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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 127 records · Page 7
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NASA NTRS1999

Monte Carlo Simulation of Alloy Design Techniques: Fracture and Welding Studied Using the BFS Method for Alloys

Large-scale simulations of dynamic processes at the atomic level have developed into one of the main areas of work in computational materials science. Until recently, severe computational restrictions, as well as the lack of accurate methods for calculating the energetics, resulted in slower growth in the area than that required by current alloy design programs. The Computational Materials Group at the NASA Lewis Research Center is devoted to the development of powerful, accurate, economical tools to aid in alloy design. These include the BFS (Bozzolo, Ferrante, and Smith) method for alloys (ref. 1) and the development of dedicated software for large-scale simulations based on Monte Carlo- Metropolis numerical techniques, as well as state-of-the-art visualization methods. Our previous effort linking theoretical and computational modeling resulted in the successful prediction of the microstructure of a five-element intermetallic alloy, in excellent agreement with experimental results (refs. 2 and 3). This effort also produced a complete description of the role of alloying additions in intermetallic binary, ternary, and higher order alloys (ref. 4).

Bozzolo, Guillermo H.↗
NASA NTRS1988

Spatial versus time hysteresis in damping mechanisms

A description is given of continuing investigations on the task of estimating internal damping mechanisms in flexible structures. Specifically, two models for internal damping in Euler-Bernoulli beams are considered: spatial hysteresis and time hysteresis. A theoretically sound computational algorithm for estimation is described, and experimental results are discussed. It is concluded that both models perform well in the sense that they accurately predict response for the experiments conducted.

Banks, H. T.↗
NASA NTRS1993

Transient Ejector Analysis (TEA) code user's guide

A FORTRAN computer program for the semi analytic prediction of unsteady thrust augmenting ejector performance has been developed, based on a theoretical analysis for ejectors. That analysis blends classic self-similar turbulent jet descriptions with control-volume mixing region elements. Division of the ejector into an inlet, diffuser, and mixing region allowed flexibility in the modeling of the physics for each region. In particular, the inlet and diffuser analyses are simplified by a quasi-steady-analysis, justified by the assumption that pressure is the forcing function in those regions. Only the mixing region is assumed to be dominated by viscous effects. The present work provides an overview of the code structure, a description of the required input and output data file formats, and the results for a test case. Since there are limitations to the code for applications outside the bounds of the test case, the user should consider TEA as a research code (not as a production code), designed specifically as an implementation of the proposed ejector theory. Program error flags are discussed, and some diagnostic routines are presented.

Drummond, Colin K.↗
NASA NTRS2000

Modeling of Ternary Element Site Substitution in NiAl

It is well recognized that ternary alloying additions can have a dramatic impact on the behavior of ordered intermetallic alloys such as nickel aluminides. Properties as diverse as yield strength, fracture strength, fracture mode, cyclic oxidation resistance, creep strength, and thermal and electrical diffusivity can change by orders of magnitude when a few percent or less of a ternary element is added. Yet our understanding of the resulting point defect structures and the simple site preferences of ternary alloying additions is poor because these are extremely difficult characteristics to determine. This disconnection between the understanding of the structure and properties in ordered alloys is at least in part responsible for the limited development and commercialization of these materials. Theoretical methods have provided useful but limited insight in this area, since most techniques suffer from constraints in the type of elements and the crystallographic structures that can be modeled. In an effort to overcome these limitations, the Bozzolo-Ferrante-Smith (BFS) method for alloys was designed. After a brief description of this approximate quantum mechanical approach, we use BFS to investigate the energetics of Si, Ti, V, Cr, Fe, Co, Cu, Zr, Nb, Mo, Ru, Hf, Ta and W additions to B2-ordered, stoichiometric NiAl. In addition to determining the site preference for these alloying additions over a range of compositions, we include results for the concentration dependence of the lattice parameter. In this introductory paper, we performed our analyses in the absence of constitutional and thermal vacancies for alloys of the form Ni50(Al,X)50. Where data exist, a comparison between experimental, theoretical, and BFS results is also included.

