arXiv subjects

Sung Min Jo

Publications and source records attributed to Sung Min Jo.

11 recordsLinked to original sources

arXiv2026

Bayesian Estimation of Spectroscopic Parameters: Application to the Atomic Nitrogen Bound-Bound System

Atomic nitrogen bound-bound radiation is a major component of the radiative heat flux on hypersonic vehicles entering nitrogen-dominated atmospheres, yet its prediction is limited by substantial parametric uncertainty in the published Einstein coefficients and Stark broadening coefficients. In the present study, these spectroscopic parameters are inferred and their uncertainty is quantified through Bayesian inversion of equilibrium spectral radiance measured in the NASA Ames Electric-Arc Shock Tube for two shots of the Test 62 campaign at shock speeds of 10.32 and 10.72 km/s. The inference is restricted to the post-shock equilibrium region, where the Boltzmann assumption closes the species population degree of freedom. The residual uncertainty in the post-shock temperature and species number densities is incorporated as a coupled nuisance parameter distribution. A hybrid principal component analysis and polynomial chaos expansion surrogate model and a likelihood formulated jointly over the two shots enable tractable Markov chain Monte Carlo sampling across multiple wavelength regions. Eighteen parameters in total, ten Einstein coefficients and eight Stark broadening coefficients, are inferred across eight wavelength regions, with posterior uncertainties significantly reduced relative to the prior literature bands. Forward propagation of the joint posterior through the stagnation-line flow field around a 3 m radius sphere at entry velocities of 10, 12, and 14 km/s demonstrates a reduction in the standard deviation of the predicted radiative heat flux by approximately a factor of five compared with the prior, in particular at 14 km/s, it drops from 10.4 to 1.94 W/cm$^{2}$.

physics.plasm-ph↗

arXiv2026

Impact of non-equilibrium radiation in a high-enthalpy inductively coupled plasma wind tunnel

High-power inductively coupled plasma (ICP) wind tunnels are widely used to reproduce high-enthalpy environments relevant to atmospheric entry and hypersonic testing. Despite their importance, radiative heat transfer in ICP facilities is commonly neglected or modeled using simplified optically thin assumptions, and the impact of non-equilibrium radiation on plasma dynamics remains poorly quantified. In this work, a loosely coupled, multi-physics framework is developed to systematically investigate radiative cooling effects in the 350 kW Plasmatron X facility at the University of Illinois Urbana-Champaign. The approach self-consistently couples a magnetohydrodynamic plasma framework with a spectral radiative transport solver, eliminating the need for optically thin or empirical models. Simulations are performed for nitrogen and air plasmas over a wide range of operating pressures (1-101 kPa) and powers (100-350 kW). The results reveal a strong pressure dependence of radiative losses, with radiation contributing negligibly at low pressures, but becoming a dominant energy sink at elevated pressures. At atmospheric pressure, radiative losses account for up to approximately 32% and 22% of the input power for nitrogen and air plasmas, respectively, leading to substantial reductions in core plasma temperatures. Nitrogen plasmas consistently exhibit higher radiative losses than air as a result of increased concentrations of radiatively active species and higher electron number densities. Pressure-power maps of radiative heat loss relative to input power are constructed to quantify combined operating effects and to provide guidance for facility operation and modeling fidelity. Finally, an assessment of self-absorption demonstrates that the Plasmatron X torch operates predominantly in an optically thin regime, even at the highest power and pressure conditions considered.

physics.plasm-ph↗

arXiv2026

Integration of local and global surrogates for failure probability estimation

This paper presents the development of an algorithm, termed the Global-Local Hybrid Surrogate (GLHS), designed to efficiently compute the probability of rare failure events in complex systems. The primary goal is to enhance the accuracy of reliability analysis while minimizing computational cost, particularly for high-dimensional problems where traditional methods, such as Monte Carlo simulations, become prohibitively expensive. The proposed GLHS builds upon the foundational work of Li et al., by integrating an adaptive strategy based on the General Domain Adaptive Strategy (Adcock et al.). The algorithm aims to approximate the failure domain of a given system, defined as the region in the input domain where the system transitions from safe to failure modes, described by a limit state surface. This failure domain is not explicitly known and must be learned iteratively during the analysis. The method employs a buffer zone, defined as the region surrounding the limit state surface. Within this buffer zone, Christoffel Adaptive Sampling is utilized to select new samples for constructing localized surrogate models, which are designed to refine the approximation in regions critical to failure probability estimation. The iterative process proceeds until convergence is reached. This results in a hybrid methodology that integrates a global surrogate to capture the overall trend with local surrogates that concentrate on critical regions near the limit state function. By adopting this strategy, the GLHS method balances computational efficiency with accuracy in estimating the failure probability.

