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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Engineering calculations for communications satellite systems planning

Computer-based techniques for optimizing communications-satellite orbit and frequency assignments are discussed. A gradient-search code was tested against a BSS scenario derived from the RARC-83 data. Improvement was obtained, but each iteration requires about 50 minutes of IBM-3081 CPU time. Gradient-search experiments on a small FSS test problem, consisting of a single service area served by 8 satellites, showed quickest convergence when the satellites were all initially placed near the center of the available orbital arc with moderate spacing. A transformation technique is proposed for investigating the surface topography of the objective function used in the gradient-search method. A new synthesis approach is based on transforming single-entry interference constraints into corresponding constraints on satellite spacings. These constraints are used with linear objective functions to formulate the co-channel orbital assignment task as a linear-programming (LP) problem or mixed integer programming (MIP) problem. Globally optimal solutions are always found with the MIP problems, but not necessarily with the LP problems. The MIP solutions can be used to evaluate the quality of the LP solutions. The initial results are very encouraging.

Reilly, C. H.↗
DOE OSTI2017

Building blocks for a digital twin of additive manufacturing

Properties and serviceability of additively manufactured components are affected by their geometry, microstructure and defects. These important attributes are now optimized by trial and error because the essential process variables cannot currently be selected from scientific principles. A recourse is to build and rigorously validate a digital twin of the additive manufacturing process that can provide accurate predictions of the spatial and temporal variations of metallurgical parameters that affect the structure and properties of components. Key building blocks of a computationally efficient first-generation digital twin of laser-based directed energy deposition additive manufacturing utilize a transient, three-dimensional model that calculates temperature and velocity fields, cooling rates, solidification parameters and deposit geometry. The measured profiles of stainless steel 316L and Alloy 800H deposits as well as the secondary dendrite arm spacing (SDAS) and Vickers hardness measurements are used to validate the proposed digital twin. The predicted cooling rates, temperature gradients, solidification rates, SDAS and micro-hardness values are shown to be more accurate than those obtained from a commonly used heat conduction calculation. Finally, these metallurgical building blocks serve as a phenomenological framework for the development of a digital twin that will make the expanding knowledge base of additive manufacturing usable in a practical way for all scientists and engineers.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Exploring gauge-fixing conditions with gradient-based optimization

Lattice gauge fixing is required to compute gauge-variant quantities, for example those used in RI-MOM renormalization schemes or as objects of comparison for model calculations. Recently, gauge-variant quantities have also been found to be more amenable to signal-to-noise optimization using contour deformations. These applications motivate systematic parameterization and exploration of gauge-fixing schemes. This work introduces a differentiable parameterization of gauge fixing which is broad enough to cover Landau gauge, Coulomb gauge, and maximal tree gauges. The adjoint state method allows gradient-based optimization to select gauge-fixing schemes that minimize an arbitrary target loss function.

Detmold, William↗
DOE OSTI2016

An inverse problem strategy based on forward model evaluations: Gradient-based optimization without adjoint solves

This study presents a new nonlinear programming formulation for the solution of inverse problems. First, a general inverse problem formulation based on the compliance error functional is presented. The proposed error functional enables the computation of the Lagrange multipliers, and thus the first order derivative information, at the expense of just one model evaluation. Therefore, the calculation of the Lagrange multipliers does not require the solution of the computationally intensive adjoint problem. This leads to significant speedups for large-scale, gradient-based inverse problems.

