Edwards Air Force Base, CA, United States
Edwards Air Force Base, CA, United States

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Le H.P.,ERC Inc. | Le H.P.,University of California at Los Angeles | Karagozian A.R.,University of California at Los Angeles | Cambier J.-L.,Air Force Research Lab
Physics of Plasmas | Year: 2013

Thermal non-equilibrium processes in partially ionized plasmas can be most accurately modeled by collisional-radiative kinetics. This level of detail is required for an accurate prediction of the plasma. However, the resultant system of equations can be prohibitively large, making multi-dimensional and unsteady simulations of non-equilibrium radiating plasma particularly challenging. In this paper, we present a scheme for model reduction of the collisional-radiative kinetics, by combining energy levels into groups and deriving the corresponding macroscopic rates for all transitions. Although level-grouping is a standard approach to this type of problem, we provide here a mechanism for achieving higher-order accuracy by accounting for the level distribution within a group. The accuracy and benefits of the scheme are demonstrated for the generic case of atomic hydrogen by comparison with the complete solution of the master rate equations and other methods. © 2013 AIP Publishing LLC.

Martin R.S.,ERC Inc. | Cambier J.-L.,Air Force Research Lab
Journal of Computational Physics | Year: 2016

The ratio of physical to computationally modeled particles is of critical importance to the fidelity of particle-based simulation methods such as Direct Simulation Monte Carlo (DSMC) and Particle-in-Cell (PIC). Like adaptive mesh refinement for continuum/grid-based simulations, particle remapping enables dynamic control of simulation fidelity in regions of interest so that computational resources can be efficiently distributed within the problem. This is particularly important for simulations involving high dynamic range in the density for one or more species such as problems involving chain-branching reactions like combustion and ionizing breakdown. In this work, a new method of particle remapping is presented which strictly conserves mass, momentum, and energy while simultaneously remaining faithful to the original velocity distribution function through the use of octree binning in velocity space. © 2016.

Clemett S.J.,ERC Inc. | Sandford S.A.,NASA | Horz F.,NASA | McKay D.S.,NASA
Meteoritics and Planetary Science | Year: 2010

The successful return of the Stardust spacecraft provides a unique opportunity to investigate the nature and distribution of organic matter in cometary dust particles collected from comet 81P/Wild 2. Analysis of individual cometary impact tracks in silica aerogel using the technique of two-step laser mass spectrometry demonstrates the presence of complex aromatic organic matter. While concerns remain as to the organic purity of the aerogel collection medium and the thermal effects associated with hypervelocity capture, the majority of the observed organic species appear indigenous to the impacting particles and are hence of cometary origin. While the aromatic fraction of the total organic matter present is believed to be small, it is notable in that it appears to be N rich. Spectral analysis in combination with instrumental detection sensitivies suggest that N is incorporated predominantly in the form of aromatic nitriles (R-C≡N). While organic species in the Stardust samples do share some similarities with those present in the matrices of carbonaceous chondrites, the closest match is found with stratospherically collected interplanetary dust particles. These findings are consistent with the notion that a fraction of interplanetary dust is of cometary origin. The presence of complex organic N containing species in comets has astrobiological implications as comets are likely to have contributed to the prebiotic chemical inventory of both the Earth and Mars. © 2010 The Meteoritical Society.

Park C.,Korea Advanced Institute of Science and Technology | Brown J.D.,ERC Inc.
Astronomical Journal | Year: 2012

The phenomenon of fragmentation and spreading of a high-speed flying object resembling a meteorite is studied experimentally and theoretically. Experimentally, a model made of graphite is launched in a ballistic range and is made to fragment and spread. The flow field produced by the cloud of the fragments is observed optically. The observed deceleration and spreading behavior is numerically reconstructed using computational-fluid-dynamic calculations, applying an improved meteoroid fragmentation theory. The existing meteoroid fragmentation theory is improved by introducing the hypothesis that the incubation process of the pressurized fluid permeating through the fragment precedes the splitting process. The incubation time is determined by the ratio of permeability of the fragment to the fluid's viscosity and is much longer than the time for splitting given by the existing theory. Agreement is obtained between the observed and calculated behavior of the fragment cloud by appropriately choosing this ratio. © 2012. The American Astronomical Society. All rights reserved.

Hollis B.R.,NASA | Prabhu D.K.,ERC Corporation | Prabhu D.K.,NASA
Journal of Spacecraft and Rockets | Year: 2013

An assessment of computational uncertainties is presented for numerical methods used by NASA to predict laminar, convective aeroheating environments for Mars-entry vehicles. A survey was conducted of existing experimental heat transfer and shock-shape data for high-enthalpy reacting-gas CO2 flows, and five relevant test series were selected for comparison with predictions. Solutions were generated at the experimental test conditions using NASA state-of-the-art computational tools and compared with these data. The comparisons were evaluated to establish predictive uncertainties as a function of total enthalpy and to provide guidance for future experimental testing requirements to help lower these uncertainties. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc.

