Aaron M. Brandis

Shock Layer Radiation Modeling and Uncertainty for Mars Entry

A model for simulating nonequilibrium radiation from Mars entry shock layers is presented. A new chemical kinetic rate model is developed that provides good agreement with recent EAST and X2 shock tube radiation measurements. This model includes a CO dissociation rate that is a factor of 13 larger than the rate used widely in previous models. Uncertainties in the proposed rates are assessed along with uncertainties in translational-vibrational relaxation modeling parameters. The stagnation point radiative ux uncertainty due to these oweld modeling parameter uncertainties is computed to vary from 50 to 200% for a range of free-stream conditions, with densities ranging from 5e-5 to 5e-4 kg/m 3 and velocities ranging from of 6.3 to 7.7 km/s. These conditions cover the range of anticipated peak radiative heating conditions for proposed hypersonic inatable aerodynamic decelerators (HIADs). Modeling parameters for the radiative spectrum are compiled along with a non-Boltzmann rate model for the dominant radiating molecules, CO, CN, and C2. A method for treating non-local absorption in the non-Boltzmann model is developed, which is shown to result in up to a 50% increase in the radiative ux through

Nonequilibrium radiation intensity measurements in simulated Titan atmospheres

This paper details the experimental work conducted at the University of Queensland to measure the nonequilibrium radiation intensity behind a shock in simulated Titan atmospheres, as would be seen during planetary entry. Radiation during Titan entry is more important at lower speeds (about 5-6 km/s) than other planetary entries due to the formation of cyanogen in above equilibrium concentrations in the shock layer, which is a highly radiative species. The experiments were focused on measuring the nonequilibrium radiation emitted from cyanogen between the wavelength range of 310-450 nm. This paper includes experimental results for radiation and spectra found in the postshock region of the flow. Experiments have been conducted at various ambient pressures, shock speeds, and chemical compositions. This leads to a comprehensive benchmark data set for Titan entry, which will be useful for validation of theoretical models. Spectra were recorded at various axial locations behind the shock, enabling the construction of radiation profiles for Titan entry. Furthermore, wavelength profiles can also be constructed to identify various radiating species, in this case, predominately cyanogen violet. Furthermore, this paper includes comparisons with experiments performed at NASA Ames Research Center on their electric arc-driven shock tube in Titan compositions. Excellent quantitative agreement has been obtained between the two facilities.

Assessment of Radiative Heating Uncertainty for Hyperbolic Earth Entry

This paper investigates the shock-layer radiative heating uncertainty for hyperbolic Earth entry, with the main focus being a Mars return. In Part I of this work, a baseline simulation approach involving the LAURA Navier-Stokes code with coupled ablation and radiation is presented, with the HARA radiation code being used for the radiation predictions. Flight cases representative of peak-heating Mars or asteroid return are de ned and the strong influence of coupled ablation and radiation on their aerothermodynamic environments are shown. Structural uncertainties inherent in the baseline simulations are identified, with turbulence modeling, precursor absorption, grid convergence, and radiation transport uncertainties combining for a +34% and ..24% structural uncertainty on the radiative heating. A parametric uncertainty analysis, which assumes interval uncertainties, is presented. This analysis accounts for uncertainties in the radiation models as well as heat of formation uncertainties in the flow field model. Discussions and references are provided to support the uncertainty range chosen for each parameter. A parametric uncertainty of +47.3% and -28.3% is computed for the stagnation-point radiative heating for the 15 km/s Mars-return case. A breakdown of the largest individual uncertainty contributors is presented, which includes C3 Swings cross-section, photoionization edge shift, and Opacity Project atomic lines. Combining the structural and parametric uncertainty components results in a total uncertainty of +81.3% and ..52.3% for the Mars-return case. In Part II, the computational technique and uncertainty analysis presented in Part I are applied to 1960s era shock-tube and constricted-arc experimental cases. It is shown that experiments contain shock layer temperatures and radiative ux values relevant to the Mars-return cases of present interest. Comparisons between the predictions and measurements, accounting for the uncertainty in both, are made for a range of experiments. A measure of comparison quality is de ned, which consists of the percent overlap of the predicted uncertainty bar with the corresponding measurement uncertainty bar. For nearly all cases, this percent overlap is greater than zero, and for most of the higher temperature cases (T >13,000 K) it is greater than 50%. These favorable comparisons provide evidence that the baseline computational technique and uncertainty analysis presented in Part I are adequate for Mars-return simulations. In Part III, the computational technique and uncertainty analysis presented in Part I are applied to EAST shock-tube cases. These experimental cases contain wavelength dependent intensity measurements in a wavelength range that covers 60% of the radiative intensity for the 11 km/s, 5 m radius flight case studied in Part I. Comparisons between the predictions and EAST measurements are made for a range of experiments. The uncertainty analysis presented in Part I is applied to each prediction, and comparisons are made using the metrics defined in Part II. The agreement between predictions and measurements is excellent for velocities greater than 10.5 km/s. Both the wavelength dependent and wavelength integrated intensities agree within 30% for nearly all cases considered. This agreement provides confidence in the computational technique and uncertainty analysis presented in Part I, and provides further evidence that this approach is adequate for Mars-return simulations. Part IV of this paper reviews existing experimental data that include the influence of massive ablation on radiative heating. It is concluded that this existing data is not sufficient for the present uncertainty analysis. Experiments to capture the influence of massive ablation on radiation are suggested as future work, along with further studies of the radiative precursor and improvements in the radiation properties of ablation products.

