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In this work, we used molecular dynamics (MD) to perform trajectory simulations of ice-like argon and amorphous silica aggregates on atomically smooth highly ordered pyrolytic graphite (HOPG) and a comparatively rougher quartz surface. It was found that at all incidence velocities, the quartz surface was stickier than the HOPG surface. The sticking probabilities and elastic moduli obtained from MD were then used to model surface evolution at a micron length scale using kinetic Monte Carlo (kMC) simulations. Rules were derived to control the number of sites available for the process execution in kMC to accurately model erosion of HOPG by atomic oxygen (AO) attack and ice-nucleation on surfaces. It was observed that the effect of defects was to increase the material erosion rate, while that of aggregate nucleation was to lower it. Similarly, simulations were performed to study the effects of AO attack and N2 adsorption-desorption on surface evolution and it was found that N2 adsorption-desorption limits the surface available for erosion by AO attack. [ABSTRACT FROM AUTHOR]

A particle simulation method to model water condensation process in a supersonic rocket plume is proposed and developed. Classic nucleation theory is used to predict nucleation, condensation, and the evaporation rates for water clusters. Microscopic kinetic models are developed to simulate collision processes between water clusters and monomers, between water clusters and foreign molecules, and evaporation of monomers from water clusters. These models are integrated into the direct simulation Monte Carlo method to simulate an axisymmetric mullispecies gas expansion coupled with condensation. The developed computational scheme, first verified by empirical scaling laws of condensation in supersonic microjets, is then applied to predict the spatial distributions of water cluster number density, size, and temperature in a rocket exhaust plume. An empirical equation is used to correct classical nucleation rate, condensation results are compared between the original and corrected nucleation rate, and the impact of the nucleation rate on a flow with condensation is discussed in detail.

Recent potential energy surfaces (PESs) for the N2 + N and N2 + N2 systems are used in molecular dynamics (MD) to simulate rates of vibrational and rotational relaxations for conditions that occur in hypersonic flows. For both chemical systems, it is found that the rotational relaxation number increases with the translational temperature and decreases as the rotational temperature approaches the translational temperature. The vibrational relaxation number is observed to decrease with translational temperature and approaches the rotational relaxation number in the high temperature region. The rotational and vibrational relaxation numbers are generally larger in the N2 + N2 system. MD-quasi-classical trajectory (QCT) with the PESs is also used to calculate the V-T transition cross sections, the collision cross section, and the dissociation cross section for each collision pair. Direct simulation Monte Carlo (DSMC) results for hypersonic flow over a blunt body with the total collision cross section from MD/QCT simulations, Larsen-Borgnakke with new relaxation numbers, and the N2 dissociation rate from MD/QCT show a profile with a decreased translational temperature and a rotational temperature close to vibrational temperature. The results demonstrate that many of the physical models employed in DSMC should be revised as fundamental potential energy surfaces suitable for high temperature conditions become available. [ABSTRACT FROM AUTHOR]

The Direct Simulation Monte Carlo (DSMC) method typically used for simulating hypersonic Earth re-entry flows requires accurate total collision cross sections and reaction probabilities. However, total cross sections are often determined from extrapolations of relatively low-temperature viscosity data, so their reliability is unknown for the high temperatures observed in hypersonic flows. Existing DSMC reaction models accurately reproduce experimental equilibrium reaction rates, but the applicability of these rates to the strong thermal nonequilibrium observed in hypersonic shocks is unknown. For hypersonic flows, these modeling issues are particularly relevant for nitrogen, the dominant species of air. To rectify this deficiency, the Molecular Dynamics/Quasi-Classical Trajectories (MD/QCT) method is used to accurately compute collision and reaction cross sections for the N2(1Σg+)-N2(1Σ+) collision pair for conditions expected in hypersonic shocks using a new potential energy surface developed using a ReaxFF fit to recent advanced ab initio calculations. The MD/QCT-computed reaction probabilities were found to exhibit better physical behavior and predict less dissociation than the baseline total collision energy reaction model for strong nonequilibrium conditions expected in a shock. The MD/QCT reaction model compared well with computed equilibrium reaction rates and shock-tube data. In addition, the MD/QCT-computed total cross sections were found to agree well with established variable hard sphere total cross sections. [ABSTRACT FROM AUTHOR]

