摘要 :
Quasiclassical trajectory analysis of oxygen dissociation is presented with conditions sampled from thermal equilibrium and nonequilibrium. Ground state O_2 + O_2 and O_2 + O interactions both occur on several degeneracies, and so...
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Quasiclassical trajectory analysis of oxygen dissociation is presented with conditions sampled from thermal equilibrium and nonequilibrium. Ground state O_2 + O_2 and O_2 + O interactions both occur on several degeneracies, and so a total of 13 potential energy surfaces are used in the investigation. Spin and spatial degeneracy is found to have a moderate effect on the dissociation rate, and a strong effect on vibrational relaxation mechanisms in O_2 + O. For a given thermal environment, the oxygen dissociation rate is found to be similar for all collision partners. The vibrational energy decrease due to dissociation, a necessary input to CFD, depends on the degree of thermal nonequilibrium, and a curve collapse with previous data for nitrogen dissociation is demonstrated. Finally, the effect of each reactive state on dissociation is quantified for both nitrogen and oxygen dissociation. The effect of collision partner's internal energy on simple dissociation is probably negligible, and vibrational energy of the dissociating molecule has the strongest effect. These rigorous statistical analyses enable the development of physics-based models for CFD.
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摘要 :
Quasiclassical trajectory analysis of oxygen dissociation is presented with conditions sampled from thermal equilibrium and nonequilibrium. Ground state O_2 + O_2 and O_2 + O interactions both occur on several degeneracies, and so...
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Quasiclassical trajectory analysis of oxygen dissociation is presented with conditions sampled from thermal equilibrium and nonequilibrium. Ground state O_2 + O_2 and O_2 + O interactions both occur on several degeneracies, and so a total of 13 potential energy surfaces are used in the investigation. Spin and spatial degeneracy is found to have a moderate effect on the dissociation rate, and a strong effect on vibrational relaxation mechanisms in O_2 + O. For a given thermal environment, the oxygen dissociation rate is found to be similar for all collision partners. The vibrational energy decrease due to dissociation, a necessary input to CFD, depends on the degree of thermal nonequilibrium, and a curve collapse with previous data for nitrogen dissociation is demonstrated. Finally, the effect of each reactive state on dissociation is quantified for both nitrogen and oxygen dissociation. The effect of collision partner's internal energy on simple dissociation is probably negligible, and vibrational energy of the dissociating molecule has the strongest effect. These rigorous statistical analyses enable the development of physics-based models for CFD.
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Existing chemical kinetics models in state-of-the-art CFD result in predictions that are not fully consistent with direct molecular simulations (DMS) based on ab-initio data. Progress toward the development and implementation of a...
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Existing chemical kinetics models in state-of-the-art CFD result in predictions that are not fully consistent with direct molecular simulations (DMS) based on ab-initio data. Progress toward the development and implementation of a chemical kinetics model based on a microscopic description of the gas behavior is presented. A consistent comparison between DMS and CFD is desired, so electronic energy is excluded from the CFD calculations. The model for change in vibrational energy due to dissociation is found to be important, especially for isothermal simulations. Several other individual factors are also examined. Incorporating rates derived from Boltzmann distributions results in an overprediction of the DMS dissociation rate. Capturing the depletion of vibrationally-excited molecules is found to be necessary.
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摘要 :
Existing chemical kinetics models in state-of-the-art CFD result in predictions that are not fully consistent with direct molecular simulations (DMS) based on ab-initio data. Progress toward the development and implementation of a...
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Existing chemical kinetics models in state-of-the-art CFD result in predictions that are not fully consistent with direct molecular simulations (DMS) based on ab-initio data. Progress toward the development and implementation of a chemical kinetics model based on a microscopic description of the gas behavior is presented. A consistent comparison between DMS and CFD is desired, so electronic energy is excluded from the CFD calculations. The model for change in vibrational energy due to dissociation is found to be important, especially for isothermal simulations. Several other individual factors are also examined. Incorporating rates derived from Boltzmann distributions results in an overprediction of the DMS dissociation rate. Capturing the depletion of vibrationally-excited molecules is found to be necessary.
