Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “structural evolution”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

New horizons in astronomy.

Besides the study of astronomy itself, other topics included cover the geological and biological developments on earth and basic physics of matter, heat, and light. Optical and radio telescopes are discussed, as well as photographic and photoelectric means of detection. The immediate solar system is described by topics on the properties and atmospheres of the inner and outer planets, the sun's energy, sunspots, and the solar spectrum. Stars both on and off the main sequence are discussed in terms of distances, intrinsic properties, lifetimes and evolution. The Milky Way is compared to other galaxies in size, star population and structure, and the different galaxy shapes are pictured. Topics of most recent interest are covered by results of the lunar explorations, new concepts of Mars, and problems of space travel. Problems of modern astronomy include pulsars, neutron stars, and quasars.

Brandt, J. C.↗
NASA NTRS1974

Preservation of lunar core samples - Preparation and interpretation of three-dimensional stratigraphic sections

Preservation and analysis of core samples collected from the lunar regolith are discussed. The objectives of core sampling are described together with the core-sampling equipment used in the Apollo program. The effects of disaggregation in core samples are summarized, and ways to preserve the stratigraphy in these samples are reviewed, particularly partial impregnation of samples with polybutyl methacrylate. It is shown that stratigraphic sections preserved in this way reveal individual fragments of the regolith in their original positions, primary and secondary depositional structures related to lunar processes, and the nature and extent of deformations caused by sampling equipment. It is concluded that since the core sedimentary structures indicate complex three-dimensional variations in the regolith, quantitative models of regolith evolution must allow for distinctive local histories as well as irregular reworking and deposition events due to impact processes.

Fryxell, R.↗
NASA NTRS1981

A comparison of the height distributions of solar flare hard X-rays in thick target and thermal models

Using recent observations of solar hard X-ray bursts having a spatial resolution of a few arcsec, a computation is made of the theoretically predicted spatial variation of hard X-ray flux versus height in both thick target nonthermal and thermal models of solar hard X-ray bursts. The work on the thick target model revises previous results in this area by adopting a more realistic model for the flaring atmospheric structure and also by taking into account energy loss and scattering processes in the evolution of the nonthermal electron beam which have previously been neglected. It is noted that in the so-called 'thermal' model currently in vogue there is a substantial nonthermal bremsstrahlung component at all photon energies; it is further demonstrated that this nonthermal component results in a hard X-ray flux versus height distribution whose maximum brightness per unit length is, similarly to the thick target model, located in the chromosphere and not in the thermal source itself. The characteristics of the hard X-ray height distributions in both thick target and thermal models are summarized and compared.

Emslie, A. G.↗
NASA NTRS1982

Understand B-type stars

When observations of B stars made from space are added to observations made from the ground and the total body of observational information is confronted with theoretical expectations about B stars, it is clear that nonthermal phenomena occur in the atmospheres of B stars. The nature of these phenomena and what they imply about the physical state of a B star and how a B star evolves are examined using knowledge of the spectrum of a B star as a key to obtaining an understanding of what a B star is like. Three approaches to modeling stellar structure (atmospheres) are considered, the characteristic properties of a mantle, and B stars and evolution are discussed.

Source record↗
NASA NTRS1983

A statistical VLBI study of milli-arcsecond cores in extragalactic radio sources

VLBI observations at 2.3 GHz with a baseline of approximately 8 x 10 to the 7th wavelengths were performed on a complete sample of 103 sources from the Parkes + or - 4 deg catalog. Compact milli-arcsec cores were found in 35 percent of all sources: in 80 percent of quasars, in 10 percent of galaxies, and in 20 percent of empty field sources. It is shown that for quasars the percentage of the source flux density coming from milli-arcsec cores increased with increasing radio spectral index, radio variability, and optical redness. It is noted that quasars with extended radio structure (not less than approximately 10 arcsec) seem more likely to have a detectable milli-arcsec core than do extended radio galaxies. The absence of a strong correlation between quasar milli-arcsec structure and redshift is found to be evidence for a lack of strong physical evolution in quasars.

Preston, R. A.↗
NASA NTRS1983

Acceleration and confinement of energetic particles in the 1980 June 7 solar flare

Pulsations with large amplitude and duration have been observed during the hard X-ray and microwave radio bursts associated with the 1980 June 7 solar flare. The high time resolution measurements of 20-800 keV X-rays were made with the X-ray spectrometers aboard the ISEE 3 and P78-1 spacecraft. The radio measurements, covering metric to microwave wavelengths, were made at the Nobeyama and Toyokawa observatories in Japan. The temporal evolution of the X-ray and radio spectra and the polarization and spatial structure of the microwave source have been examined. The following interpretation is found to be consistent with the observations: (1) the variations in the electron acceleration/injection spectrum are responsible for the observed variations in the hard X-ray and microwave emissions; (2) the locations of the hard X-ray and microwave sources are probably different, the X-ray source being located at a lower altitude.