Analysis of air radiation measurements obtained in the EAST and X2 shocktube facilities

This paper presents measurements of equilibrium radiation obtained in the NASA Ames Research Centers EAST facility and the University of Queenslands X2 facility. These experiments were aimed at measuring the level of radiation encountered during conditions relevant to Orion lunar return into Earths atmosphere. The facilities have targeted the same nominal test conditions of 10 km/s and 26.6 Pa (0.2 Torr). In addition, variations on the nominal shock speed have also been the focus of recent testing in the EAST facility. A comprehensive comparison between the EAST data and NEQAIR is presented in this paper with preliminary X2 comparisons where appropriate. Since the two facilities have different dimensions, and the tests have different shock speeds, NEQAIR simulations are used as a point of reference for the EAST and X2 comparison. Results obtained by independently reducing the data from both facilities are compared. The present analysis endeavors to provide a better understanding of the uncertainty in the measurements, as well as provide an initial comparison between EAST and X2. Furthermore, the present analysis explores various radiative mechanisms to determine if they are due to physical processes relevant to flight, or are just facility dependent phenomena. These phenomena include effects such as the magnitude of the background continuum.

Analysis of nonequilibrium CN radiation encountered during titan atmospheric entry

The focus of this paper is to analyze the Titan reaction scheme and determine the mechanisms that control the formation and decay of the radiation emitted by the cyanogen molecule (CN) during entry into Titans atmosphere. Through a parametric study of important reactions combined with an investigation into reaction pathways, it has been concluded that the coupling between the dissociation of N(2) and the formation of the CN (through the reaction N(2) + C CN + N) controls the radiation decay rate. The reason for the super equilibrium concentrations (approximately 50% higher than equilibrium) was identified to be a result of the N(2) + C CN +N reaction continuing to overproduce the CN after nominal equilibrium values were reached. This is due to the slow buildup of N to drive the reverse reaction. The absolute level of emitted radiation has been shown to be controlled by the lifetime of the CN(B) state and the excitation of CN(X) to CN(B) by heavy particle impact. Thus, it is the conclusion of this paper that CN radiation is primarily controlled by N(2) dissociation.

Measurement and Characterization of Mid-wave Infrared Radiation in CO2 Shocks

We present the characterization of infrared radiation obtained in the NASA Ames Electric Arc Shock Tube at velocities from 3-7.5 km/s and freestream densities from 4.724 g/m (corresponding to ground test pressures of 0.2 to 1.0 Torr), which are relevant to Mars entry conditions. The IR radiation is shown to decrease with increasing velocity over this range, and is expected to be largest at 3 km/s. Based on this data, an estimated relationship for (near) peak radiative heating is given as 20 W/cm for Mars Science Laboratory, which compares well to the 18 W/cm discrepancy (out of 35 W/cm) observed on its stagnation line sensor. Analysis of the experimental data shows that spectral profiles are predicted well by NEQAIR for most conditions. The only exception is the 2.7 μm band at high temperature, which is underpredicted. Spatially resolved data are used for comparison against proposed kinetic models for CO2 shock environments. No one model matches the data at all conditions, but each may agree over some velocity range. A simplified three reaction model is shown to predict the post-shock decay well, while matching the radiative magnitude to within 30-70% in the velocity range 3.0-5.7 km/s. Attempts to extract reaction rates directly from the data shows the dissociation rate to be controlled by O atom exchange, and show good consistency with published kinetic rates for a velocity of 3 km/s. At higher velocity, the initial kinetics occur in thermal non-equilibrium, suggesting further study of these mechanisms are warranted.