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In this paper we study the vibrational energy transfer process and dissociation due to N_2 + N_2 collisions using the state-to-state method. We discuss vibrational state transition rates and vibrational state specific dissociation...
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In this paper we study the vibrational energy transfer process and dissociation due to N_2 + N_2 collisions using the state-to-state method. We discuss vibrational state transition rates and vibrational state specific dissociation rates obtained from QCT using the PES developed at the University of Minnesota. We first simulate an isothermal heat bath without dissociation to study the vibrational energy transfer process in isolation and then investigate nonequilib-rium dissociation under QSS conditions to examine the impact of state-transitions and state-specific dissociation rates in a coupled simulation. Lastly, this paper delineates the role of mul-tiquantum effects on vibrational energy transfer and dissociation to establish an engineering criteria to limit multi quantum effects so as to make the state-to-state approach computationally feasible for realistic geometries. The analysis was carried out over a temperature range of T = 8000 K to 30000 K. The multi quantum truncation criteria proposed can reduce state-to-state computational cost up to ～ 65% at the low temperature end of T= 8000 K and up to ～ 12% at the high temperature end of T = 30000 K while being within 10% of the accuracy of the full vibrational state-to-state solution.
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摘要 :
In this paper we study the vibrational energy transfer process and dissociation due to N_2 + N_2 collisions using the state-to-state method. We discuss vibrational state transition rates and vibrational state specific dissociation...
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In this paper we study the vibrational energy transfer process and dissociation due to N_2 + N_2 collisions using the state-to-state method. We discuss vibrational state transition rates and vibrational state specific dissociation rates obtained from QCT using the PES developed at the University of Minnesota. We first simulate an isothermal heat bath without dissociation to study the vibrational energy transfer process in isolation and then investigate nonequilib-rium dissociation under QSS conditions to examine the impact of state-transitions and state-specific dissociation rates in a coupled simulation. Lastly, this paper delineates the role of mul-tiquantum effects on vibrational energy transfer and dissociation to establish an engineering criteria to limit multi quantum effects so as to make the state-to-state approach computationally feasible for realistic geometries. The analysis was carried out over a temperature range of T = 8000 K to 30000 K. The multi quantum truncation criteria proposed can reduce state-to-state computational cost up to ～ 65% at the low temperature end of T= 8000 K and up to ～ 12% at the high temperature end of T = 30000 K while being within 10% of the accuracy of the full vibrational state-to-state solution.
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In this article we present a detailed discussion of hypersonic two-dimensional nitrogen flow past a cylinder. Flowfields are simulated with the direct molecular simulation (DMS) method, using a high fidelity ab initio potential en...
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In this article we present a detailed discussion of hypersonic two-dimensional nitrogen flow past a cylinder. Flowfields are simulated with the direct molecular simulation (DMS) method, using a high fidelity ab initio potential energy surface (PES). We study rotational and vibrational excitation of nitrogen due to strong shocks waves and relaxation of internal energy modes in expanding shock heated flows with DMS. Furthermore, we compare flowflelds obtained using a high fidelity (ab initio) PES with those obtained by using an analytical Lennard Jones PES with DMS. It is found that the Lennard Jones PES can capture flowfield physics at low temperatures, however, it does not perform well at high temperature conditions. Additionally, a comparison of the DMS flowfield is carried out with solutions obtained from the direct simulation Monte-Carlo (DSMC) method.
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We describe the application of feed-forward artificial neural networks (ANN) to potential energy surface (PES) fitting for application to trajectory calculations or direct molecular simulation. The relevant physical symmetries are...