Plasma–surface interactions caused by electric propulsion devices are an important spacecraft aspect of the design that is difficult to measure in ground-based facilities. The negatively biased solar panel surfaces attract the slow-moving charge exchange (CEX) ions generated inside an ion core plume, which can cause surface sputtering on the protective coatings of the solar panels. We use a fully kinetic particle-in-cell direct simulation Monte Carlo (PIC-DSMC) approach that models both electron and ion trajectories to allow us to fully characterize the plasma sheath formed near these surfaces and to understand how the plasma sheath affects the trajectories of CEX ions, their incident energies and angles, and surface sputtering rates. We find that, outside the plasma core, the ion and electron distribution functions are highly non-Maxwellian, and the assumption of electron temperatures is questionable. We introduce a novel floating potential ground boundary condition that enables us to emulate the spacecraft ground for a high range of plasma number densities and surface charging conditions. Finally, we estimate the erosion of the surface using the kinetic results and surface yield empirical relations. [ABSTRACT FROM AUTHOR]

Three-dimensional, fully kinetic, coupled particle-in-cell direct simulation Monte Carlo (PIC-DSMC) simulations are performed to accurately predict the self-consistent electric field and plume and backflow characteristics of an ion thruster, and are compared with results based on the usual Boltzmann approximation. The fully kinetic calculations are performed for the first time using the operational thruster exit number densities without geometric scaling and the actual electron-to-ion mass ratio. It was found that kinetic treatment of both electrons and ions significantly affects the self-consistent radial electric field as well as the backflow ion characteristics. For the xenon ion plume with a colocated electron source, increase in the thruster exit number density strengthens the radial electric field which, in turn, increases the energy of the backflow ions by an order of magnitude. In addition, the spanwise variation of the ion flux on the solar panel showed that charge-exchange ions mostly impinge at the tip of the solar panel, furthest from the thruster geometry, for the fully kinetic cases. In comparison, the Boltzmann relation predicts a lower radial electric field for the operational high number density case, resulting in near-normal ion incidence angles in the backflow region causing significant flux closer to the thruster geometry. [ABSTRACT FROM AUTHOR]

Using molecular dynamics (MD), we have studied the mechanism of heat accommodation between carbon dioxide clusters and monomers for temperatures and cluster size conditions that exist in homogeneous condensing supersonic expansion plumes. The work was motivated by our meso-scale direct simulation Monte Carlo and Bhatnagar-Gross-Krook based condensation simulations where we found that the heat accommodation model plays a key role in the near-field of the nozzle expansion particularly as the degree of condensation increases [R. Kumar, Z. Li, and D. Levin, Phys. Fluids 23, 052001 (2011)]. The heat released by nucleation and condensation and the heat removed by cluster evaporation can be transferred or removed from either the kinetic or translational modes of the carbon dioxide monomers. The molecular dynamics results show that the time required for gas-cluster interactions to establish an equilibrium from an initial state of non-equilibrium is less than the time step used in meso-scale analyses [R. Kumar, Z. Li, and D. Levin, Phys. Fluids 23, 052001 (2011)]. Therefore, the good agreement obtained between the measured cluster and gas number density and gas temperature profiles with the meso-scale modeling using the second energy exchange mechanism is not fortuitous but is physically based. Our MD simulations also showed that a dynamic equilibrium is established by the gas-cluster interactions in which condensation and evaporation processes take place constantly to and from a cluster. [ABSTRACT FROM AUTHOR]