Kane, S. R.↗
NASA NTRS1983

The formation of galaxies

Current models of galaxy formation are examined in a review of recent observational and theoretical studies. Observational data on elliptical galaxies, disk galaxies, luminosity functions, clustering, and angular fluctuations in the cosmic microwave background are summarized. Theoretical aspects discussed include the origin and early evolution of small fluctuations, matter and radiation fluctuations, the formation of large-scale structure, dissipationless galaxy formation, galaxy mergers, dissipational galaxy formation, and the implications of particle physics (GUTs, massive neutrinos, and gravitinos) for cosmology.

Efstathiou, G.↗
NASA NTRS1987

A processing method and results of meteor shower radar observations

Studies of meteor showers permit the solving of some principal problems of meteor astronomy: to obtain the structure of a stream in cross section and along its orbits; to retrace the evolution of particle orbits of the stream taking into account gravitational and nongravitational forces and to discover the orbital elements of its parent body; to find out the total mass of solid particles ejected from the parent body taking into account physical and chemical evolution of meteor bodies; and to use meteor streams as natural probes for investigation of the average characteristics of the meteor complex in the solar system. A simple and effective method of determining the flux density and mass exponent parameter was worked out. This method and its results are discussed.

Belkovich, O. I.↗
NASA NTRS1988

Ultraviolet Spectra of Two Magnetic White Dwarfs and Ultraviolet Spectra of Subluminous Objects Found in the Kiso Schmidt Survey and Ultraviolet Absorptions in the Spectra of DA White Dwarfds

Research under NASA Grant NAG5-287 has carried out a number of projects in conjunction with the International Ultraviolet Explorer (IUE) satellite. These include: (1) studies of the UV spectra of DA white dwarfs which show quasi-molecular bands of H2 and H2(+); (2) the peculiar star HR6560; (3) the UV spectra of two magnetic white dwarfs that also show the quasi-molecular features; (4) investigations of the UV spectra of subluminous stars, primarily identified from visual wavelength spectroscopy in the Kiso survey of UV excess stars, some of which show interesting metal lines in their UV spectra; and (5) completion of studies of UV spectra of DB stars. The main result of this research has been to further knowledge of the structure and compositions of subluminous stars which helps cast light on their formation and evolution.

Wegner, Gary A.↗
NASA NTRS1987

Radio propagation experiments in the outer solar system with Voyager

The outer solar system's planetary atmospheres, ionospheres, rings, and magnetic fields are under study in light of microwave telecommunications from the two Voyager spacecraft. The use of the hydrogen maser frequency standards on the ground, in conjunction with thermally controlled quartz oscillators aboard the spacecraft, ensures long coherence intervals and allows the application of novel signal processing methods. On this basis, studies of atmospheric structure and scintillation parameters, planetary ring structure, and magnetic control of small ionospheric irregularities have been undertaken; information concerning planetary evolution, composition, and dynamics is thereby obtained.

Tyler, G. Leonard↗
NASA NTRS1988

Materials and formation of the Imbrium basin

A study of Imbrium basin deposits has been conducted, including geologic mapping, analysis of orbital geochemical data, earth-based near-IR spectroscopy, and analytical modeling. The morphologic facies of the deposits display an unusual bilateral symmetry with respect to the main rim of the basin, which is thought to be 1160 km in diameter and composed of Apennine-Carpathian-Alpes mountain ranges. The Imbrium ring structure consists of six concentric rings ranging from 550-3200 km in diameter. The evolution of the target for the Imbrium basin impact is discussed. It is suggested that the Imbrium impact, which occurred 3.85 Gyrs ago, formed a basin predominantly by a proportional-growth crater-forming mechanism.

Spudis, Paul D.↗
NASA NTRS1991

Mars aerobrake assembly demonstration

NASA has identified aerobraking as a potentially critical technology for the Space Exploration Initiative (SEI). The size of Mars aerobrakes may be beyond the capabilities of future launch vehicles to place them into orbit in one launch. On-orbit assembly using facilities and operations developed under the Space Station Freedom (SSF) Program represent one approach for realizing such large structures. the results of early testing in this subject can help influence the future evolution of the SSF. The objectives are to: (1) generate empirical data on operational procedures for on-orbit assembly of a large Mars aerobrake; (2) develop aerobrake design concepts; (3) identify critical issues and requirements associated with SSF utilization; and (4) to stimulate student participation in the SEI.