42 ENGINEERING↗
DOE OSTI2015

Generalized and efficient algorithm for computing multipole energies and gradients based on Cartesian tensors

Accurate representation of intermolecular forces has been the central task of classical atomic simulations, known as molecular mechanics. Recent advancements in molecular mechanics models have put forward the explicit representation of permanent and/or induced electric multipole (EMP) moments. The formulas developed so far to calculate EMP interactions tend to have complicated expressions, especially in Cartesian coordinates, which can only be applied to a specific kernel potential function. For example, one needs to develop a new formula each time a new kernel function is encountered. The complication of these formalisms arises from an intriguing and yet obscured mathematical relation between the kernel functions and the gradient operators. Here, I uncover this relation via rigorous derivation and find that the formula to calculate EMP interactions is basically invariant to the potential kernel functions as long as they are of the form f(r), i.e., any Green’s function that depends on inter-particle distance. I provide an algorithm for efficient evaluation of EMP interaction energies, forces, and torques for any kernel f(r) up to any arbitrary rank of EMP moments in Cartesian coordinates. The working equations of this algorithm are essentially the same for any kernel f(r). Recently, a few recursive algorithms were proposed to calculate EMP interactions. Depending on the kernel functions, the algorithm here is about 4–16 times faster than these algorithms in terms of the required number of floating point operations and is much more memory efficient. I show that it is even faster than a theoretically ideal recursion scheme, i.e., one that requires 1 floating point multiplication and 1 addition per recursion step. This algorithm has a compact vector-based expression that is optimal for computer programming. The Cartesian nature of this algorithm makes it fit easily into modern molecular simulation packages as compared with spherical coordinate-based algorithms. A software library based on this algorithm has been implemented in C++11 and has been released.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Exploiting graphical processing units to enable quantum chemistry calculation of large solvated molecules with conductor-like polarizable continuum models

The conductor–like polarizable continuum model (C–PCM) with switching/Gaussian smooth discretization is a widely used implicit solvation model in quantum chemistry. We have previously implemented C–PCM solvation for Hartree–Fock (HF) and density functional theory (DFT) on graphical processing units (GPUs), enabling the quantum mechanical treatment of large solvated biomolecules. Here, we first propose a GPU–based algorithm for the PCM conjugate gradient linear solver that greatly improves the performance for very large molecules. The overhead for PCM–related evaluations now consumes less than 15% of the total runtime for DFT calculations on large molecules. Second, we demonstrate that our algorithms tailored for ground state C–PCM are transferable to excited state properties. Using a single GPU, our method evaluates the analytic gradient of the linear response PCM time–dependent density functional theory energy up to 80× faster than a conventional central processing unit (CPU)–based implementation. In addition, our C–PCM algorithms are transferable to other methods that require electrostatic potential (ESP) evaluations. For example, we achieve speed–ups of up to 130× for restricted ESP–based atomic charge evaluations, when compared to CPU–based codes. As a result, we also summarize and compare the different PCM cavity discretization schemes used in some popular quantum chemistry packages as a reference for both users and developers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Impact of plasma response on plasma displacements in DIII-D during application of external 3D perturbations

The effects of applied non-axisymmetric resonant magnetic perturbations (RMPs) are predicted without and with self-consistent plasma response by modeling of the magnetic field structure and two-fluid MHD simulations, respectively. A synthetic diagnostic is used to simulate soft X-ray (SXR) emission within the steep gradient region of the pedestal, 0.98 > ψ > 0.94. The entire pedestal and edge region is characterized by large changes in plasma rotation and current density. Those parameters are expected to strongly affect the plasma response to RMPs. The M3D-C1 code takes into account this response self-consistently. The plasma response is investigated in detail and used in the forward modeling of the simulated local SXR emission, within the framework of the synthetic diagnostic. The resulting synthetic emission is compared to measured SXR data. The latter clearly shows helical m = 11 ± 1 displacements around the 11/3 rational surface of sizes up to 5 cm, which change with the poloidal angle. The synthetic emission with plasma response is used to explain the nature of the measured displacements. Different approaches are tested. One approach is based on the magnetic field structure to simulate local emission, which shows additional structures at the separatrix, that are caused by the lobes. Especially without plasma response, almost only separatrix structures are generated while no significant displacements are found further inside. Another approach to model local emission uses the fluid quantities electron density and temperature, as calculated by M3D-C1. Compared to the previous approach, based on the magnetic field structure, the emission simulated by the fluid approach with plasma response shows better agreement with the measured SXR data. To be specific, it has comparable displacements in the steep gradient region and no lobe structures at all. The helical displacements around the 11/3 surface are identified to be directly related to the kink response, caused by non-resonant amplification of various poloidal RMP modes due to plasma response. Regarding the latter, the role of different plasma parameters is investigated, but it appears that the electron rotation plays a key role in the formation of screening and resonant amplification, while the kinking appears to be sensitive to the edge current density. As a result, it is also hypothesised that the strength of the kink response is also correlated to edge-localized-mode (ELM) stability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

