Deepak Bose

Data-Parallel Line Relaxation Method for the Navier -Stokes Equations

The Gauss‐Seidel line relaxation method is modie ed for the simulation of viscous e ows on massively parallel computers. The resulting data-parallel line relaxation method is shown to have good convergence properties for a seriesoftestcases.Thenewmethodrequiressignie cantlymorememorythanthepreviouslydevelopeddata-parallel relaxation methods, but it reaches a steady-state solution in much less time for all cases tested to date. In addition, the data-parallel line relaxation method shows good convergence properties even on the high-cell-aspect-ratio grids required to simulate high-Reynolds-number e ows. The new method is implemented using message passing on the Cray T3E, and the parallel performance of the method on this machine is discussed. The data-parallel line relaxation method combines the fast convergence of the Gauss ‐Seidel line relaxation method with a high parallel efe ciency and thus shows promise for large-scale simulation of viscous e ows.

Recommended Collision Integrals for Transport Property Computations, Part 1: Air Species

Ar eview of the best-available data for calculating a complete set of binary collision integral data for the computation of the mixture transport properties (viscosity, thermal conductivity, and ordinary and thermal diffusion) of 13-species weakly ionized air is presented. Although the fidelity of the data varies, all collision integrals presented herein, except for electron-neutral interactions, are estimated to be accurate to within 25% over the temperature range of interest (300‐15,000 K) for reentry and laboratory plasmas. In addition, most of the dominant atom‐atom and atom‐ion interactions for dissociated weakly ionized air were derived from ab initio methods that are estimated to be accurate to within 10%. The accuracy and valid temperature range for electron-neutral interactions vary because of scarcity of the required cross-sectional data.

Thermal rate constants of the N2+O→NO+N reaction using ab initio 3A″ and 3A′ potential energy surfaces

Theoretical determinations of the thermal rate constants and product energy distributions of the N2+O→NO+N reaction, which plays a crucial role in hydrocarbon air combustion and high temperature air chemistry, are carried out using a quasiclassical trajectory method. An analytical fit of the lowest 3A′ potential energy surface of this reaction based on the CCI ab initio data is obtained. The trajectory study is done on this surface and an analytical 3A″ surface proposed by Gilibert et al. [J. Chem. Phys. 97, 5542 (1992)]. The thermal rate constants computed from 3000 to 20 000 K are in good agreement with the available experimental data. In addition, the dependence of the rate constant on the N2 internal state is studied. It is found that a low vibrational excitation can reduce the rate constant of this reaction by a factor of 3. Also, we investigate the effect of the N2 vibrational state on the product NO vibrational distribution, and it is found that at low N2 vibrational states, the NO vibrational dist...

Uncertainty Analysis of Laminar Aeroheating Predictions for Mars Entries

A Monte Carlo sensitivity and uncertainty analysis is performed for a laminar convective heating prediction in a moderate Mars atmospheric entry condition using a nonequilibrium reacting Navier-Stokes computational fluid dynamics code. The objectives are to isolate the rate limiting mechanisms and identify the chief sources of aeroheating uncertainty. A flux-based wall catalysis formulation is developed and used to define four different catalytic regimes that are then individually analyzed at three different trajectory points. A total of 130 input parameters are statistically varied to short list a handful of parameters that essentially control the heat flux prediction. The uncertainties in these key input parameters are estimated, and a full Monte Carlo uncertainty analysis is performed. The results obtained provide the quantitative contribution of uncertainties in key modeling parameters, such as collision integrals, wall catalysis, and reaction rates to the final heat flux uncertainty. It is found that in high and low catalytic regimes, the collision integrals (which govern the transport properties of the mixture) contribute a large portion of the uncertainty, whereas in the moderately catalytic regime the catalytic properties of the surface contribute almost all of the uncertainty.

Thermal rate constants of the O2+N→NO+O reaction based on the A2′ and A4′ potential-energy surfaces

A detailed quasiclassical trajectory study of the O2+N→NO+O reaction is performed based on ab initio potential-energy surfaces of the 2A′ and 4A′ states. The study is aimed at generating a database of thermally averaged and O2 state-specific rate constants needed for accurate simulations of NO kinetics in high-temperature flow processes. The rate constants obtained show good agreement with the available experimental data and with other quasiclassical trajectory calculations. It is found that the reactant internal energy of the O2+N→NO+O reaction is less effective in enhancing the rate than in the N2+O→NO+N reaction. An analysis of the product vibrational energy shows that NO formed by the O2+N→NO+O reaction has a non-Boltzmann distribution. It is also found that the most populated NO vibrational level is determined by the reactant vibrational energy, while the terminal slope of the NO vibrational distribution is a strong function of the reactant translational temperature.

