Marat Kulakhmetov

Ab initio-informed maximum entropy modeling of rovibrational relaxation and state-specific dissociation with application to the O2 + O system

Quasi-classical trajectory (QCT) calculations are used to study state-specific ro-vibrational energy exchange and dissociation in the O2 + O system. Atom-diatom collisions with energy between 0.1 and 20 eV are calculated with a double many body expansion potential energy surface by Varandas and Pais [Mol. Phys. 65, 843 (1988)]. Inelastic collisions favor mono-quantum vibrational transitions at translational energies above 1.3 eV although multi-quantum transitions are also important. Post-collision vibrational favoring decreases first exponentially and then linearly as Δv increases. Vibrationally elastic collisions (Δv = 0) favor small ΔJ transitions while vibrationally inelastic collisions have equilibrium post-collision rotational distributions. Dissociation exhibits both vibrational and rotational favoring. New vibrational-translational (VT), vibrational-rotational-translational (VRT) energy exchange, and dissociation models are developed based on QCT observations and maximum entropy considerations. Full set of parameters for state-to-state modeling of oxygen is presented. The VT energy exchange model describes 22 000 state-to-state vibrational cross sections using 11 parameters and reproduces vibrational relaxation rates within 30% in the 2500-20 000 K temperature range. The VRT model captures 80 × 10(6) state-to-state ro-vibrational cross sections using 19 parameters and reproduces vibrational relaxation rates within 60% in the 5000-15 000 K temperature range. The developed dissociation model reproduces state-specific and equilibrium dissociation rates within 25% using just 48 parameters. The maximum entropy framework makes it feasible to upscale ab initio simulation to full nonequilibrium flow calculations.

Effect of O2 + O ab initio and Morse additive pairwise potentials on dissociation and relaxation rates for nonequilibrium flow calculations

This work quantifies the sensitivity of O2 + O dissociation rates and relaxation to interatomic potential energy surfaces at high-enthalpy, nonequilibrium flow conditions. State-to-state cross sections are obtained by quasi-classical trajectory (QCT) calculations with two potential surfaces. The first is a Morse additive pairwise potential for O3 that is constructed based on O2(3Σg−) spectroscopic measurements and the second is a double many-body expansion potential by Varandas and Pais [Mol. Phys. 65, 843–860 (1988)]. The QCT calculations of cross sections and rates with the two surfaces are compared to each other and shock tube measurements. It is found that, at temperatures between 2500 K and 20 000 K, the equilibrium dissociation rates predicted by the two potentials agree within 12%, and they are bound by experimental dissociation measurements. In contrast, above 10 000 K, ab initio based equilibrium dissociation rates are about a factor of two higher than the widely used Park’s model. The nonequilib...

NOx production and rainout from Chicxulub impact ejecta reentry

The Chicxulub impact 66.0 Ma ago initiated the second biggest extinction in the Phanerozoic Eon. The cause of the concurrent oceanic nitrogen isotopic anomaly, however, remains elusive. The Chicxulub impactor struck the Yucatan peninsula, ejecting 2 × 1015 kg of molten and vaporized rock that reentered globally as approximately 1023 microscopic spherules. Here we report that modern techniques indicate that this ejecta generates 1.5 × 1014 moles of NOx, which is enough to cause the observed nitrogen enrichment of the basal layer. Additionally, reentry-based NO production would explain the anomalously heavy isotopic composition of the observed nitrogen. We include N, O, N2, O2, and NO species in simulations of nonequilibrium chemically reacting flow around a reentering spherule. We then determine the net production of NO from all the spherules and use turbulence models to determine how quickly this yield diffuses through the atmosphere. Upon reaching the stratosphere and troposphere, cloud moisture absorbs the NOx and forms nitric acid. We model this process and determine the acidity of the resulting precipitation, which peaks about 1 year after the impact. The precipitation ultimately reaches the upper ocean, where we assume that the well-mixed surface layer is 100 m deep. We then model the naturally occurring carbonate/bicarbonate buffer and determine the net pH. We find that insufficient NOx reaches the ocean to directly cause the observed end-Cretaceous oceanic extinction via acidification and buffer removal. However, the resulting nitrates are sufficient to explain the concurrent nitrogen isotopic anomaly and facilitate an end-Cretaceous algae bloom.

DSMC study of oxygen shockwaves based on high-fidelity vibrational relaxation and dissociation models

This work evaluates high-fidelity vibrational-translational (VT) energy relaxation and dissociation models for pure O2 normal shockwave simulations with the direct simulation Monte Carlo (DSMC) method. The O2–O collisions are described using ab initio state-specific relaxation and dissociation models. The Macheret-Fridman (MF) dissociation model is adapted to the DSMC framework by modifying the standard implementation of the total collision energy (TCE) model. The O2–O2 dissociation is modeled with this TCE+MF approach, which is calibrated with O2–O ab initio data and experimental equilibrium dissociation rates. The O2–O2 vibrational relaxation is modeled via the Larsen-Borgnakke model, calibrated to experimental VT rates. All the present results are compared to experimental data and previous calculations available in the literature. It is found that, in general, the ab initio dissociation model is better than the TCE model at matching the shock experiments. Therefore, when available, efficient ab initio ...