First-Principles Investigation of Ferromagnetic, Thermodynamic, Elastic, and Half-Metallic Behavior of ReCrTe and RuCrTe Half-Heusler Alloys

Calculations are realized within the full-potential linearized augmented plane wave (FP-LAPW) computational approach with the generalized gradient approximation (GGA) for the exchange-correlation potential. In this work, we present investigation of structural, electronic, ferromagnetic, elastic, and thermodynamic properties and half-metallic behavior of ReCrTe and RuCrTe compounds. Analysis of band structures and densities of states show the half-metallic ferromagnetic behavior of these compounds. The values of total magnetic moments µ{sub tot} are 1 µ{sub B} per unit cell for ReCrTe and 3 µ{sub B} per unit cell for RuCrTe. The contribution of Cr is the most important. The calculated values of a total magnetic moment nicely follow the rule µ{sub tot} = Z{sub t}− 18. The study of the impacts of strains on the magnetic moment reveals that the two compounds are stable ferromagnetic compounds. RuCrTe compound preserves the half-metallic behavior when the unit cell volume is changed in the range of [ − 4%, 4%], whereas, weak strain destroys the half metallic nature for ReCrTe compound. Using the quasi-harmonic Debye model, the effects of pressure P and temperature T on thermal expansion coefficient, Debye temperature, heat capacity and Grüneisen parameter for these two compounds are investigated for the first time. The study of the elastic constants shows that ReCrTe and RuCrTe are mechanically stable. RuCrTe is stiffer than ReCrTe compounds. The obtained values of Pugh’s ratio B/G and Cauchy pressure (C{sub 12} − C{sub 44}) reveal that ReCrTe and RuCrTe are brittle. The study of the electron charge density and Poisson’s ratio proves that the ionic character dominates the atomic bonding for the two compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Low-pressure inductively coupled plasmas in hydrogen: impact of gas heating on the spatial distribution of atomic hydrogen and vibrationally excited states

Non-equilibrium inductively coupled plasmas (ICPs) operating in hydrogen are of significant interest for applications including large-area materials processing. Increasing control of spatial gas heating, which drives the formation of neutral species density gradients and the rate of gas-temperature-dependent reactions, is critical. In this study, we use 2D fluid-kinetic simulations with the Hybrid Plasma Equipment Model to investigate the spatially resolved production of atomic hydrogen in a low-pressure planar ICP operating in pure hydrogen (10–20 Pa or 0.075–0.15 Torr, 300 W). The reaction set incorporates self-consistent calculation of the spatially resolved gas temperature and 14 vibrationally excited states. We find that the formation of neutral-gas density gradients, which result from spatially non-uniform electrical power deposition at constant pressure, can drive significant variations in the vibrational distribution function and density of atomic hydrogen when gas heating is spatially resolved. This highlights the significance of spatial gas heating on the production of reactive species in relatively high-power-density plasma processing sources.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

First-principles calculations of the electronic and structural properties of GaSb