cs.CE↗

arXiv2026

First-Principle-Inspired Reduced-Order Models of Chemical-Kinetics in $\text{H}_2\left(\text{X}^1Σ_g^+\right)$+$\text{H}\left({}^2\text{S}\right)$ System

In the present study, two-different reduced-order models are proposed for $\text{H}_2\left(\text{X}^1Σ_g^+\right)$+$\text{H}\left({}^2\text{S}\right)$ system by leveraging first-principle quasi-classical trajectory simulations and in-depth master equation analyses. The most recent available ab-initio potential energy surface is adopted to construct a new set of rovibrational state-to-state kinetic database valid over a wide range of temperatures. Firstly, a modified two-temperature model is proposed by incorporating the master equation-informed model parameters, enabling the advanced treatment of the internal energy coupling and the nonequilibrium dissociation predictions. Secondly, a hybrid coarse-graining model is proposed by combining a graph-based approach optimized globally for a wide range of temperatures with a centrifugal-barrier-based coarse-graining method. The proposed reduced-order models offer significantly improved accuracy in predicting the nonequilibrium energy transfer and dissociation dynamics compared to the existing coarse-graining and 2T models in previous studies. In addition, aerothermal heating prediction relevant to Uranus planetary entry reveals 16.5% of convective heat flux discrepancy compared to the present modified 2T approach with the existing 2T, demonstrating the importance of accurate modeling of the chemical-kinetics in the $\text{H}_2\left(\text{X}^1Σ_g^+\right)$+$\text{H}\left({}^2\text{S}\right)$ system.

physics.chem-ph↗

arXiv2025

An Euler-Lagrangian Multiphysics Coupling Framework for Particle-Laden High-Speed Flows

Particle-laden effects in high-speed flows require a coupled Euler and Lagrangian prediction technique with varying fidelity of thermochemical models, depending on the simulation conditions of interest. This requirement makes the development of a conventional monolithic solver challenging to manage the different fidelity of the thermochemical models within a single computational framework. To address this, the present study proposes a multi-solver framework for the coupled Euler-Lagrangian predictions applicable to various particle-laden high-speed flow conditions. Volumetric and surface couplings are established between a particle solver ORACLE (OpenFOAM-based lagRAngian CoupLEr) and a thermochemical nonequilibrium flow solver based on an adaptable data exchange algorithm. The developed framework is then validated by predicting particle-laden supersonic nozzle flows and aerothermal heating around a hypersonic Martian atmospheric entry capsule. Finally, a quasi-1D approximation is proposed in conjunction with a surrogate method to efficiently and accurately predict particle-laden surface erosion, with quantified parametric uncertainty, for hypersonic aerothermal characterization.

physics.flu-dyn↗

arXiv2025

Master equation study of three-body recombination of nitrogen and oxygen in non-equilibrium hypersonic flows

This work aims to study the energy transfer and recombination processes in N$_{2}$$\left(^{1}\sum^{+}_{g}\right)$+N$\left(^{4}S_{u}\right)$ and O$_{2}$$\left(^{3}\sum^{+}_{g}\right)$+O$\left(^{3}P_{2}\right)$ chemical systems when the system is suddenly cooled in a 0-D isothermal reactor thereby inducing strong non-equilibrium. A state-to-state (StS) study of the non-equilibrium phenomenon is crucial for developing accurate and efficient reduced-order models that can accurately capture the thermophysics involved. The gas mixture, consisting primarily of atoms at a high initial temperature of 10,000 K, is suddenly plunged into a low-temperature heat bath to simulate non-equilibrium recombination conditions. The population distribution of microscopic energy levels for each system is determined by solving a system of master equations. The conventional assumption of faster equilibration of rotational mode as compared to the vibrational mode holds for $ N_2 +N$, while it is not a very strong assumption for $ O_2 +O$ as the two relaxation time scales are comparable. Effective recombination rate constants for the quasi-steady state (QSS) period are calculated using the population distribution obtained by solving the master equations. It was also observed that the relaxation time constants for heating and cooling are different, with the time constant being lower for the cooling case due to anharmonicity effects in expanding flows. An attempt has also been made to use the insights from the StS analysis to determine an accurate binning strategy for the recombination processes involved in the two chemical systems.