Bozzolo, Guillermo↗
NASA NTRS1976

A depolarization and attenuation experiment using the CTS satellite. Volume 1: Experiment description

An experiment for measuring precipitation attenuation and depolarization on the Communications Technology Satellite (CTS) 11.7 GHz downlink is described. Attenuation and depolarization of the signal received from the spacecraft is monitored on a 24 hour basis. Data is correlated with ground weather conditions. Theoretical models for millimeter wave propagation through rain are refined for maximum agreement with observed data. Techniques are developed for predicting and mimimizing the effects of rain scatter and depolarization on future satellite communication systems.

Bostian, C. W.↗
NASA NTRS1984

Calculations of radar backscattering coefficient of vegetation-covered soils

The present investigation has the objective to develop a simple 'user's' model for simulating the measured radar backscattering coefficients from vegetation-covered fields in conjunction with the data obtained by Jackson et al. (1980, 1982). The theoretical work reported by Fung and Eom (1981) provides the basis for the model. Certain modifications are related to a consideration of the effect of a vegetation canopy. The first part of the model is concerned with a description of scatter from rough bare soil, while the second part takes into account the effect of a vegetation cover. It is shown that the measured angular distribution of the backscattering coefficient of vegetation-covered fields can be satisfactory reproduced by using the developed model.

Mo, T.↗
NASA NTRS1984

Density fluctuations in the interstellar medium: Evidence for anisotropic magnetogasdynamic turbulence. I - Model and astrophysical sites

A self-consistent theoretical model of turbulent fluctuations in a compressible electrically conducting gas is developed in order to quantitatively study a turbulent fluid origin for the small spiral scale density fluctuations observed by Armstrong et al. (1981). A heuristic description of turbulent flows in compressible fluids is offered as an analogy for the small-scale turbulent velocity and magnetic field variations which derive density variations in the presence of a strong local magnetic field.

Higdon, J. C.↗
NASA NTRS1995

Rotational And Rovibrational Energy Transfer In Electron Collisions With Molecules

Air flows around a hypervelocity reentry vehicle undergo dissociation, rovibrational excitation and ionization. More specifically the air, initially 80% N2 and 20% O2, in the shock layer consists of species such as N, O, N2, O2, NO, N+, O+, N+, O+, NO+ and 2 free electrons. It was pointed out in multi temperature models'' that the temperature of the rotational energy modes and the gas-kinetic translational temperature are quickly equilibrated by a few collisions and rise rapidly to high temperatures as 50000K before falling off to equilibrium value of 10000K. Contrary, the electronic and vibrational temperatures state energy distributions remain low (less than 15000K) because of the slow equilibration. Electron vibrational energy transfer is thought to play a crucial role in such a ionizing flow regime since chemical reaction rates and dissociation depend strongly on the vibrational temperatures. Modeling of these flowfields in principle require the rovibrational excitation and de-excitation cross section data for average electron energies from threshold up to several eV (leV=11605.4 K). In this lecture we focus on theoretical description of rotational effects i.e. energy transfer of electrons to molecules such that the molecular rotational (vojo goes to voj) or vibrational and rotational (v(sub 0)j(sub 0) goes to vj) states are changed. Excitation and de-excitation of electronic states was discussed in a previous talk at this conference.

Thuemmel, Helmar T.↗
NASA NTRS1978

Some properties of upper atmosphere dynamics

A unifying description is provided of some important dynamic properties of the upper atmosphere in composition and temperature characteristic of a variety of phenomena, including diurnal and seasonal tides, magnetic storms, and momentum coupling with the magnetosphere. A theoretical multiconstituent model is used which can link the large-scale variations of composition and temperature to the dynamics and energetics of the thermosphere. Global mean properties of the thermosphere are reviewed, and an attempt is made to convey some understanding of the dynamic properties of energy and diffusive mass transport in the thermosphere. Attention is given to sources of energy for the thermosphere, the transport processes involved in the solar diurnal tide of the thermosphere, energy and particle sources for the annual tide, feedback from composition changes to wind-field and temperature variations, energy deposition in the thermosphere during magnetic storms and substorms, and momentum source signatures in the thermosphere.

