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Teodoro L.F.A.,Eloret Corporation | Eke V.R.,Durham University | Elphic R.C.,NASA
Geophysical Research Letters | Year: 2010

Although controversial in its physical form, there is mounting evidence of hydrogen enhancements at the lunar poles. The permanently shadowed locales are potential sites for significant concentrations of cold-trapped volatiles, including water ice. We derive maps of the lunar hydrogen distribution near the poles by applying a pixon image reconstruction algorithm to the Lunar Prospector epithermal neutron data coupled with a new map of cold trap locations derived from the KAGUYA (SELENE) altimetry measurements. The results presented in this article require the hydrogen to be concentrated into "cold traps.". Copyright 2010 by the American Geophysical Union.

Young A.P.,University of California at Santa Cruz | Knysh S.,Eloret Corporation | Smelyanskiy V.N.,NASA
Physical Review Letters | Year: 2010

We simulate the quantum adiabatic algorithm (QAA) for the exact cover problem for sizes up to N=256 using quantum Monte Carlo simulations incorporating parallel tempering. At large N, we find that some instances have a discontinuous (first-order) quantum phase transition during the evolution of the QAA. This fraction increases with increasing N and may tend to 1 for N→. © 2010 The American Physical Society.

Norman P.,University of Minnesota | Schwarztentruber T.,University of Minnesota | Cozmuta I.,Eloret Corporation
10th AIAA/ASME Joint Thermophysics and Heat Transfer Conference | Year: 2010

The goal of this work is to model surface catalysis in partially dissociated Air-SiO2 systems, which is of interest for accurately predicting heating on hypersonic vehicles. This is accomplished through molecular dynamics simulations using the ReaxFF potential, which is able to model chemical reactions. The ReaxFF potential is found to accurately reproduce experimental results for the bulk structure of β-quartz SiO2. Potential energy surfaces for oxygen adsorption on β-quartz show that the ReaxFF potential may need further training to reproduce results from quantum chemical calculations. A numerical method for measuring recombination coefficients on a silica surface is developed, and tested for gases at 10 atm and 100 atm over the temperature range (500-2000 K). We find that recombination coefficients for oxygen on quartz are higher than those measured experimentally, however, the trend in recombination coefficients is exponential with temperature as seen in experiment. © 2010 by the American Institute of Aeronautics and Astronautics, Inc.

Lachaud J.,Reacting Flow Environments Branch | Cozmuta I.,Eloret Corporation | Mansour N.N.,Mail Stop
Journal of Spacecraft and Rockets | Year: 2010

Amultiscale approach is used to model and analyze the ablation of porous materials. Models are developed for the oxidation of a carbon preform and of the char layer of two phenolic impregnated carbon ablators with the same chemical composition but with different structures. Oxygen diffusion through the pores of the materials and in depth oxidation and mass loss are first modeled at the microscopic scale. The microscopic model is then averaged to yield a set of partial differential equations describing the macroscopic behavior of the material. Microscopic and macroscopic approaches are applied with progressive degrees of complexity to gain a comprehensive understanding of the ablation process. Porous medium ablation is found to occur in a zone of the char layer that we call the ablation zone. The thickness of the ablation zone is a decreasing function of the Thiele number. The studied materials are shown to display different ablation behaviors, a fact not captured by current models that are based on chemical composition only. Applied to Stardust's phenolic impregnated carbon ablator, the models explain and reproduce the unexpecteddrop in density measured in the char layer during Stardust postflight analyses [Stackpoole, M., Sepka, S., Cozmuta, I., and Kontinos, D., "Post-Flight Evaluation of StardustSample Return Capsule Forebody Heat-Shield Material," AIAA Paper 2008-1202, Jan. 2008]. © Clearance Center, Inc.