physics.chem-ph↗

arXiv2025

Reaction Dynamics for the [NNO] System from State-Resolved and Coarse-Grained Models

The dynamics for the NO($X^2 Π$) + N($^4$S) $\leftrightarrow$ N$_{2}(X^{1}Σ_{g}^{+}$) + O($^{3}$P) reaction was followed in the $^3$A' electronic state using state-to-state (STS) and Arrhenius-based rates from two different high-level potential energy surfaces represented as a reproducing kernel (RKHS) and permutationally invariant polynomials (PIPs). Despite the different number of bound states supported by the RKHS- and PIP-PESs the ignition points from STS and Arrhenius rates are at $\sim 10^{-6}$ s whether or not reverse rates are from assuming microreversibility or explicitly given. Conversion from NO to N$_2$ is incomplete if Arrhenius-rates are used but complete turnover is observed if STS-information is used. This is due to non-equilibrium energy flow and state dynamics which requires a state-based description. Including full dissociation leads asymptotically to the correct 2:1 [N]:[O] concentration with little differences for the species' dynamics depending on the PES used for the STS-information. In conclusion, concentration profiles from coarse-grained simulations are consistent over 14 orders of magnitude in time using STS-information based on two different high-level PESs.

physics.chem-ph↗

arXiv2024

Numerical Investigation of Radiative Transfers Interactions with Material Ablative Response for Hypersonic Atmospheric Entry

Radiative transfer interactions with material ablation are critical contributors to vehicle heating during high-altitude, high-velocity atmospheric entry. However, the inherent complexity of fully coupled multi-physics models often necessitates simplifying assumptions, which may overlook key phenomena that significantly affect heat loads, particularly radiative heating. Common approximations include neglecting the contribution of ablation products, applying simplified frozen wall boundary conditions, or treating radiative transfer in a loosely coupled manner. This study introduces a high-fidelity, tightly coupled multi-solver framework designed to accurately capture the multi-physics challenges of hypersonic flow around an ablative body. The proposed approach consistently accounts for the interactions between shock-heated gases, surface material response, and radiative transfer. Our results demonstrate that including radiative heating in the surface energy balance substantially influences the ablation rate. Ablation products are shown to absorb radiative heat flux in the vacuum-ultraviolet spectrum along the stagnation line, while strongly emitting in off-stagnation regions. These findings emphasize the necessity of a tightly coupled multiphysics framework to faithfully capture the complex, multidimensional interactions in hypersonic flow environments, which conventional, loosely coupled models fail to represent accurately.

physics.comp-ph↗

arXiv2023

Multi-physics modeling of non-equilibrium phenomena in inductively coupled plasma discharges: Part I. A state-to-state approach

This work presents a vibrational and electronic state-to-state model for nitrogen plasma implemented within a multi-physics modular computational framework to study non-equilibrium effects in inductively coupled plasma (ICP) discharges. Within the computational framework, the set of vibronic (i.e., vibrational and electronic) master equations are solved in a tightly coupled fashion with the flow governing equations. This tight coupling eliminates the need for invoking any simplifying assumptions when computing the state of the plasma, thereby ensuring a higher degree of physical fidelity. To mitigate computational complexity, a maximum entropy coarse-graining strategy is deployed, effectively truncating the internal state space. The efficacy of this reduced StS model is empirically substantiated through zero-dimensional isochoric simulations. In these simulations, the results obtained from the reduced-order model are rigorously compared against those obtained from the full StS model, thereby confirming the accuracy of the reduced StS framework. The developed Coarse-grained StS model was employed to study the plasma discharge within the VKI Plasmatron facility. Our results reveal pronounced discrepancies between the plasma flow fields obtained from StS simulations and those derived from Local Thermodynamic Equilibrium (LTE) models, which are conventionally used in the simulation of such facilities. The analysis demonstrates a substantial departure of the internal state populations of atoms and molecules from the Boltzmann distribution. These nonequilibrium effects have important consequences on the energy coupling dynamics, thereby impacting the overall morphology of the plasma discharge. A deeper analysis of the results demonstrates that the population distribution is in a Quasi-Steady-State in the hot plasma core.

