Takashi Ozawa

Development of kinetic-based energy exchange models for noncontinuum, ionized hypersonic flows

Ultrahigh Mach number re-entry vehicles create sufficiently energetic flow conditions with substantial ionization occurring in the noncontinuum flow regime. To model these noncontinuum, ionized, and thermochemically nonequilibrium flows, a direct simulation Monte Carlo (DSMC) approach is investigated. Energy exchange models that have been developed for Navier–Stokes computational fluid dynamics computations are examined and revised for application to the DSMC method. Since the electron–heavy particle collision rate is approximately two orders of magnitude higher than that between heavy particles, a new model is developed for electron scattering collision processes and electron-vibrational energy exchange based on the electron-molecule shape resonance phenomena. It is found that the flow electron and vibrational temperatures are sensitive to the electron-vibrational relaxation model because the relaxation time changes by orders of magnitude. The DSMC calculations of the bow-shock region of a blunt body wer...

Modeling of Stardust Reentry Ablation Flows in the Near-Continuum Flight Regime

The ablation process of the Stardust thermal protection material is designed to reduce aerodynamic heating during reentry for extreme conditions. The coupling of ablation species with the flowfield is investigated in this work using the direct simulation Monte Carlo method for transitional to near-continuum flows. To model surface thermal and chemical ablation processes, a variable surface temperature wall is obtained assuming a radiative heat flux balanced by convective heat flux. It is found that chemical ablation due to the reaction between thermal protection system carbon materials and gaseous oxygen and nitrogen atoms is dominant compared with thermal ablation. As the altitude decreases, the forebody surface temperature increases, the ablation process becomes more intensive, and the influence of ionization reactions on the flowfield becomes more important due to denser freestream conditions.

Comparison of High-Altitude Hypersonic Wake Flows of Slender and Blunt Bodies

The gas dynamic features of the laminar, near-wake flow behind slender and blunt hypersonic vehicles are studied using the direct simulation Monte Carlo method. Near-wake flows are characterized by features of low density, low Reynolds number, high temperature, thermal nonequilibrium, species separation, and recirculation. The impact of freestream number density and velocity on the near-wake flowfield is considered and compared for slender and blunt bodies. The near-wake structure postulated by theory and observed in numerical continuum calculations is also observed in the kinetic simulations, which are more accurate in the high-altitude, rarefied near-wake flow. The paper discusses the validation of the direct simulation Monte Carlo computational tool with experimental data for slender and blunt shapes and a previously published blunt direct simulation Monte Carlo geometry case. Then, the near-wake flows generated by a 10 deg slender cone and a 70 deg blunt body are analyzed. The near-wake flows behind slender and blunt bodies are similar in that the freestream Mach number has little impact on the near-wake flow structure and the recirculation length is not found to be related to the local Reynolds number. For both geometries, the base radius was found to be the characteristic length in the near-wake flow. Significant differences in the near-wake flow for the two geometries were observed in the spatial distribution of gas temperatures, the degree of chemical dissociation, and the sensitivity of recirculation length to freestream number density.

Particle and continuum method comparison of a high-altitude, extreme-mach-number reentry flow

Stardust reentry flows have been simulated at an altitude of 80 km for a freestream velocity of 12.8 km/s using direct simulation Monte Carlo (DSMC) and computational fluid dynamics (CFD). Five ions and electrons were considered in the flowfield, and ionization processes were modeled in DSMC. The ion-averaged velocity method in DSMC was validated to maintain charge neutrality in the shock. Collision and energy-exchange models for DSMC were reviewed to ensure adequacy for the high-energy flow regime. Accurate electron-heavy particle collision cross sections and an electron-vibration relaxation model using Lees relaxation time were implemented in DSMC. Although the DSMC results agreed well with CFD for the collision-only case, discrepancies between DSMC and CFD were observed in the shock with the relaxation model activated. Furthermore, with full chemical reactions and ionization processes, DSMC results were compared with CFD. It was found that the assumption of electron temperature is crucial for the prediction of degree of ionization. At 80 km, the degree of ionization predicted by DSMC was found to be approximately 5 %, but in CFD, the degree of ionization is greater than 25 % for the case of T e = T tr and 9% for the case of T e = T vib· In DSMC, the electron-vibration relaxation model was found to be important to predict electron and vibrational temperatures at this altitude, and the electron temperature is the same order as the vibrational temperature. Therefore, compared to the DSMC solution, the assumption of T e = T vib is preferable in CFD. In addition, using the Mott-Smith model, good agreement was obtained between the analytical bimodal distribution functions and DSMC velocity distributions. An effective temperature correction in the relaxation and chemical reaction models using the Mott-Smith model may reduce the continuum breakdown discrepancy between DSMC and CFD inside the shock in terms of degree of ionization and temperatures, but a general implementation is not clear.