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We describe the application of feed-forward artificial neural networks (ANN) to potential energy surface (PES) fitting for application to trajectory calculations or direct molecular simulation. The relevant physical symmetries are correctly enforced by the appropriate selection of the input variables. In particular, permutation invariance is guaranteed by mapping the interatomic distances onto a set of permutation invariant inputs, known as fundamental invariants, that generate the permutation invariant polynomial ring. By doing an appropriate energy decomposition, we impose the correct 2-body energy contribution. Furthermore, spurious long-distance interactions between diatoms are removed by using an apodization function to smoothly cut off the interaction below an energy threshold at which the many-body energy term is to be considered negligible. This guarantees the correct long-range asymptotic behavior of the resulting ANN PES needed in trajectory integration. In this work, the application of neural networks to PES training is specialized for the N_2+N_2 system using the ab initio data set of Bender and co-workers. The ANN PES is shown to be as accurate or to have superior accuracy than the standard description based on permutation invariant polynomials (PIP). The root-mean-squared error on the training set single-point energies is generally lower than that of the PIP PES. Preliminary results from quasi-classical trajectory calculations show excellent agreement between dissociation probabilities obtained from trajectories perfomed on the ANN PES and on the PIP PES. Similar agreement is seen at the level of vibrational energy distributions. For the test case considered, the ANN PES is also generally more computationally efficient than the PIP PES at comparable root-mean-squared error levels.
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摘要 :
We describe the application of feed-forward artificial neural networks (ANN) to potential energy surface (PES) fitting for application to trajectory calculations or direct molecular simulation. The relevant physical symmetries are...
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We describe the application of feed-forward artificial neural networks (ANN) to potential energy surface (PES) fitting for application to trajectory calculations or direct molecular simulation. The relevant physical symmetries are correctly enforced by the appropriate selection of the input variables. In particular, permutation invariance is guaranteed by mapping the interatomic distances onto a set of permutation invariant inputs, known as fundamental invariants, that generate the permutation invariant polynomial ring. By doing an appropriate energy decomposition, we impose the correct 2-body energy contribution. Furthermore, spurious long-distance interactions between diatoms are removed by using an apodization function to smoothly cut off the interaction below an energy threshold at which the many-body energy term is to be considered negligible. This guarantees the correct long-range asymptotic behavior of the resulting ANN PES needed in trajectory integration. In this work, the application of neural networks to PES training is specialized for the N_2+N_2 system using the ab initio data set of Bender and co-workers. The ANN PES is shown to be as accurate or to have superior accuracy than the standard description based on permutation invariant polynomials (PIP). The root-mean-squared error on the training set single-point energies is generally lower than that of the PIP PES. Preliminary results from quasi-classical trajectory calculations show excellent agreement between dissociation probabilities obtained from trajectories perfomed on the ANN PES and on the PIP PES. Similar agreement is seen at the level of vibrational energy distributions. For the test case considered, the ANN PES is also generally more computationally efficient than the PIP PES at comparable root-mean-squared error levels.
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In this paper we explore the impact of different rate coefficients on the widely used two-temperature nonequilibrium model on the flowfield dynamics of hypersonic flows. Equilibrium dissociation rate coefficients for O_2 + O colli...
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In this paper we explore the impact of different rate coefficients on the widely used two-temperature nonequilibrium model on the flowfield dynamics of hypersonic flows. Equilibrium dissociation rate coefficients for O_2 + O collisions are calculated based on the earlier work of Grover et al. We propose curve fits for ab initio-based characteristic excitation times and dissociation rate coefficients for N_2 + N_2, N + N_2, O_2 + O_2, and O + O_2 for accurate predictions of aerodynamic coefficients and surface properties. We consider an oxygen shock wave corresponding to the experiments by Ibraguimova et al, where the free stream velocity is v = 4400 m/s and free stream pressure is p = 0.8 torn We also consider Mach 11 and 15 oxygen flow over a hemisphere. Finally we compare shock stand-off distance with experiments conducted by Nonaka et al for freestream velocities of v = 3850 m/s, 3490 m/s, and 3160 m/s and the binary scaling for these cases is ρR = 1 × 10~(-4), 4 × 10~(-4), 1.7 × 10~(-3) respectively. We compare translational and vibrational temperatures and the composition of the gas for the two-temperature Park dissociation model with rate coefficients proposed by Park and the newly proposed ab initio-based. Furthermore, we compare computational predictions of the shock-standoff distance with experiments by Nonaka et al. to delineate the role of rates from the Park two-temperature model.
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