A coalescence model for homogeneous condensation of ammonia in supersonic expansions to vacuum has been developed using molecular dynamics trajectory calculations. The MD calculations show that the sticking probability increases as the ammonia cluster size increases or the cluster temperature decreases. In addition, the sensitivity of the sticking probability to cluster size decreases as the temperature decreases. Comparison of the Ashgriz-Poo semiempirical coalescence model with MD simulations show that for cluster sizes larger than 100 the former model may be used. To model homogeneous nucleation in an ammonia jet, direct simulation Monte Carlo (DSMC) simulations were performed for different stagnation pressure conditions using the MD simulation outcomes for smaller cluster-cluster collisions and the Ashgriz-Poo model for cluster sizes larger than 100. We found that, by including the combined coalescence model, the average cluster sizes and size distributions predicted by DSMC agree reasonably well with experiment. [ABSTRACT FROM AUTHOR]

In the present work, we have simulated the homogeneous condensation of carbon dioxide and ethanol using the Bhatnagar-Gross-Krook based approach. In an earlier work of Gallagher-Rogers et al. [J. Thermophys. Heat Transfer 22, 695 (2008)], it was found that it was not possible to simulate condensation experiments of Wegener et al. [Phys. Fluids 15, 1869 (1972)] using the direct simulation Monte Carlo method. Therefore, in this work, we have used the statistical Bhatnagar-Gross-Krook approach, which was found to be numerically more efficient than direct simulation Monte Carlo method in our previous studies [Kumar et al., AIAA J. 48, 1531 (2010)], to model homogeneous condensation of two small polyatomic systems, carbon dioxide and ethanol. A new weighting scheme is developed in the Bhatnagar-Gross-Krook framework to reduce the computational load associated with the study of homogeneous condensation flows. The solutions obtained by the use of the new scheme are compared with those obtained by the baseline Bhatnagar-Gross-Krook condensation model (without the species weighting scheme) for the condensing flow of carbon dioxide in the stagnation pressure range of 1-5 bars. Use of the new weighting scheme in the present work makes the simulation of homogeneous condensation of ethanol possible. We obtain good agreement between our simulated predictions for homogeneous condensation of ethanol and experiments in terms of the point of condensation onset and the distribution of mass fraction of ethanol condensed along the nozzle centerline. [ABSTRACT FROM AUTHOR]

Recent direct simulation Monte Carlo (DSMC) simulations of homogeneous condensation in free expansion water plumes [Z. Li, J. Zhong, D. A. Levin, and B. Garrison, AIAA J. 47, 1241 (2009)] show that the nucleation rate is a key factor for accurately modeling condensation phenomenon. In this work, we use molecular dynamics (MD) simulations of a free expansion to explore the microscopic mechanisms of water dimer formation and develop collision models required by DSMC. Bimolecular and termolecular dimer cluster formation mechanisms are considered and the former is found to be the main mechanism in expanding flows to vacuum. MD simulations between two water molecules using the simple point charge intermolecular potential were performed to predict the bimolecular dimer formation probability and the probability was found to decrease with collision energy. The formation probabilities and postcollisional velocity and energy distributions were then integrated into DSMC simulations of a free expansion of an orifice condensation plume with different chamber stagnation temperatures and pressures. The dimer mole fraction was found to increase with distance from the orifice and become constant after a distance of about two orifice diameters. Similar to experiment, the terminal dimer mole fraction was found to decrease with chamber stagnation temperatures and increase linearly with chamber stagnation pressures which is consistent with a bimolecular nucleation mechanism. [ABSTRACT FROM AUTHOR]

Highlights • Predicted gas transport properties using an integrated kinetic computational framework. • Analyzed the effect of material anisotropy on the permeability and pore-scale velocity. • Pore-diameter computed in this work agreed well with tomography derived data. • Determined representative elementary volume (REV) required for material characterization studies. Abstract We use the Cuda-based Hybrid Approach for Octree Simulations (CHAOS) DSMC solver to predict gas transport coefficients of Morgan felt and FiberForm TPS materials with sample size of (1 × 1 × 1) mm3. The detailed velocity flow-field of the pressure-driven flow through these materials is studied to compare the effect of material microstructures on gas transport. It is found that the effective flow path traversed by the gas is more circuitous and longer for FiberForm compared to the more porous felt. The obstruction offered by the material and the circuitous flow path is quantified by the Klinkenberg-derived permeability and hydraulic tortuosity factor, which are key material properties that govern the momentum transport through porous media. We also compute the hydraulic pore diameter of these materials and find that the through-thickness and in-plane pore diameter is equal to 86.94 and 98.7 μ m for felt and 36.25 and 60.9 μ m for Fiberform, which is within 5–6% of the average pore-size obtained from the tomography images. [ABSTRACT FROM AUTHOR]