In this paper, we carried out first-principles calculations in order to investigate the structural and electronic properties of the binary compound gallium antimonide (GaSb). This theoretical study was carried out using the Density Functional Theory within the plane-wave pseudopotential method. The effects of exchange and correlation (XC) were treated using the functional Local Density Approximation (LDA), generalized gradient approximation (GGA): Perdew–Burke–Ernzerhof (PBE), Perdew-Burke-Ernzerhof revised for solids (PBEsol), Perdew-Wang91 (PW91), revised Perdew–Burke–Ernzerhof (rPBE), Armiento–Mattson 2005 (AM05) and meta-generalized gradient approximation (meta-GGA): Tao–Perdew–Staroverov–Scuseria (TPSS) and revised Tao–Perdew–Staroverov–Scuseria (RTPSS) and modified Becke-Johnson (MBJ). We calculated the densities of state (DOS) and band structure with different XC potentials identified and compared them with the theoretical and experimental results reported in the literature. It was discovered that functional: LDA, PBEsol, AM05 and RTPSS provide the best results to calculate the lattice parameters (a) and bulk modulus (B{sub 0}); while for the cohesive energy (E{sub coh}), functional: AM05, RTPSS and PW91 are closer to the values obtained experimentally. The MBJ, Rtpss and AM05 values found for the band gap energy is slightly underestimated with those values reported experimentally.

36 MATERIALS SCIENCE↗
NASA NTRS1994

Thermal-infrared remote sensing and Kirchhoff's Law. 1: Laboratory measurements

By the end of this century the Earth Observing System (EOS) will provide worldwide, thermal infrared, multispectral images of the Earth, presenting geologists with a new kind of remote sensing data for interpretation. Thus it has become essential to understand the spectral emittance behavior of terrestrial surface materials. Perhaps the most fundamental question to be answereed is the extent to which such materials follow Kirchhoff's law (epsilon = 1 -R) under laboratory and field conditions, especially when a sample displays a thermal gradient. We present the first rigorous quantitative comparison of directional and hemispherical reflectance and directional emittance of rock and soil samples in the laboratory, with thermal gradients induced by heating them from below and allowing them to radiate to a colder background. The results show that only an extemeley low density sample composed of fine particles sifted into a 'fairy castle' structure displays a thermal gradient steep enough within the infrared skin depth to cause significant (6%) departure from Kirchhoff's law. There is no detectable effect on the more normal terrestrial samples, such as soils and rocks measured in the laboratory, even when semitransparent coatings are involved. Thus both emittance and reflectance measurements can be used to calculate sample emissivity for most terrestrial surface materieals. However, the effect on Kirchhoffian behavior of different field environments, which may induce a steeper thermal gradient in particulate samples, has yet to be determined, and some low-density surface materials like newly fallen snow, frost, and efflorescent salts on playas have yet to be measured in emittance.

Salisbury, John W.↗
NASA NTRS1995

Simultaneous concentration and purification through gradient deformation chromatography

Mobile-phase additives, commonly used to modulate absorbate retention in gradient elution chromatography, are usually assumed to be either linearly retained or unretained. Previous theoretical work from our laboratory has shown that these modulators, such as salts in ion-exchange and hydrophobic interaction chromatography and organic modifiers in reversed-phase chromatography, can absorb nonlinearly, giving rise to gradient deformation. Consequently, adsorbate peaks that elute in the vicinity of the head of the deformed gradient may exhibit unusual shapes, form shoulders, and/or be concentrated. These effects for a reversed-phase sorbent with aqueous acetonitrile (ACN) as the modulator are verified experimentally. Gradient deformation is demonstrated experimentally and agrees with simulations based on ACN isotherm parameters that are independently determined from batch equilibrium studies using the layer model. Unusual absorbate peak shapes were found experimentally for single-component injections of phenylalanine, similar to those calculated by the simulations. A binary mixture of tryptophan and phenylalanine is used to demonstrate simultaneous concentration and separation, again in agreement with simulations. The possibility of gradient deformation in ion-exchange and hydrophobic interaction chromatography is discussed.