Mayr, H. G.↗
NASA NTRS1983

Acoustic fluidization and the scale dependence of impact crater morphology

A phenomenological Bingham plastic model has previously been shown to provide an adequate description of the collapse of impact craters. This paper demonstrates that the Bingham parameters may be derived from a model in which acoustic energy generated during excavation fluidizes the rock debris surrounding the crater. Experimental support for the theoretical flow law is presented. Although the Bingham yield stress cannot be computed without detailed knowledge of the initial acoustic field, the Bingham viscosity is derived from a simple argument which shows that it increases as the 3/2 power of crater diameter, consistent with observation. Crater collapse may occur in material with internal dissipation Q as low as 100, comparable to laboratory observations of dissipation in granular materials. Crater collapse thus does not require that the acoustic field be regenerated during flow.

Melosh, H. J.↗
NASA NTRS1993

Use of spectral analogy to evaluate canopy reflectance sensitivity to leaf optical property

The spectral variation of canopy reflectance is mostly governed by the absorption properties of the elements, hence the leaves, since their intrinsic scattering properties show very little spectral variation. The relationship between canopy reflectance and leaf reflectance measured at the red edge over sugar beet canopies was used to simulate canopy reflectance from leaf reflectance spectra measured over the whole spectral domain. The results show that the spectral analogies found allows accurate reconstruction of canopy reflectance spectra. Explicit assumptions about the very low spectral variation of leaf intrinsic scattering properties are thus indirectly justified. The sensitivity of canopy reflectance (rho(sub c)) to leaf optical properties can then be investigated from concurrent spectral variations of canopy (delta rho(sub c)/delta lambda) and leaf reflectance (delta rho(sub l)/delta lambda): (delta rho(sub c))/(delta rho(sub l)) = ((delta rho(sub c))/(delta lambda) ((delta rho( sub l))/(delta lambda))(sup -1)). This expression is strictly valid only when the optical properties of the soil background or the other vegetation elements such as bark are either spectrally flat or do not contribute significantly to canopy reflectance. Simulations using the SAIL and PROSPECT models demonstrate that the sensitivity of canopy reflectance to leaf reflectance is significant for large vegetation cover fractions in spectral domains where absorption is low. In these conditions, multiple, scattering enhances the leaf absorption features by a factor that can be greater than 2.0. To override the limitations of the SAIL model for the description of the canopy architecture, we tested the previous findings on experimental data. Concurrent canopy and leaf reflectance spectra were measured for a range of sugar beet canopies. The results show good agreement with the theoretical findings. Conclusions are drawn about the applicability of these findings, with particular attention to the potential detectability of leaf biochemical composition from canopy reflectance sensed from space.

Baret, Frederic↗
NASA NTRS1995

Modeling and numerical simulation of microwave pulse propagation in air breakdown environment