Simulation of radiating CO2-N2 shock layer experiments at hyperbolic entry conditions

Numerical simulations supporting radiating shock layer experiments with a CO2 – N2 test gas in the X2 free-piston impulse facility are presented. A ueq = 9.7 km/s, 1L = 5.4×10−5 kg/m2 expansion tunnel condition and a u1 = 8.5 km/s, 1 = 2.35×10−4 kg/m3 shock tube condition are investigated. Shock layer simulations with the Euler equations, a two-temperature thermal model and coupled nonequilibrium radiation are compared with radiant intensity and temperature profiles derived from emission spectroscopy measurements of the CN Violet band system. Applying the CO2 – N2 reaction scheme modifications proposed by Lee, Park and Chang1 and omitting translation-electron energy exchange is found to give the closest agreement with calibrated intensity measurements.

Numerical Simulation of Radiation Measurements taken in the X2 Facility for Mars and Titan Gas Mixtures

Thermochemical relaxation behind a normal shock in Mars and Titan gas mixtures is simulated using a CFD solver, DPLR, for a hemisphere of 1 m radius; the thermochemical relaxation along the stagnation streamline is considered equivalent to the flow behind a normal shock. Flow simulations are performed for a Titan gas mixture (98% N2, 2% CH4 by volume) for shock speeds of 5.7 and 7.6 km/s and pressures ranging from 20 to 1000 Pa, and a Mars gas mixture (96% CO2, and 4% N2 by volume) for a shock speed of 8.6 km/s and freestream pressure of 13 Pa. For each case, the temperatures and number densities of chemical species obtained from the CFD flow predictions are used as an input to a line-by-line radiation code, NEQAIR. The NEQAIR code is then used to compute the spatial distribution of volumetric radiance starting from the shock front to the point where thermochemical equilibrium is nominally established. Computations of volumetric spectral radiance assume Boltzmann distributions over radiatively linked electronic states of atoms and molecules. The CFD treats metastable states independently. The results of these simulations are compared against experimental data acquired in the X2 facility at the University of Queensland, Australia. The experimental measurements were taken over a spectral range of 310-450 nm where the dominant contributor to radiation is the CN violet band system. In almost all cases, the present approach of computing the spatial variation of post-shock volumetric radiance by applying NEQAIR along a stagnation line computed using a high-fidelity flow solver with good spatial resolution of the relaxation zone is shown to replicate trends in measured relaxation of radiance for both Mars and Titan gas mixtures.

Radiative Heating for MSL Entry: Verification of Simulations from Ground Test to Flight Data

The heat shield of the Mars Science Laboratory (MSL) was equipped with thermocouple stacks to measure in-depth heating of the thermal protection system during atmospheric entry. The heat load derived from the thermocouples in the stagnation region was found to be 33% lower than corresponding post-flight predictions of convective heating alone. It was hypothesized that this difference could be attributed to radiation from the shock-heated gas, a mechanism not considered in pre-flight analyses of flow fields. In order to test the hypothesis and quantify the contribution of shock-layer radiation to total surface heating, ground tests and simulations (both flow and radiation) were performed at several points along the best-estimated entry trajectory of MSL. The present paper provides an assessment of the quality of the radiation model and its impact to stagnation point heating. The impact of radiative heating is shown to account for 43% of the heat load discrepancy. Additional possible factors behind the remaining discrepancy are discussed.

Compositional Dependence of Radiance in CO2/N2/Ar Systems

We report measurements of radiance behind a shock wave in CO2/N2/Ar systems relevant for Martian entry. In particular, the concentration of N2 and Ar are varied to explore the dependencies of chemistry and radiance level on N2 concentration. The study is primarily motivated by the need to improve upon earlier measurements containing 96/4 CO2/N2, while the major constituents of the Martian atmosphere are actually 95.7/2.7/1.6 CO2/N2/Ar by volume. Results show the radiance, except for that of CN, to be weakly sensitive to N2 level in the gas mixture. Electronic temperature is extracted from Planck-limited portions of the spectrum and analyzed for fast and slow relaxation times which are associated with the decomposition of CO2 and CO, respectively. Analysis of CN relaxation is performed via the CN Violet radiation. Relaxation times, found to be weakly dependent upon composition, are compared to times extracted from stagnation line calculations performed with reaction rates recommended by Johnston et al. Disagreements in experimentally inferred relaxation times and those from stagnation line computations prompt a re-examination of rates of certain reactions. Alternate reaction rates are proposed for these reactions which generally produce a better agreement with experiment, but further adjustment is still required.