Unsteadiness of axisymmetric shock-dominated hypersonic laminar separated flow over a double cone is studied for the first time using a combination of time accurate Direct Simulation Monte Carlo (DSMC) calculations, linear global instability analysis, and momentum potential theory (MPT). Close to steady state linear analysis reveals the spatial structure of the underlying temporally stable global modes. At all Reynolds numbers examined, the amplitude functions demonstrate the strong coupling between the separated flow region at the cone junction with the entire shock system, including pressure and temperature waves generated behind the shock and spatially amplified Kelvin-Helmholtz waves. In addition, as the Reynolds number is increased, temporally damped harmonic shock oscillations and multiple-reflected λ-shock patterns emerge in the eigenfunctions. Application of the MPT (valid for both linear and nonlinear signals) to the highest Reynolds number DSMC results shows that large acoustic and thermal potential variations exist in the vicinity of the separation shock, the λ-shock patterns, and the shear layers. It is further shown that the motion of the bow shock system is highly affected by non-uniformities in the acoustic field. At the highest Reynolds number considered here, the unsteadiness is characterized by Strouhal numbers in the shear layer and bow-shock regions and is found to be in qualitative agreement with earlier experimental and numerical work. [ABSTRACT FROM AUTHOR]

There is significant scientific interest in simulating the unique atmospheric conditions on the Jovian moon Io that range from cold surface temperatures to hyperthermal interactions which possibly supply the Jovian plasma torus. The Direct Simulation Monte Carlo (DSMC) method is well suited to model the rarefied, predominantly SO2, Ionian atmosphere. High speed collisions between SO2 and the hypervelocity O atoms and ions that compose the plasma torus are a significant mechanism in determining the composition of the atmosphere; therefore, high-fidelity modeling of their interactions is crucial to the accuracy of such simulations. Typically, the Total Collision Energy (TCE) model is used to determine molecular dissociation probabilities and the Variable Hard Sphere (VHS) model is used to determine collision cross sections. However, the parameters for each of these baseline models are based on low-temperature experimental data and thus have unknown reliability for the hyperthermal conditions in the Ionian atmosphere. Recently, Molecular Dynamics/Quasi-Classical Trajectory (MD/QCT) studies have been conducted to generate accurate collision and chemistry models for the SO2-O collision pair in order to replace the baseline models. However, the influence of MD/QCT models on Ionian simulations compared to the previously used models is not well understood. In this work, 1D simulations are conducted using both the MD/QCT-based and baseline models in order to determine the effect of MD/QCT models on Ionian simulations. It is found that atmospheric structure predictions are highly sensitive to the chemistry and collision models. Specifically, the MD/QCT model predicts approximately half the SO2 atmospheric dissociation due to O and O+ bombardment compared to TCE models, and also predicts a temperature rise due to plasma heating further from the Ionian surface than the existing baseline methodologies. These findings indicate that the accurate MD/QCT chemistry and collision models provide a significant improvement over the baseline models for DSMC simulations of the Ionian atmosphere. [ABSTRACT FROM AUTHOR]