Chemical Reaction Modeling for Hypervelocity Collisions between O and HCl

The sensitivity of a rarefied-to-transitional flow to the fidelity of the chemical reaction model is investigated for a new molecular dynamics/quasiclassical trajectory (MD/QCT)-derived model and compared with the widely used total collision energy (TCE) model of Bird. For hypervelocity collisions that occur in the space environment, it is not clear, a priori, that the TCE model will provide reasonable results for the required high energy range and, particularly, if strong favoring of the reaction among different forms of reactant energy occurs. In fact, in previous work, the TCE model, using available Arrhenius parameters, has been found, for these flow conditions, to give unphysical probabilities. A chemical reaction model, suitable for use in the direct simulation Monte Carlo (DSMC) method, is developed to simulate the hypervelocity collisions of O(P3)+HCl(Σ+1)→OH(Π2)+Cl(P2), an example of an important reaction in high-altitude atmospheric-jet interactions. The model utilizes the MD/QCT method with a n...

Development of a Coupled DSMC - Particle Photon Monte Carlo Method for Simulating Atomic Radiation in Hypersonic Reentry Flows

With a fast reentry speed, the Stardust vehicle generates a strong shock region ahead of its blunt body with a temperature above 60,000 K. These extreme Mach number o ws are sucien tly energetic to initiate gas ionization processes and thermal and chemical ablation processes. The generated charged particles aect nonequilibrium atomic and molecular energy distributions and shock layer radiation. In this work, we present the rst loosely coupled direct simulation Monte Carlo (DSMC) simulations with the particle-based photon Monte Carlo (p-PMC) method to simulate Stardust reentry o ws in the transitional o w regime. Eleven species including 5 ionization processes were modeled in DSMC, and the average ion velocity model was used to simulate electron motion. At these altitudes, the degree of ionization is predicted to be between 3 - 7 % along the stagnation line, and the maximum translational and electron temperatures are approximately 60,000 K and 20,000 K, respectively. To ecien tly capture high nonequilibrium eects, emission and absorption cross section databases using the Nonequilibrium Air Radiation (NEQAIR) were generated, and N and O radiation was calculated by the p-PMC method. It was found that the N emission is approximately one order of magnitude higher than the O emission along the stagnation line due to the higher concentration of N at this altitude. The radiation energy change calculated by the p-PMC method has been coupled in the DSMC calculations. It was found that while the N and O atomic radiation does not have impact on the o w eld at 81 km, at 68.9 km, the stronger radiation aected the o w eld. The radiation resulted in lowering the gas temperatures in the high emission region, but slight increase of temperatures near the body as well as convective heat ux to the Stardust surface.

Modeling of the Stardust Reentry Flows with Ionization in DSMC

Ionized hypersonic o ws for the Stardust blunt body at 80 km altitude were simulated using the direct simulation Monte Carlo (DSMC) method, at a speed of 12.8 km/s, the fastest man-made object to enter the Earth’s atmosphere. The modeling of ionization processes and energy exchange between translational and internal modes in DSMC is discussed. Eleven species including 5 ions and electron and chemical reactions are considered in the o weld. It was found that the charge neutrality assumption held in the bow-shock region and using this assumption, ionization processes were modeled. The electron-vibration energy exchange model is important for the prediction of electron and vibrational temperature proles, and this process was modeled by Lee’s relaxation time for the rst time in DSMC. DSMC results were compared with CFD(DPLR) results, with the main result that DSMC predicted lower energy exchange rates between translational and internal modes than CFD. The lower energy exchange rates resulted in lower dissociation rates and a lower degree of ionization in DSMC. Furthermore, radiation was calculated using the NEQAIR code from the CFD(DPLR) and DSMC o welds along the stagnation line for NO, N + , N, and O. The development of the DSMC models signican tly aected the N radiation and O radiation in ultraviolet range.