physics.plasm-ph↗

arXiv2022

Rovibrational Internal Energy Transfer and Dissociation of High-Temperature Oxygen Mixture

This work constructs a rovibrational state-to-state model for the $\text{O}_2$+$\text{O}_2$ system leveraging high-fidelity potential energy surfaces and quasi-classical trajectory calculations. The model is used to investigate internal energy transfer and non-equilibrium reactive processes in dissociating environment using a master equation approach, whereby the kinetics of each internal rovibrational state is explicitly computed. To cope with the exponentially large number of elementary processes that characterize reactive bimolecular collisions, the internal states of the collision partner are assumed to follow a Boltzmann distribution at a prescribed internal temperature. This procedure makes the problem tractable, reducing the computational cost to a comparable scale with the $\text{O}_2$+O system. The constructed rovibrational-specific kinetic database covers the temperature range of 7500-20000 K. The analysis of the energy transfer and dissociation process in isochoric and isothermal conditions reveals that significant departures from the equilibrium Boltzmann distribution occur during the energy transfer and dissociation phase. Comparing the population distribution of the $\text{O}_2$ molecules against the $\text{O}_2$+O demonstrates a more significant extent of non-equilibrium characterized by a more diffuse distribution whereby the vibrational strands are more clearly identifiable. This is partly due to a less efficient mixing of the rovibrational states, which results in more diffuse rovibrational distributions in the quasi-steady-state distribution. The master equation analysis for the combined $\text{O}_3$+$\text{O}_4$ system reveals that the $\text{O}_2$+$\text{O}_2$ governs the early stage of energy transfer, while the $\text{O}_2$+O takes control of the dissociation dynamics. The findings will provide strong physical foundations for future development of oxygen chemistry.

physics.chem-ph↗

arXiv2021

Rovibrational-Specific QCT and Master Equation Study on $\text{N}_2(\text{X}^1Σ_g^+)$+$\text{O}({}^3\text{P})$ and $\text{NO}(\text{X}^2Π)$+$\text{N}({}^4\text{S})$ Systems in High-Energy Collisions

This work presents a detailed investigation of the energy transfer and dissociation mechanisms in $\text{N}_2(\text{X}^1Σ_g^+)$+$\text{O}({}^3\text{P})$ and $\text{NO}(\text{X}^2Π)$+$\text{N}({}^4\text{S})$ systems using rovibrational-specific quasi-classical trajectory (QCT) and master equation analyses. The complete set of state-to-state kinetic data, obtained via QCT, allows for an in-depth investigation of the Zel'dovich mechanism leading to the formation of $\text{NO}$ molecules at microscopic and macroscopic scales. The master equation analysis demonstrates that the low-lying vibrational states of $\text{N}_2$ and $\text{NO}$ have dominant contributions to the $\text{NO}$ formation and the corresponding extinction of $\text{N}_2$ through the exchange process. For the considered temperature range, it is found that while nearly 50% of the dissociation processes for $\text{N}_2$ and $\text{NO}$ occurs in the molecular quasi-steady-state (QSS) regime, the amount of the Zel'dovich reaction is zero. Using the QSS approximation to model the Zel'dovich mechanism leads to an overestimation of $\text{NO}$ production by more than a factor of 4 in the high-temperature range. The breakdown of this well-known approximation has profound consequences for the approaches that heavily rely on the validity of QSS assumption in hypersonic applications. The investigation of the rovibrational state population dynamics reveals substantial similarity among different chemical systems for the energy transfer and the dissociation processes, providing promising physical foundations for the use of reduced-order strategies to other chemical systems without significant loss of accuracy.

physics.chem-ph↗