator and anharmonic oscillator potentials it is found that ro-vibrational coupling increases with the degree of anharmonicity. For relevant shock wave conditions for high-speed, high-altitude flight, the overshoot in rotational temperature behind the shocks is decreased, while vibrational excitation rate is increased. This reduction in rotational temperature overshoot and increase in vibrational relaxation rate were found to become more pronounced as the post shock temperature increases. Additionally, CTCDSMC simulations of reflected shock waves are verified with simulations of standing shock waves.