Cornella B.M.,ERC Inc. | Gimelshein S.F.,ERC Inc. | Lilly T.C.,University of Colorado at Colorado Springs | Ketsdever A.D.,Air Force Research Lab
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

Molecular nitrogen at 0.8 atm and 300 and 500 K and methane at 0.8 atm and 300 K were subjected to optical lattices formed by narrow-band 532-nm laser pulses with intensities on the optical axis near, but below, the gas ionization limit. A third pulse was introduced to experimentally probe the response, as a function of the lattice velocity, of the gas to the deep monochromatic potential wells formed by the lasers. Coherent Rayleigh-Brillouin scattering (CRBS) line shapes were recorded and compared to numerically predicted magnitudes of the density perturbations induced in the gas. Both experimental results and those from direct simulation Monte Carlo simulations show a deviation from previously published low-intensity CRBS line-shape models. The deviation indicates a trend, as a function of lattice velocity, similar to that relating to previously published energy and momentum transfer calculations for high-intensity lattices. Furthermore, the deviation indicates a maximum intensity at which current CRBS theory is valid. © 2013 American Physical Society.

Weber E.,Air Force Research Lab | Fernandez M.,Air Force Research Lab | Wapner P.,ERC Inc | Hoffman W.,Air Force Research Lab
Carbon | Year: 2010

X-ray Micro-Tomography (μCT) applied to carbon-carbon composites is shown to be able to quantify the amount, shape, and distribution in three dimensions of both open and closed porosity with a minimum dimension greater than 10 μm. Being a non-destructive technique, it is also able to track these values following each densification cycle. It is also demonstrated that μCT is able to obtain bulk density values for non-uniform samples as well as the same results for skeletal density as other techniques used conventionally. Furthermore, values for open porosity comparable to those obtained by mercury porosimetry can be obtained by X-ray Micro-Tomography if the value obtained by the mercury porosimeter is truncated below the resolution of the μCT. Finally, it is shown that in conjunction with data from the mercury porosimetry, μCT is also able to demonstrate the presence of "bottle-neck" pores i.e. open pores with restricted pore access dimensions.

Chen Y.-K.,NASA | Gokcen T.,ERC Inc.
Journal of Spacecraft and Rockets | Year: 2014

This study demonstrates that coupling of a material thermal response code and a flow solver with nonequilibrium gas-surface interaction for simulation of charring carbon ablators can be performed using an implicit approach. The material thermal response code used in this study is the three-dimensional version of fully implicit ablation and thermal response program, which predicts charring material thermal response and shape change on hypersonic space vehicles. The flow code solves the reacting Navier-Stokes equations using data-parallel line relaxation method. Coupling between the material response and flow codes is performed by solving the surface mass balance in the flow solver and the surface energy balance in the material response code. Thus, the material surface recession is predicted in the flow code, and the surface temperature and pyrolysis gas injection rate are computed in the material response code. It is demonstrated that the time-lagged explicit approach is sufficient for simulations at low surface heating conditions, in which the surface ablation rate is not a strong function of the surface temperature. At elevated surface heating conditions, the implicit approach has to be taken because the carbon ablation rate becomes a stiff function of the surface temperature, and thus the explicit approach appears to be inappropriate, resulting in severe numerical oscillations of predicted surface temperature. Implicit coupling for simulation of arc-jet models is performed, and the predictions are compared with measured data. Implicit coupling for trajectory-based simulation of Stardust forebody heat shield is also conducted. The predicted stagnation point total recession is compared with that predicted using the chemical equilibrium surface assumption. Copyright © 2013 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Pekker L.,ERC Inc.
International Journal of Heat and Mass Transfer | Year: 2010

We develop an analytical Knudsen layer model at the ablative surface in gas flow; this model takes into account the temperature and velocity gradients in the bulk gas and the rebounding of gas molecules by the ablative wall back into the gas region. In addition, this model uses a bimodal velocity distribution function which preserves the laws of conservation of mass, momentum and energy within the Knudsen layer and converges to the Chapman-Enskog velocity distribution function at the outer boundary of the Knudsen layer. This model enables us to obtain the boundary conditions at the ablative surface in gas flow for arbitrary condensation and accommodation coefficients, which can be used for computational fluid dynamics simulation of ablation avoiding "micro" modelling of the evaporation process at the mean free path scale. © 2009 Elsevier Ltd.

Reid M.R.,U.S. Air force | Scharfe D.B.,ERC Inc. | Webb R.N.,University of Colorado at Colorado Springs
Solar Energy | Year: 2013

A system capable of receiving, absorbing, and converting solar energy was designed for use on a satellite in low Earth orbit. The proposed system, an alternative to conventional photovoltaic panels paired with electrochemical batteries, has at the core of its design a latent heat based energy storage system that employs silicon as the phase change material. Thermal to electric conversion is achieved by thermophotovoltaic cells that then provide electrical power for various satellite components. The system was evaluated computationally. Through prediction of the melt and solidification fronts the amount of solar irradiation required to fully utilize the phase change material was determined to be between 4 and 5. kW depending on the orbit. The average temperature of the emitter, used to power the thermophotovoltaic cells, was also predicted throughout an orbit. The emitter temperature range, 1450-1850. K, is well-suited for use with commercially available gallium antimony cells. © 2013 Elsevier Ltd.

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