Trumble K.A.,NASA | Cozmuta I.,Eloret Corporation | Sepka S.,Eloret Corporation | Jenniskens P.,Search for Extraterrestrial Intelligence Institute | Winter M.,NASA
Journal of Spacecraft and Rockets | Year: 2010

The reentry of the Stardust sample return capsule was captured by several optical instruments through an observation campaign aboard the NASA DC-8 airborne observatory. Flow environments obtained from computational fluid dynamics solutions are loosely coupled with material response modeling to predict the surface temperature and the observed continuum emission of Stardust throughout the reentry. The calculated surface temperatures are compared with the data from several spectral instruments onboard the airborne observatory, including the ECHELLE (echelle-based spectrograph for the crisp and high efficient detection of low light emission) camera and conventional spectrometer in Czerny-Turner configuration. The ECHELLE camera recorded spectral intensity at a period in the trajectory before peak heating. The graybody curves corresponding to the average and area-averaged surface temperatures predicted by the computational fluid dynamics and material response coupled simulation have excellent agreement with the recorded data at altitudes lower than 74 km. At these altitudes, the computational fluid dynamics and material response coupling agrees with the surface temperature to within 50 K. The computational fluid dynamics calculation without the material response modeling overestimates surface temperatures because it does not take into account such things as ablation. The overprediction of the computational fluid dynamics and material response simulated surface temperature early in the trajectory coincides with high- emission intensity lines corresponding to thermal paint products. The presence ofpaint on the heat shield could have contributed to the lower observed surface temperatures and could explain the overprediction by the simulated data, which does not account for the paint. The average surface temperatures resulting from the spectrometer in Czerny- Turner configuration telescope analysis agree to within less than 5% with the average surface temperatures predicted by the material response. This observation period included the point of peak heating. The calculated flux based on the surface temperature agrees well with the observed flux. Surface temperature is one of the critical parameters used in the design of thermal protection systems, because it is an indicator of material performance. The coupled computational fluid dynamics and material response approach employed in the present analysis increases confidence for future missions such as the crew exploration vehicle Orion.

Chen Y.-K.,NASA | Miles F.S.,NASA | Gokcen T.,Eloret Corporation
Journal of Spacecraft and Rockets | Year: 2010

The central focus of this study is to demonstrate that time-accurate solutions for multidimensional ablation and shape change of thermal protection system materials may be obtained by loose coupling of a high-fidelity flow solver with a material thermal response code. In this study, the flow code solves the nonequilibrium Navier-Stokes equations using the data-parallel line-relaxation (DPLR) method. The material response code is the latest version of the Two-dimensional Implicit Thermal Response and Ablation Program (TITAN). In TITAN, the governing equations, which include a three-component decomposition model and a surface energy balance with thermo- chemical ablation, are solved with a robust moving-grid scheme to predict the shape change caused by surface recession. Coupling between the material response and flow codes is required for many multidimensional ablation simulations, because the magnitude and distribution of the surface heat flux are very sensitive to shape change. This paper demonstrates the application of the TITAN-DPLR system to problems with large-scale recession and shape change. Ablation and thermal response simulations are presented for iso-q and flat-faced arc-jet test models and also for a wedge with a cylindrical leading edge exposed to hypersonic flow at various angles of attack.

Reda D.C.,NASA | Wilder M.C.,NASA | Prabhu D.K.,Eloret Corporation
Journal of Spacecraft and Rockets | Year: 2010

Smooth titanium hemispheres with isolated three-dimensional surface-roughness elements were flown in the NASA Ames Research Center hypersonic ballistic range through quiescent CO2 and air environments. Global surface intensity (temperature) distributions were optically measured and thermal wakes behind individual roughness elements were analyzed to define tripping effectiveness. Real-gas Navier-Stokes calculations of model flowfields, including laminar boundary-layer development in these flowfields, were conducted to predict key dimensionless parameters used to correlate transition on blunt bodies in hypersonic flow. For isolated roughness elements totally immersed within the laminar boundary layer, critical roughness Reynolds numbers for flights in air were found to be higher than those measured for flights in CO2; i.e., it was easier to trip the CO2 boundary layer to turbulence. Tripping effectiveness was found to be dependent on trip location within the subsonic region of the blunt- body flowfield, with effective tripping being most difficult to achieve for elements positioned closest to the stagnation point. Direct comparisons of critical roughness Reynolds numbers for three-dimensional isolated versus three- dimensional distributed roughness elements for flights in air showed that distributed roughness patterns were significantly more effective at tripping the blunt-body laminar boundary layer to turbulence.