The modeling of hypersonic radiation in non-equilibrium, non-continuum flows is considered in the framework of the direct simulation Monte Carlo (DSMC) approach. The study explores the influence of electronic states on the flow chemistry and degree of ionization as well as the assumption that the electronic states can be described by a steady state solution to a system of rate equations of excitation, de-excitation, and radiative transfer processes. The work implements selected excited levels of atomic nitrogen and oxygen and the corresponding electron impact excitation/de-excitation and ionization processes in DSMC. The simulations show that when excitation models are included, the degree of ionization in the Stardust transitional re-entry flow increases due to additional intermediate steps to ionization. The extra ionization reactions consume the electron energy to reduce the electron temperature. The DSMC predicted excited state level populations are lower than those predicted by a quasi steady state calculation, but the differences can be understood in terms of the flow distribution functions. [ABSTRACT FROM AUTHOR]

In this work, we propose a new heat accommodation model to simulate freely expanding homogeneous condensation flows of gaseous carbon dioxide using a new approach, the statistical Bhatnagar-Gross-Krook method. The motivation for the present work comes from the earlier work of Li et al. [J. Phys. Chem. 114, 5276 (2010)] in which condensation models were proposed and used in the direct simulation Monte Carlo method to simulate the flow of carbon dioxide from supersonic expansions of small nozzles into near-vacuum conditions. Simulations conducted for stagnation pressures of one and three bar were compared with the measurements of gas and cluster number densities, cluster size, and carbon dioxide rotational temperature obtained by Ramos et al. [Phys. Rev. A 72, 3204 (2005)]. Due to the high computational cost of direct simulation Monte Carlo method, comparison between simulations and data could only be performed for these stagnation pressures, with good agreement obtained beyond the condensation onset point, in the farfield. As the stagnation pressure increases, the degree of condensation also increases; therefore, to improve the modeling of condensation onset, one must be able to simulate higher stagnation pressures. In simulations of an expanding flow of argon through a nozzle, Kumar et al. [AIAA J. 48, 1531 (2010)] found that the statistical Bhatnagar-Gross-Krook method provides the same accuracy as direct simulation Monte Carlo method, but, at one half of the computational cost. In this work, the statistical Bhatnagar-Gross-Krook method was modified to account for internal degrees of freedom for multi-species polyatomic gases. With the computational approach in hand, we developed and tested a new heat accommodation model for a polyatomic system to properly account for the heat release of condensation. We then developed condensation models in the framework of the statistical Bhatnagar-Gross-Krook method. Simulations were found to agree well with the experiment for all stagnation pressure cases (1-5 bar), validating the accuracy of the Bhatnagar-Gross-Krook based condensation model in capturing the physics of condensation. [ABSTRACT FROM AUTHOR]

A kinetic-based quasi-steady-state model is developed to study sodium radiation intensity for reentry high-altitude flows of the blunt-body Stardust and a slender body. The sodium detected in the high-speed Stardust-reentry flow was from an impurity of the ablative thermal protection material. In this work, the particle-based direct simulation Monte Carlo method is used to obtain flow solutions, and a quasi-steady-state model is used to derive the sodium number-density distributions in the ground and excited electronic states. The sodium radiation intensity is calculated based on the sodium number-density distributions in the excited states and agrees reasonably well with the observation data for the Stardust reentry at 81 km. It was also found that sodium radiation in the slender-body reentry flow will be orders of magnitude less than the value observed for the blunt body. [ABSTRACT FROM AUTHOR]

Abstract: Validation of global numerical models of planetary atmospheres requires simulating images and spectra from the IR to UV spectral regions in order to compare them with remote observations. This paper describes Rassvet, a 3-D spherical-shell backward Monte Carlo radiative transfer model developed for such simulations. It utilizes a new methodology for calculating atmospheric brightness in scattered sunlight by introducing the concept of an “effective emission source”. This allows for the accumulation of the scattered contribution along the entire path of a ray and the calculation of the atmospheric radiation when both scattered sunlight and thermal emission contribute to the remote measurement – which was not possible in previous models. A “polychromatic” algorithm is extended for applications with the backward Monte Carlo method and implemented in the model. It allows for the calculation of radiative intensity for several wavelengths simultaneously, resulting in improved efficiency. The capabilities of the model are demonstrated by simulating remote measurements from the atmosphere of Io. [Copyright &y& Elsevier]

