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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. Source

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. Source

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. Source

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. Source

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. Source

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