Valentini P.,University of Minnesota | Schwartzentruber T.E.,University of Minnesota | Cozmuta I.,Eloret Corporation
Journal of Chemical Physics | Year: 2010

The molecular dynamics technique with the ab initio based classical reactive force field ReaxFF is used to study the adsorption dynamics of O 2 on Pt(111) for both normal and oblique impacts. Overall, good quantitative agreement with the experimental data is found at low incident energies. Specifically, our simulations reproduce the characteristic minimum of the trapping probability at kinetic incident energies around 0.1 eV. This feature is determined by the presence of a physisorption well in the ReaxFF potential energy surface (PES) and the progressive suppression of a steering mechanism when increasing the translational kinetic energy (or the molecule's rotational energy) because of steric hindrance. In the energy range between 0.1 and 0.4 eV, the sticking probability increases, similar to molecular beam sticking data. For very energetic impacts (above 0.4 eV), ReaxFF predicts sticking probabilities lower than experimental sticking data by almost a factor of 3 due to an overall less attractive ReaxFF PES compared to experiments and density functional theory. For oblique impacts, the trapping probability is reduced by the nonzero parallel momentum because of the PES corrugation and does not scale with the total incident kinetic energy. Furthermore, our simulations predict quasispecular (slightly supraspecular) distributions of angles of reflection, in accordance with molecular beam experiments. Increasing the beam energy (between 1.2 and 1.7 eV) causes the angular distributions to broaden and to exhibit a tail toward the surface normal because molecules have enough momentum to get very near the surface and thus probe more corrugated repulsive regions of the PES. © 2010 American Institute of Physics.

Cruden B.A.,Eloret Corporation
AIP Conference Proceedings | Year: 2011

During planetary entry, a shock-heated plasma that imparts significant heating to the structure is formed in front of the space vehicle. At high velocities, a significant portion of that energy transfer originates from radiation from the shock-heated plasma. Shock tubes are capable of simulating the high velocity and low density conditions typical of planetary entry and thus are able to recreate the radiative environment encountered by spacecraft. The Electric Arc Shock Tube (EAST) at NASA Ames Research Center is one of the few shock tubes in the world that is capable of reaching the high velocities that are necessary to study more extreme entry conditions. The EAST is presently being utilized to simulate radiation in a variety of planetary atmospheres. It is presently the only facility in which radiation originating in the vacuum ultraviolet is being quantified. This paper briefly describes recent tests in the EAST facility relevant to Earth, Mars, and Venus entry conditions, and outlines the issues in relating ground test data to flight relevant condition via predictive radiation simulations. © 2011 American Institute of Physics.

Kim B.,Eloret Corporation | Lu Y.,Eloret Corporation | Hannon A.,NASA | Meyyappan M.,NASA | Li J.,NASA
Sensors and Actuators, B: Chemical | Year: 2013

The development of a tin oxide nanoparticle based sensor for detecting carbon monoxide at low temperature, 60 °C is presented. A combination of three approaches namely, (1) addition of a catalytic metal-1.5% palladium, (2) optimization of organic binder content, and (3) a proper design of electrodes, leads to high sensitivity, excellent repeatability, and long-term stability in sensor response. The sensors have been tested in dry (<1% RH) and humid (>70% RH) conditions, and no humidity effect on the sensor performance was noticed. The sensors using 15% hydroxypropyl cellulose (HPC) mixed with Pd/SnO2 show sensitivity to CO gas in the parts per million (ppm) level of concentration, 5-10% repeatability in 6-18 ppm CO exposures, and active response for more than 40 days. In addition, the fatigued sensors were recoverable with a brief heating process.

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