In previous work, a cluster evaporation model was derived from the classical nucleation theory (CNT) to simulate condensation in free expanding plumes using the direct simulation Monte Carlo (DSMC) method. However, the use of a CNT evaporation model, especially in a low temperature environment, is problematic because macroparameters such as cluster surface tension and vapor saturation pressure are not physical for small cluster sizes. In this work, we propose a kinetic based evaporation model obtained from unimolecular dissociation theory (UDT) to model argon cluster evaporation processes in a free expanding plume. The UDT argon cluster evaporation model has been directly verified by molecular dynamics and quasiclassical trajectory simulations. It is found that although there is about one order of magnitude difference in the CNT and UDT evaporation rates, these two theories predict similar cluster evaporation rate trends as a function of cluster size and temperature. The verified new UDT evaporation model, as well as the previous CNT model, are applied to a free expanding argon condensation plume simulated with DSMC. The simulation results show that although there are some differences in cluster number density and average cluster size using the CNT and UDT evaporation models, the condensation onset conditions and Rayleigh scattering intensity for both models agree reasonably well with experimental data. [ABSTRACT FROM AUTHOR]

A collision-limiter method, designated as equilibrium direct simulation Monte Carlo (eDSMC), is proposed to extend the DSMC technique to high pressure flows. The method is similar to collision-limiter schemes considered in the past with the important distinction that for inviscid flows, equilibrium is enforced in the entire flow by providing a sufficient number of collisions, based on pre-simulation testing. To test the method with standard DSMC and Navier-Stokes (NS) methods, axi-symmetric nozzle and embedded-channel flows are simulated and compared with experimental temperature data and pre-existing calculations, respectively. The method is shown to agree with third-order Eulerian nozzle flows and first-order channel flows. Chapman-Enskog theory is utilized to predict the range of initial conditions where eDSMC is potentially useful for modeling flows that contain viscous boundary layer regions. Comparison with supersonic nozzle data suggests that the eDSMC method is not adequate for capturing the large variation in flow length scales occurring in supersonic expansions into a vacuum. However, when eDSMC is used in combination with the baseline-DSMC method a near-exact solution is obtained with a considerable computational savings compared to the exact DSMC solution. Viscous flow channel calculations are found to agree well with an exact Navier-Stokes (NS) calculation for a small Knudsen number case as predicted by Chapman-Enskog theory. [ABSTRACT FROM AUTHOR]

Time-dependent performance of a high-temperature MEMS-based thruster is studied in detail by a coupled thermal-fluid analysis. The material thermal response governed by the transient heat conduction equation is obtained using the finite element method. The low-Reynolds number gas flow in the microthruster is modeled by the direct simulation Monte Carlo (DSMC) approach. The temporal variation of the thruster material temperature and gas flowfields are obtained as well as the thruster operational time limits for thermally insulated and convectively cooled thrusters. The predicted thrust and mass discharge coefficient of both two-dimensional (2-D) and three-dimensional (3-D) micronozzles decreases in time as the viscous losses increase for higher wall temperatures. [ABSTRACT FROM AUTHOR]

Gaseous flow through a microchannel is treated numerically and analytically in order to assess pressure and mass flow losses due to a constriction of a finite length. Numerical modeling of two-dimensional (2-D) microchannel flow in the slip and transitional regimes is carried out using the direct simulation Monte Carlo (DSMC) method. The prediction of pressure losses and mass flow based on a simple analytic model for constriction microchannel flow are found to be in excellent agreement with DSMC simulations. Constriction has a dramatic effect on pressure loss and mass flow rate for the considered cases. The DSMC results indicate that the flow in the constriction microchannel separates in the transition section, but the separation does not significantly impact pressure distributions. [ABSTRACT FROM AUTHOR]