NASA Discipline Number 93-10↗
DOE OSTI2015

Orientation gradients in relation to grain boundaries at varying strain level and spatial resolution

The resolution dependence of orientation gradients was studied in a well-annealed 99.9995% pure polycrystalline copper pulled to failure in tension. Owing to the well developed neck, different regions in the sample correspond to different tensile strains. Post-mortem characterization was performed using EBSD on cross-sections containing the tensile axis. Kernel average misorientation (KAM) was calculated as a metric to establish correlation between defect accumulation and microstructural features, with a threshold of 5° to focus on intra-granular gradients. The region with the lowest strain (2%) showed high KAM values adjacent to grain boundaries compared to the grain interior, regardless of the point spacing, i.e. the spatial resolution. However, in the region with the highest strain (13%) a strong dependence on resolution was found. For point spacings of 0.5 μm or smaller, the same correlation of high KAM with locations near boundaries was found. At coarse spacings i.e. low spatial resolution, by contrast, the reverse was found in that the highest KAM values appear in the grain interiors, as previously observed in X-ray microscopy on the same sample which had a similar coarse resolution. An analysis of orientation gradients parallel to, and perpendicular to boundaries suggested that the latter tend to be the larger of the two. This helps to explain why boundary-adjacent points have low KAM values. The conclusion is that measurement of local orientation gradient requires a resolution that is comparable to the dislocation substructure.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Luminescence Properties of Undoped Langasite Crystals

The optical and luminescence properties of La{sub 3}Ga{sub 5}SiO{sub 14} lanthanum–gallium silicate crystals grown in atmospheres of argon and argon with the addition of oxygen are investigated. The results of calculations of the structure of energy bands are presented, obtained using the CASTEP module in the framework of the generalized gradient approximation and the local density approximation. The width of the optical band gap of the crystal is determined to be E{sub g}{sup opt} = 5.1 eV. Upon interband excitation, La{sub 3}Ga{sub 5}SiO{sub 14} crystals grown in argon atmosphere show a luminescence band with a maximum at 430 nm, whereas for a crystal grown in argon with addition of oxygen, two luminescence bands with maxima at 470 and 530 nm dominate in the luminescence spectrum. The nature of the luminescence centers responsible for these bands is discussed with the help of the data for electronic structure calculations. The effect of temperature on the luminescent properties of La{sub 3}Ga{sub 5}SiO{sub 14} is demonstrated. The presence of traps in La{sub 3}Ga{sub 5}SiO{sub 14} is shown using thermally stimulated luminescence, and their activation energy is determined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1988

Electrostatic forces in planetary rings

The average charge on a particle in a particle-plasma cloud, the plasma potential inside the cloud, and the Coulomb force acting on the particle are calculated. The net repulsive electrostatic force on a particle depends on the plasma density, temperature, density of particles, particle size, and the gradient of the particle density. In a uniformly dense ring the electrostatic repulsion is zero. It is also shown that the electrostatic force acts like a pressure force, that even a collisionless ring can be stable against gravitational collapse, and that a finite ring thickness does not necessarily imply a finite velocity dispersion. A simple criterion for the importance of electrostatic forces in planetary rings is derived which involves the calculation of the vertical ring thickness which would result if only electrostatic repulsion were responsible for the finite ring thickness. Electrostatic forces are entirely negligible in the main rings of Saturn and the E and G rings. They may also be negligible in the F ring. However, the Uranian rings and Jupiter's ring seem to be very much influenced by electrostatic repulsion. In fact, electrostatic forces could support a Jovian ring which is an order of magnitude more dense than observed.

Goertz, C. K.↗
NASA NTRS1987

Calculating Transport Of Moisture Through Honeycomb Panels

Diffusion equations solved for steady-state and transient conditions. HUMID is computer program for predicting moisture gradients and internal bursting pressure in composite honeycomb-sandwich panels. Designed to monitor ebb and flow of moisture in Space Shuttle Orbiter payload bay doors, but applicable for general use. Written in FORTRAN.