Impact of Flowfield-Radiation Coupling on Aeroheating for Titan Aerocapture

A methodology is developed that enables fully coupled computation of three-dimensional flow fields including radiation, assuming an optically thin shock layer. The method can easily be incorporated into existing computational fluid dynamics codes and does not appreciably increase the cost or affect the robustness of the resulting simulations. Further improvements in the accuracy of radiative heating predictions in an optically thin gas can be achieved by using a view-factor method rather than the standard tangent slab approach. These techniques are applied to the Titan aerocapture aeroheating problem, which is dominated by strong radiative heating. For this application, neglecting the nonadiabatic effects caused by radiation coupling results in an overprediction of radiative heating levels by about a factor of 2. Radiative coupling effects also significantly lower predicted convective heating by reducing boundary-layer edge temperatures. In addition, it is shown that the tangent slab approximation overpredicts radiative heating levels by a minimum of 20% in the stagnation region for this application. Over an entire design trajectory, correctly modeling radiative heat transfer results in a more than a factor of 2 reduction in total stagnation-region heat load over an uncoupled analysis.

Probabilistic Modeling of Aerothermal and Thermal Protection Material Response Uncertainties

A Monte-Carlo-based methodology is presented for physics-based probabilistic uncertainty analysis of aerothermodynamics and thermal protection system (TPS) material response modeling for aerocapture or direct entry missions. The objective of the methodology is to identify and quantify the most important sources of uncertainty in aeroheating and the resulting thermal protection material selection, design, and sizing based on inaccuracies in current knowledge of the parametric input modeling parameters. The resulting parametric modeling uncertainty would be combined with other uncertainty sources to determine the final aeroheating and TPS response modeling uncertainty for a given application, which can then be used to define appropriate margins and factors of safety that should be applied to the TPS. These techniques facilitate a risk-based probabilistic design approach, whereby the thermal protection system can be designed to a desired risk tolerance, and any remaining risk can be effectively compared to that of other subsystems via a system-level risk mitigation analysis. Modeling sensitivities, which are a byproduct of the uncertainty analysis, can be used to rank input uncertainty drivers. Key input uncertainties can then be prioritized and targeted for further analysis or testing. The strengths and limitations of this technique are discussed. Sample results are presented for two cases: Titan aerocapture and Mars Pathfinder. These cases demonstrate the utility of the methodology to quantify the uncertainty levels, rank sources of input uncertainty, and assist in the identification of structural uncertainties in the models employed.

Uncertainty and Sensitivity Analysis of Thermochemical Modeling for Titan Atmospheric Entry

A Monte Carlo uncertainty and sensitivity analysis technique is presented to i) identify the major sources of uncertainty in the thermochemical models used for aerothermal analysis, and ii) track the propagation of these uncertainties through the system into the predicted quantities of interest, such as the vehicle heating, shock layer properties, etc. The technique is applied to the aerothermal analysis of Titan aerocapture, where CN shock layer radiation is the dominant source of vehicle heating. Several hundred model input parameters, including reaction rate constants, vibration-chemistry coupling parameters, vibrational relaxation times, and transport properties, are independently sampled over their range of uncertainties, and the vehicle heating is determined probabilistically. A massively parallel, axisymmetric CFD (Data-Parallel Line Relaxation) code was used to make the several thousand runs needed to statistically describe the variability in the heating predictions. It is found that major contributions to the uncertainty in the predicted heating originates from the uncertainties in the rates of N2 dissociation by H atom impact, and some atomic exchange reactions: N2+H→NH+N and N2+C→CN+N.

Analysis and Model Validation of Shock Layer Radiation in Air

This paper analyzes the shock layer radiative heating environment for a large entry vehicle on a lunar return trajectory. Modeling results show that much of the shock layer plasma is in local thermodynamic equilibrium (LTE) and is not optically thin. The ionization level is generally high (15%) and the air is almost fully dissociated. A significant amount of vacuum ultraviolet (VUV) radiation is produced due to bound-bound and bound-free transitions of N and O atoms. The sensitivity of total radiation to Stark broadening, which dominates over other line broadening mechanisms, is quantified. The latter part of this paper reports the status of ongoing validation of the current radiation models with measurements in the Electric-Arc Shock Tube (EAST) facility at NASA Ames Research Center. Model predictions are compared with the calibrated radiation spectra measured in the equilibrium portion of the shock layer at 0.3 Torr. The reasons for discrepancy between model and measurements are also discussed with possible hypotheses presented for further investigation.

Mars Science Laboratory Heat Shield Aerothermodynamics: Design and Reconstruction

The Mars Science Laboratory heat shield was designed to withstand a fully turbulent heat pulse using information from ground testing and computational analysis on a preflight design trajectory. Instrumentation on the flight heat shield measured in-depth temperatures to permit reconstruction of the surface heating. The data indicate that boundary-layer transition occurred at five of seven measurement locations before peak heating. Data oscillations at three pressure measurement locations may also indicate transition. This paper presents the heat shield temperature and pressure data, possible explanations for the timing of boundary-layer transition, and a comparison of reconstructed and computational heating on the actual trajectory. A smooth-wall boundary-layer Reynolds number that was used to predict transition is compared with observed transition at various heat shield locations. A single transition Reynolds number criterion does not uniformly explain the timing of boundary-layer transition observed duri...