Maximum entropy modeling of vibrational-translational energy exchange in O2+O collisions

This work proposes a vibrational-translational energy exchange model based on quasiclassical trajectory (QCT) calculations of O2 + O and Maximum Entropy (ME) considerations. QCT calculations show that collisions favor small vibrational transitions. This favoring drops off first exponentially and then linearly as ∆v increases. Such trends are captured by the new ME-QCT-VT model. The model is verified by comparing its stateto-state cross sections and rates to those calculated directly by QCT. The state-to-state rates match within 30% using 11 fitted parameters for the O2 +O system that has approximately 3,000 ro-vibrational states. In addition, the implementation of the model in direct simulation Monte Carlo (DSMC) method is discussed. Adiabatic DSMC calculations show that the model satisfies detailed balance. The vibrational temperatures and distribution functions predicted by DSMC calculations with the ME-QCT-VT model match masterequation calculations with the complete set of state-to-state rates. The proposed model makes it feasible to upscale ab-initio simulation to DSMC and CFD calculations of a full flowfield.

Climatic effects of the Chicxulub impact ejecta

Examining the short and long term effects of the Chicxulub impact is critical for understanding how life developed on Earth. While the aftermath of the initial impact would have produced harmful levels of radiation sufficient for eradicating a large portion of terrestrial life, this process does not explain the concurrent marine extinction. Following the primary impact, a large quantity of smaller spherules would de-orbit and re-enter the earths atmosphere, dispersed nearly uniformly across the planet. This secondary wave of debris would re-enter at high velocities, altering the chemical composition of the atmosphere. Furthermore, the combined surface area for the spherules would be much larger than for the original asteroid, resulting in considerably more potential reactions. For this reason, a new method was developed for predicting the total amount of toxic species produced by the spherule re-entry phase of the Chicxulub event. Using non-equilibrium properties obtained from direct simulation Monte Carlo (DSMC) methods coupled with spherule trajectory integration, the most likely cause of the observed marine extinction was determined.

Calculation and Sampling of Quasi-Classical Trajectories for Nonequilibrium Reacting Flow Simulations

Since it takes multiple collisions for molecules in the flow to return back to equilibrium, if these molecules are significantly perturbed from the equilibrium or the bulk flow velocity is high compared to geometry feature sizes, the flow may be in nonequilibrium. Under such conditions a detailed understanding of collision processes becomes important. Quasiclassical trajectory calculations make it possible to study internal energy exchange and reactions of individual collisions. This work attempts to extend the results from Quasiclassical trajectory calculations to larger flow simulations. This is done by averaging Quasiclassical trajectory calculations to produce reaction and energy exchange probabilities that are dependent on the impact parameter, relative velocities and ro-vibrational energies.

Modeling Uncertainties in Direct Simulation Monte Carlo Calculations of Hypersonic Leading-Edge Flow

The effects of uncertainties in the gas–surface and intermolecular interaction models on a hypersonic boundarylayer development are investigated by propagating these uncertainties through direct simulation Monte Carlo calculations of Mach 10 and 20 flows. The model input uncertainties considered are the momentum accommodation coefficient in the Maxwellian gas–surface interaction model, surface temperature and the viscosity exponent in the variable hard sphere molecular model. The effects of the input uncertainties are quantified by computing produced uncertainties in the flowfield temperature, flowfield density, surface shear, pressure, and heat flux. A nonintrusive generalized polynomial chaos expansion is used to propagate the uncertainties; reconstruct the probability density functions; and calculate the mean, standard deviation, and skewness of the output variables. It is shown that the polynomial chaos expansion with just three flowfield samples can propagate uncertainties with an accuracy equivalent to Monte Carlo methods with 10 million samples. The uncertainty analysis shows that the surface fluxes and the flowfields in the hypersonic boundary layer are more sensitive to the accommodation coefficient uncertainty than surface temperature or viscosity exponent uncertainty. An input uncertainty of 19% in the accommodation coefficient results in a 20% uncertainty in the flowfield temperature at Mach 10 and a 31% uncertainty at Mach 20. This input uncertainty results in 22 and 28% uncertainties in the surface fluxes at the two Mach numbers. The produced uncertainties generally increase with Mach number, and the effect of introduced uncertainty diminishes away from the leading edge.

Comparison between phenomenological and ab-initio reaction and relaxation models in DSMC

New state-specific vibrational-translational energy exchange and dissociation models, based on ab-initio data, are implemented in direct simulation Monte Carlo (DSMC) method and compared to the established Larsen-Borgnakke (LB) and total collision energy (TCE) phenomenological models. For consistency, both the LB and TCE models are calibrated with QCT-calculated O2+O data. The model comparison test cases include 0-D thermochemical relaxation under adiabatic conditions and 1-D normal shockwave calculations. The results show that both the ME-QCT-VT and LB models can reproduce vibrational relaxation accurately but the TCE model is unable to reproduce nonequilibrium rates even when it is calibrated to accurate equilibrium rates. The new reaction model does capture QCT-calculated nonequilibrium rates. For all investigated cases, we discuss the prediction differences based on the new model features.

Effects of Uncertainty in Gas‐Surface Interaction on DSMC Simulations of Hypersonic Flows

This study uses the non‐intrusive generalized polynomial chaos method to investigate the effects of uncertainties in the gas‐surface interaction model on the hypersonic boundary layer flow over a flat plate. In particular, the polynomial chaos method is applied to assess uncertainties in the surface shear, normal stress, heat flux, flowfield temperature and density resulting from uncertain surface temperature and accommodation coefficient. The polynomial chaos approach allows us to estimate probability density functions from fewer flowfield samples than the traditional random Monte Carlo sampling. The flowfield solutions are computed by the DSMC code SMILE. The analysis shows that surface fluxes and flowfields in the hypersonic boundary layer are more sensitive to the accommodation coefficient than surface temperature uncertainty.