Spectral Module for Photon Monte Carlo Calculations in Hypersonic Nonequilibrium Radiation

In this paper, efficient spectral modules and random number databases are developed for atomic and diatomic species for use in photon Monte Carlo (PMC) modeling of hypersonic nonequilibrium flow radiation. To model nonequilibrium flow conditions, the quasi steady state (QSS) assumption was used to generate electronic state populations of atomic and diatomic gas species in the databases. For atomic species (N and O), both bound-bound transitions and continuum radiation were included, and were separately databased as a function of electron temperature and number density as well as the ratio of atomic ion to neutral number density. For the radiating diatomic species of N2+, N2 , O2 , and NO, databases were generated for each electronic molecular electronic system. In each molecular electronic system, the ro-vibrational transition lines were separately databased for each electronic upper state population forming the electronic system. The spectral module for the PMC method was optimized toward computational efficiency for emission calculations, wavelength selections of photon bundles and absorption coefficient calculations in the ray tracing scheme.Copyright

Analysis of Chemistry Models for DSMC Simulations of the Atmosphere of Io

Hypervelocity chemical reactions between SO2 and O are studied using the molecular dynamic/quasi-classicaltrajectory method for conditions relevant to the modeling of the rarefied atmosphere of Io, a moon of Jupiter. The implementation of both molecular dynamic/quasi-classical-trajectory and total-collisional-energy chemistry reactionmodelsindirectsimulationMonteCarloisstudiedinazero-dimensionaltime-dependentanalysisandatwodimensionalaxisymmetricdirectsimulationMonteCarlosimulationtomodelasimpleplanetary flowcondition.The molecular dynamic/quasi-classical-trajectory simulations were found to result in lower reaction rate constants and reactionprobabilitiesthanthetotal-collisional-energymodel,andthevibrationalfavoringfeatureoftheSO2 � O ! SO � 2O reaction was revealed. The total collision cross section using the more general viscosity cross section was also obtained through the molecular dynamic/quasi-classical-trajectory simulations and was found to have a significantlydifferentenergydependencecomparedwiththeoriginalvariablehardspherecrosssection.Forthetwodimensional direct simulation Monte Carlo simulations it was found that the structure of the flow as well as the chemically formed sulfur oxide were different for the molecular dynamic/quasi-classical-trajectory and the totalcollisional-energy reaction probabilities and total cross sections. In addition, the reaction region was found to be highly nonequilibrium, which suggests that molecular dynamic/quasi-classical trajectory is a more suitable chemistry model for the simulation of Io’s atmosphere.

Accurate Molecular and Soot Infrared Radiation Model for High-Temperature Flows

The accurate computation of infrared spectral radiation from high-temperature, nonequilibrium flows remains a challenging problem, particularly for polyatomic species and particulates. A versatile computer model for calculating the infrared radiation of water and carbon dioxide and soot in generic flowfields is proposed and examined. Molecular radiation is calculated using the high-resolution transmission absorption molecular database/ high-temperature spectroscopic absorption parameters and carbon dioxide spectroscopic databank-1000 line-by-line databases to provide high-resolution, accurate spectra between 200 and 8200 cm -1 . Soot particulate radiation is modeled using the first-term approximation of Mie scattering theory, and the pseudogas approximation is used. The program was parallelized in spectral increments to run efficiently on a message-passing interface equipped cluster. Validation was performed against well-documented radiation models such as ATHENA and nonequilibrium air radiation-infrared. Soot radiative properties were validated against measurements made on a sooting diffuse laminar flame, and carbon dioxide spectra were validated against experimental data. The computer radiation model is applied to two situations: a nonequilibrium bow shock and an Atlas-like sooting plume.