Using classical nucleation theory (CNT), a homogeneous condensation model has been developed in previous work to simulate condensation in a free-expanding plume using the direct simulation Monte Carlo (DSMC) method. However, the accuracy of the CNT nucleation theory is questionable due to unphysical assumptions such as the use of macroscopic cluster surface tension for small clusters and the lack of an incubation time for nucleation. Molecular dynamics (MD) simulations performed in previous work confirm that the fundamental mechanism for the initiation of condensation is through dimer formation in two-stage ternary collisions of monomers. In this work we propose a kinetic nucleation model based on the mechanism whereby stable dimers are created from triple collisions. The new hybrid MD-DSMC kinetic nucleation model is implemented in a DSMC simulation of cluster growth processes, starting from dimers, in a condensation plume. Comparison of the distributions of steady-state plume cluster number density and size using the kinetic and CNT nucleation models shows that the condensation physics is significantly different for the two nucleation models investigated. The use of a physically more realistic nucleation model is shown to generate terminal cluster properties consistent with experiments in free-expanding jets. [ABSTRACT FROM AUTHOR]

Abstract: Conflicting observations regarding the dominance of either sublimation or volcanism as the source of the atmosphere on Io and disparate reports on the extent of its spatial distribution and the absolute column abundance invite the development of detailed computational models capable of improving our understanding of Io’s unique atmospheric structure and origin. Improving upon previous models, Walker et al. (Walker, A.C., Gratiy, S.L., Levin, D.A., Goldstein, D.B., Varghese, P.L., Trafton, L.M., Moore, C.H., Stewart, B. [2009]. Icarus) developed a fully 3-D global rarefied gas dynamics model of Io’s atmosphere including both sublimation and volcanic sources of SO2 gas. The fidelity of the model is tested by simulating remote observations at selected wavelength bands and comparing them to the corresponding astronomical observations of Io’s atmosphere. The simulations are performed with a new 3-D spherical-shell radiative transfer code utilizing a backward Monte Carlo method. We present: (1) simulations of the mid-infrared disk-integrated spectra of Io’s sunlit hemisphere at 19μm, obtained with TEXES during 2001–2004; (2) simulations of disk-resolved images at Lyman-α obtained with the Hubble Space Telescope (HST), Space Telescope Imaging Spectrograph (STIS) during 1997–2001; and (3) disk-integrated simulations of emission line profiles in the millimeter wavelength range obtained with the IRAM-30m telescope in October–November 1999. We found that the atmospheric model generally reproduces the longitudinal variation in band depth from the mid-infrared data; however, the best match is obtained when our simulation results are shifted ∼30° toward lower orbital longitudes. The simulations of Lyman-α images do not reproduce the mid-to-high latitude bright patches seen in the observations, suggesting that the model atmosphere sustains columns that are too high at those latitudes. The simulations of emission line profiles in the millimeter spectral region support the hypothesis that the atmospheric dynamics favorably explains the observed line widths, which are too wide to be formed by thermal Doppler broadening alone. [Copyright &y& Elsevier]

Highlights • DSMC study of the effect of AVCOAT-like microstructure on internal gas transport. • Permeability and tortuosity computation for AVCOAT and comparison with fibrous TPS. • Coupled convection, conduction, and radiation using a random walk model. • Study of AVCOAT with spatially varying thermophysical properties at high temperatures. • Comparison of thermal response predicted by stochastic versus finite-volume approaches. Abstract A multi-scale modeling approach based on the stochastic Direct Simulation Monte Carlo (DSMC) and walker methods is developed to understand the complex flows and thermophysical phenomena through a syntactic foam TPS, similar to AVCOAT. Using novel, unstructured adaptive mesh refinement/Octree grids and newly developed subsonic boundary conditions, the counter-flow transport of boundary layer and pyrolysis gases through the porous microstructure is modeled for the first time. Permeability of the microstructure models having porosities of 0.71 and 0.86 is computed in the DSMC simulations and compared to a fibrous TPS material. The rigorous development of a stochastic based thermal response model that can couple convective, conductive, and radiative heat transfer through a porous material having non-uniform thermophysical properties and high temperature gradients is presented and compared with a one-dimensional finite-volume approach. The material thermal response is found to be dominated by conduction, yet, the interactions between boundary layer and pyrolysis species on the actual 3-D geometry, which cannot be considered in traditional material response solvers, may in fact cause them to underpredict the TPS material temperature. [ABSTRACT FROM AUTHOR]