Garvey, John M.↗
NASA NTRS1993

The multicomponent structure of bulges

The morphology of disk galaxies is usually described by two major components, the centrally concentrated spheroidal component, called the bulge, and an oblate disk. The ratio of there contribution to the total luminosity - the bulge-to-disk ratio - is one of the parameters characterizing the Hubble sequence. Following de Vaucouleurs (1948), for most galaxies the radial distribution of the outer spheroid is fairly well described by the r exp 1/4 law I(r)=I(sub 0) exp(-(alpha)r), whereas the radial luminosity distribution of the disk follows an exponential law: I(r)=I(sub 0) exp(-alpha(r exp 1/4)) (Freeman 1970), with r the radial distance from the center. I(sub 0) and alpha are characteristic constants for each individual galaxy. Parameters for the structural properties of these components give important constraints for models of the formation and evolution of galaxies. Therefore we have tried to decompose disk and bulge components from high S/N CCD observations of a sample of edge-on disk galaxies. A common procedure for the decomposition is to model one component in a region where it dominates and subtract it from the combined light distribution. This technique was successfully carried out e.g. by van der Kruit & Earl (1981, 1982) and Wakamatsu & Hamabe (1984, 1989). Here we present two more examples of bulge-dominated edge-on SO galaxies, namely ESO 506-G33 and NGC 7123, which show an additional small and concentrated central component besides disk and 'bulge'.

Dettmar, Ralf-Juergen↗
NASA NTRS1992

Lunar and planetary landers for human exploration missions

This paper describes characteristics of lunar and planetary landers and mission profiles. Requirements are discussed, and propulsion and aerodynamic features are described. The paper covers recent history of concepts, configuration design drivers, propulsion, aerodynamics and aeroheating as applicable, structural and launch packaging factors, a summary of mission profiles, a description of the evolution of concepts, and reasons why the evolution has gone as it has.

Woodcock, Gordon↗
NASA NTRS1998

Ionospheric Instabilities and Shock Capturing Algorithms

We describe the basic physics of the gradient-drift instability and the collisional Rayleigh-Taylor instability, the two plasma instabilities which dominate the production of small-scale structure in the earth's ionosphere. We then focus our attention on the nonlinear evolution of these instabilities for two particular cases: 1) the recursive bifurcation and "freezing" of artificial ion clouds released in the ionosphere; and 2) equatorial spread F. Finally, we discuss the numerical challenges faced in attempting to numerically simulate these instabilities, and the role played by shock-capturing algorithms therein.

Zalesak, Steven T.↗
NASA NTRS2001

Interface Pattern Selection in Directional Solidification

The central focus of this research is to establish key scientific concepts that govern the selection of cellular and dendritic patterns during the directional solidification of alloys. Ground-based studies have established that the conditions under which cellular and dendritic microstructures form are precisely where convection effects are dominant in bulk samples. Thus, experimental data can not be obtained terrestrially under pure diffusive regime. Furthermore, reliable theoretical models are not yet possible which can quantitatively incorporate fluid flow in the pattern selection criterion. Consequently, microgravity experiments on cellular and dendritic growth are designed to obtain benchmark data under diffusive growth conditions that can be quantitatively analyzed and compared with the rigorous theoretical model to establish the fundamental principles that govern the selection of specific microstructure and its length scales. In the cellular structure, different cells in an array are strongly coupled so that the cellular pattern evolution is controlled by complex interactions between thermal diffusion, solute diffusion and interface effects. These interactions give infinity of solutions, and the system selects only a narrow band of solutions. The aim of this investigation is to obtain benchmark data and develop a rigorous theoretical model that will allow us to quantitatively establish the physics of this selection process.

Trivedi, Rohit↗
NASA NTRS2003

Radio Science Concepts and Approaches for Jupiter Icy Moons Orbiter

Radio Science experiments have been conducted on most deep space missions leading to numerous scientific discoveries. A set of concepts and approaches are proposed for the Jupiter Icy Moons Orbiter (JIMO) to apply Radio Science tools to investigate the interior structures of the Galilean Satellites and address key questions on their thermal and dynamical evolution. Measurements are identified that utilize the spacecraft's telecommunication system. Additional instruments can augment these measurements in order to leverage observational synergies. Experiments are also offered for the purpose of investigating the atmospheres and surfaces of the satellites.

Anderson, J. D.↗