It is shown that ionization occurs wherever the field intensity of the pulse exceeds the local breakdown threshold field of the background air. The produced plasma then attenuates the pulse and gives rise to a tail erosion phenomenon that plays the primary role in limiting the energy transfer of the pulse from source to destination. A theoretical model describing the propagation of an intense microwave pulse in an air breakdown environment is developed that includes the possible focusing effect introduced by either using phase array antennas or the other arrangements. The self-consistent description of the propagation process is provided by a set of two modal equations. These include a continuity equation (Poynting's equation) for the energy density of the pulse and a rate equation of the electron density. A forward wave approximation is used to simplify Poynting's equation, and a semiempirical formula is used for the ionization frequency, nu(sub i). This frequency provides the coupling between the two modal equations, and is used to express the electron rate equation explicitly. In terms of the relevant parameters of the atmosphere, these two equations are normalized for numerical analysis of pulse propagation in the atmosphere. The dependencies of the propagation characteristics of the pulse on intensity, frequency, width, and shape of the pulse are determined. The numerical simulations lead to a useful empirical relation p(exp 3)w = alpha = constant, where p and w are the incident power and width of the pulse and alpha depends on the percentage of pulse energy transferred from the source point to a destined position. The density distribution of the pulse's self-generated plasma is also evaluated. The results also show that for ionization caused by a single unfocused microwave pulse transmitted upwards from the ground, the maximum electron density produced at, for example, 50 km altitude is limited by the tail erosion effect to below 10(exp 6) cm(exp -3). Repetitive pulse and focused beam approaches are also examined. Both approaches can increase the maximum electron density by no more than an order of magnitude. A scheme using two obliquely propagating pulses intersecting at the destined height, e.g. 50 km, is considered. It is shown that the electron density generated at the lowest intersecting position can easily reach a value of 6.6 x 10(exp 8) cm(exp -3), which is considered to be high enough for artificial ionospheric mirror (AIM) application.

Kim, J.↗
NASA NTRS1988

Biaxial experiments supporting the development of constitutive theories for advanced high-temperature materials

Complex states of stress and strain are introduced into components during service in engineering applications. It follows that analysis of such components requires material descriptions, or constitutive theories, which reflect the tensorial nature of stress and strain. For applications involving stress levels above yield, the situation is more complex in that material response is both nonlinear and history dependent. This has led to the development of viscoplastic constitutive theories which introduce time by expressing the flow and evolutionary equation in the form of time derivatives. Models were developed here which can be used to analyze high temperature components manufactured from advanced composite materials. In parallel with these studies, effort was directed at developing multiaxial testing techniques to verify the various theories. Recent progress in the development of constitutive theories from both the theoretical and experimental viewpoints are outlined. One important aspect is that material descriptions for advanced composite materials which can be implemented in general purpose finite element codes and used for practical design are verified.

Ellis, J. R.↗
NASA NTRS1972

Conjoint-measurement framework for the study of probabilistic information processing.

The theory of conjoint measurement described by Krantz et al. (1971) is shown to indicate how a descriptive model of human processing of probabilistic information built around Bayes' rule is to be tested and how it is to be used to obtain subjective scale values. Specific relationships concerning these scale values are shown to emerge, and the theoretical prospects resulting from this development are discussed.

Wallsten, T. S.↗
NASA NTRS2016

CGRO/BATSE Data Support the New Paradigm For GRB Prompt Emission and the New L-i(nTh)-E-peak,i(nTh,rest) Relation

The paradigm for gamma-ray burst (GRB) prompt emission is changing. Since early in the Compton Gamma RayObservatory (CGRO) era, the empirical Band function has been considered a good description of the keV-MeV-gamma-ray prompt emission spectra despite the fact that its shape was very often inconsistent with the theoretical predictions, especially those expected in pure synchrotron emission scenarios. We have recently established a new observational model analyzing data of the NASA Fermi Gamma-ray Space Telescope. In this model, GRB prompt emission would be a combination of three main emission components: (i) a thermal-like component that we have interpreted so far as emission from the jet photosphere, (ii) a non-thermal component that we have interpreted so far as either synchrotron radiation from the propagating and accelerated charged particles within the jet or reprocessed jet photospheric emission, and (iii) an additional non-thermal (cutoff) power law (PL) extending from low to high energies in gamma-rays and most likely of inverse Compton origin. In this article we reanalyze some of the bright GRBs, namely GRBs 941017, 970111, and 990123, observed with the Burst And Transient Source Experiment (BATSE) on board CGRO with the new model. We conclude that BATSE data for these three GRBs are fully consistent with the recent results obtained with Fermi: some bright BATSE GRBs exhibit three separate components during the prompt phase with similar spectral parameters as those reported from Fermi data. In addition, the analysis of the BATSE GRBs with the new prompt emission model results in a relation between the time-resolved energy flux of the non-thermal component, F(in)(Th), and its corresponding nuFnu spectral peak energy,Epeak,inTh (i.e., FinThEpeak,inTh ), which has a similar index when fitted to a PL as the one initially derived from Fermi data. For GRBs with known redshifts (z) this results in a possible universal relation between the luminosity of the non-thermal component, LinTh, and its corresponding nuFnu spectral peak energy in the rest frame, Epeak,iNT,rest (i.e.,LinThEpeak,iNT,rest). We estimated the redshifts of GRBs 941017 and 970111 using GRB 990123 with z = 1.61as a reference. The estimated redshift for GRB 941017 is typical for long GRBs and the estimated redshift for GRB970111 is right in the range of the expected values for this burst.