Martin, R. T.↗
DOE OSTI2016

Modeling thermochemistry of fuel and coupling to fuel performance codes

A thermochemical library, Thermochimica, utilizes new algorithms and solvers designed for calculating equilibria of multicomponent and multiphase systems and has been coupled to the nuclear fuel performance code BISON. Thermochimica utilizes a generalized thermochemical database for urania-based fuels that has been under development for a number of years and continues to be expanded. One of the key motivations for use of thermodynamic models is that they can better represent the physics of oxygen migration. The coupling of the BISON code with Thermochimica and examples of its use in representing fuel phenomena are described. In the current implementation in BISON, fluxes are driven by the elemental concentration gradient. However, equilibrium is characterized by the absence of spatial variations of chemical potentials of the system species/components. The driving force to equilibrium (species flux) is proportional to the deviation from equilibrium, i.e., the gradient in chemical potential. It is therefore necessary to integrate chemical potential driven diffusion in the BISON representation of oxygen transport. The diffusion kernels based on thermodynamic models have been developed to simulate multicomponent transport in light water reactor (LWR) materials. The Thermochimica library was used to calculate various thermodynamics properties needed for transport calculations. The library also can calculate properties that are specific to various transport mechanisms for different fuel materials. For example, it can calculate defect site fractions among the sublattices in oxide fuels as a function of temperature, burnup, and stoichiometry. The transport and thermodynamic models are being integrated with other simulation efforts such as calculation of mobility functions that couple fluxes and chemical gradients. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
NASA NTRS1996

CFD Analysis of Modular Thrusters Performance

The effective performance of modular thrusters in an aerospike configuration is difficult to determine. Standard analytical tools are applicable to conventional nozzle shapes, but are limited when applied to an aerospike nozzle (An aerospike nozzle is an altitude compensating external nozzle). Three baseline nozzle shapes are derived using standard analytical procedures. The baseline nozzle sizes are restricted to fill a volume envelope. The three shapes are an axi-symmetric round nozzle, a two dimensional planar square exit nozzle, and a super elliptic round to nearly square nozzle. The integrated (thruster/aerospike) performance of the three nozzles is determined through the use of three dimensional viscous computational fluid dynamic (CFD) calculation where complex features of the flow field can be accurately captured. The resulting installed performance is then used to evaluate the efficiency of these nozzle shapes for aerospike applications. The determination of effective performance of a thruster nozzle integrated into an aerospike nozzle require the solution of the three dimensional turbulent Navier-Stokes equations. The model used in this study consisted of two zones; one of the upstream thruster cowl surface so freestream conditions can be accurately predicted, and two, the aerospike surface beginning with the thruster outflow and extending to the end of the aerospike surface. The numerical grid consisted of over 120,000 nodes and used symmetry on the thruster centerline and edge. A two species non-reacting chemistry model was used to capture the variation of fluid properties between the hot plume base and freestream air. From the results of the three baseline nozzle aerospike calculations, the effictive performance of the nozzle was determined. The flow fields of these calculations do show some variation between the cases. Recirculation zones on the cowl surface is predicted for the two dimensional planar nozzle and a smaller one for the super elliptic nozzle. The recirculation is caused by the strong pressure gradient between the plume and freestream flows. The axi-symmetric nozzle results indicate recirculation zones on the thruster face. These recirculation zones smooth the pressure gradient between the plume and freestream flow limiting the formation of recirculation on the cowl surface. Thruster to thruster interaction is evident for the axi-symmetric and supper elliptic calculation while the two dimensional planar nozzle did not have any lateral expansion in the nozzle, so thruster to thruster interaction is limited. The integrated performance results, at the altitude choosen, show very little variation between the three thruster shapes. This result allows for nozzle shape determination based on additional considerations (thermal, structural, weight) besides performance.

Ungewitter, Ronald J.↗