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 quasisteady state 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 rovibrational 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 nearwake 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 nearwake 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. [ABSTRACT FROM AUTHOR]

Abstract: Io’s sublimation-driven atmosphere is modeled using the direct simulation Monte Carlo (DSMC) method. These rarefied gas dynamics simulations improve upon earlier models by using a three-dimensional domain encompassing the entire planet computed in parallel. The effects of plasma heating, planetary rotation, inhomogeneous surface frost, molecular residence time of SO2 on the exposed (non-volatile) rocky surface, and surface temperature distribution are investigated. Circumplanetary flow is predicted to develop from the warm dayside toward the cooler nightside. Io’s rotation leads to a highly asymmetric frost surface temperature distribution (due to the frost’s high thermal inertia) which results in circumplanetary flow that is not axi-symmetric about the subsolar point. The non-equilibrium thermal structure of the atmosphere, specifically vibrational and rotational temperatures, is also examined. Plasma heating is found to significantly inflate the atmosphere on both the dayside and nightside. The plasma energy flux causes high temperatures at high altitudes but plasma energy depletion through the dense gas column above the warmest frost permits gas temperatures cooler than the surface at low altitudes. A frost map (Douté, S., Schmitt, B., Lopes-Gautier, R., Carlson, R., Soderblom, L., Shirley, J., and the Galileo NIMS Team [2001]. Icarus 149, 107–132) is used to control the sublimated flux of SO2 which can result in inhomogeneous column densities that vary by nearly a factor of four for the same surface temperature. A short residence time for SO2 molecules on the “rock” component is found to smooth lateral atmospheric inhomogeneities caused by variations in the surface frost distribution, creating an atmosphere that looks nearly identical to one with uniform frost coverage. A longer residence time is found to agree better with mid-infrared observations (Spencer, J.R., Lellouch, E., Richter, M.J., López-Valverde, M.A., Jessup, K.L, Greathouse, T.K., Flaud, J. [2005]. Icarus 176, 283–304) and reproduce the observed anti-jovian/sub-jovian column density asymmetry. The computed peak dayside column density for Io assuming a surface frost temperature of 115K agrees with those suggested by Lyman-α observations (Feaga, L.M., McGrath, M., Feldman, P.D. [2009]. Icarus 201, 570–584). On the other hand, the peak dayside column density at 120K is a factor of five larger and is higher than the upper range of observations (Jessup, K.L., Spencer, J.R., Ballester, G.E., Howell, R.R., Roesler, F., Vigel, M., Yelle, R. [2004]. Icarus 169, 197–215; ). [Copyright &y& Elsevier]

A numerical model is developed to consider for the first time droplet coalescence along with transport, heating and desolvation in an argon inductively coupled plasma (Ar ICP). The direct simulation Monte Carlo (DSMC) method and the Ashgriz–Poo model are used, respectively, to compute droplet–droplet interactions and to determine the outcome of droplet collisions. Molecular dynamics (MD) simulations support the use of the Ashgriz–Poo coalescence model for small droplet coalescence. Simulations predict spatial maps of droplet number and mass densities within an Ar ICP for a conventional nebulizer-spray chamber arrangement, a direct injection high efficiency nebulizer (DIHEN), and a large bore DIHEN (LB-DIHEN). The primary findings are: (1) even at 1500 W, the collisions of the droplets in the plasma lead primarily to coalescence, particularly for direct aerosol injection; (2) the importance of coalescence in a spray simulation exhibits a complex relationship with the gas temperature and droplet size; (3) DIHEN droplets penetrate further into the Ar ICP when coalescence is considered; and (4) droplets from a spray chamber or the LB-DIHEN coalesce less frequently than those from a DIHEN. The implications of these predictions in spectrochemical analysis in ICP spectrometry are discussed. [Copyright &y& Elsevier]