acceleration of particles – black hole physics â↗
NASA NTRS2001

The Structure and Dynamics of the Solar Corona and Inner Heliosphere

This report covers technical progress during the third quarter of the second year of NASA Sun-Earth Connections Theory Program (SECTP) contract 'The Structure and Dynamics of the Solar Corona and Inner Heliosphere,' NAS5-99188, between NASA and Science Applications International Corporation, and covers the period February 16, 2001 to May 15, 2001. Under this contract SAIC and the University of California, Irvine (UCI) have conducted research into theoretical modeling of active regions, the solar corona, and the inner heliosphere, using the MHD model.In this report we summarize the accomplishments made by our group during the first seven quarters of our Sun-Earth Connection Theory Program contract. The descriptions are intended to illustrate our principal results. A full account can be found in the referenced publications.

Mikic, Zoran↗
NASA NTRS2004

Random Matrix Approach to Quantum Adiabatic Evolution Algorithms

We analyze the power of quantum adiabatic evolution algorithms (Q-QA) for solving random NP-hard optimization problems within a theoretical framework based on the random matrix theory (RMT). We present two types of the driven RMT models. In the first model, the driving Hamiltonian is represented by Brownian motion in the matrix space. We use the Brownian motion model to obtain a description of multiple avoided crossing phenomena. We show that the failure mechanism of the QAA is due to the interaction of the ground state with the "cloud" formed by all the excited states, confirming that in the driven RMT models. the Landau-Zener mechanism of dissipation is not important. We show that the QAEA has a finite probability of success in a certain range of parameters. implying the polynomial complexity of the algorithm. The second model corresponds to the standard QAEA with the problem Hamiltonian taken from the Gaussian Unitary RMT ensemble (GUE). We show that the level dynamics in this model can be mapped onto the dynamics in the Brownian motion model. However, the driven RMT model always leads to the exponential complexity of the algorithm due to the presence of the long-range intertemporal correlations of the eigenvalues. Our results indicate that the weakness of effective transitions is the leading effect that can make the Markovian type QAEA successful.

Boulatov, Alexei↗
NASA NTRS2000

Theoretical Analysis of the Electron Spiral Toroid Concept

This report describes the analysis of the Electron Spiral Toroid (EST) concept being promoted by Electron Power Systems Inc. (EPS). The EST is described as a toroidal plasma structure composed Of ion and electron shells. It is claimed that the EST requires little or no external confinement, despite the extraordinarily large energy densities resulting from the self-generating magnetic fields. The present analysis is based upon documentation made available by EPS, a previous description of the model by the Massachusetts Institute of Technology (MIT), and direct discussions with EPS and MIT. It is found that claims of absolute stability and large energy storage capacities of the EST concept have not been substantiated. Notably, it can be demonstrated that the ion fluid is fundamentally unstable. Although various scenarios for ion confinement were subsequently suggested by EPS and MIT, none were found to be plausible. Although the experimental data does not prove the existence of EST configurations, there is undeniable experimental evidence that some type of plasma structures whose characteristics remain to be determined are observed. However, more realistic theoretical models must first be developed to explain their existence and properties before applications of interest to NASA can he assessed and developed.

Cambier, Jean-Luc↗