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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Further evidence for a color gradient in M15

Using data obtained at the Canada-France-Hawaii Telescope (CFHT) in subarcsecond seeing color maps are constructed in B - V and H-alpha - R of the central regions of the globular cluster M15. Comparison in B - V of the region within 0.31 pc of the cluster center with a coadded sample taken from the surrounding region demonstrates a color gradient, in the sense that the central region is bluer than the surrounding region at a high level of statistical significance. Analysis of the H-alpha - R color map indicates that the central region is 'H-alpha deficient', i.e., has a stronger H-alpha absorption line on average than the coadded region. This result argues against the possibility that the excess blue light from the central region is due to cataclysmic variables. Instead, two possible causes of the color gradient are (1) it may be due to a centrally concentrated population of faint blue stars, possibly resulting from binary-single star interactions, and (2) it may result from a central deficit of low-mass main-sequence stars.

Cederbloom, S. E.↗
NASA NTRS1990

Hypersonic flow simulations using DSMC (direct simulation Monte Carlo)

A review of the DSMC method of Bird is presented. The DSMC method provides the capability of simulating real gas flows in the rarefied flow regime. Recent developments and applications of the method for hypersonic flows are reported for both ground-based tests and during entry. Results obtained using both axisymmetric and 3D codes are included.

Moss, James N.↗
NASA NTRS1992

Hypersonic rarefied flow past spheres including wake structure

Results of a numerical study using the direct simulation Monte Carlo method are presented for hypersonic rarefied flow past spheres. The flow conditions considered are those corresponding to low density wind tunnel test conditions. The set of the experimental conditions for the calculations encompasses the transitional to near-continuum flow regimes. Comparison of the calculated drag with experimental results shows good agreement to well within the experimental error. Particular attention is focused on the wake structure. Calculations show that the wake is very rarefied with considerable thermal nonequilibrium for all the cases considered. No flow separation is observed in the wake for the near-continuum case where a vortex has been predicted by Navier-Stokes type calculations.

Dogra, Virendra K.↗
NASA NTRS1991

Diurnal changes in the dielectric properties and water status of eastern hemlock and red spruce from Howland, ME

The diurnal characteristics of microwave dielectric properties and water potential of two conifer species were investigated in July and September, 1990. P-band and C-band radial dielectric profiles of hemlock and red spruce, as well as hemlock diurnal water potential and dielectric profiles, are presented. The resulting radial dielectric profiles matched the regions of the functional sapwood (water transport component of the active xylem) in both species such that the sapwood was characterized by a higher dielectric than the bark and heartwood tissues. This is probably due to characteristic differences in the water content of each tissue. As the hemlocks progressed through their diurnal water potential pattern, the dielectric profile remained static until mid-afternoon. As the tension in the water column relaxed (2 to 3 bars) the dielectric constant decreased by 30 to 40 percent. There are several possible explanations for this phenomenon, and these may relate to the dependency of the dielectric measurements on temperature, salinity, and volumetric water content.

Salas, W. A.↗
NASA NTRS1991

Enhancements and applications of DSMC for hypersonic rarefied flows

A review of the direct simulation Monte Carlo (DSMC) method of Bird is presented. The DSMC method provides the capability of simulating real gas flows in the rarefied flow regime. Recent developments and applications of the method for hypersonic flows are reported for both ground-based tests and during entry. The flows considered were three-dimensional, and both an existing general code and a new code under development were used.

Moss, James N.↗
NASA NTRS1991

Application of the DSMC method to hypersonic flow about a delta wing

The direct simulation Monte Carlo method was applied to a hypersonic rarefied flow about a delta wing at angle of incidence. Three-dimensional calculations were performed using a single-species gas model that accounts for energy exchange with the internal modes. Computed results were compared with the available experimental data obtained in a nitrogen wind-tunnel experiment. Discrepancies were shown to exist between the experimental and calculated results. These differences may be caused by the inadequately defined freestream conditions.

Celenligil, M. C.↗
NASA NTRS1992

Gamma Ray and Neutron Spectrometer for the Lunar Resource Mapper

One of the early Space Exploration Initiatives will be a lunar orbiter to map the elemental composition of the Moon. This mission will support further lunar exploration and habitation and will provide a valuable dataset for understanding lunar geological processes. The proposed payload will consist of the gamma ray and neutron spectrometers which are discussed, an x ray fluorescence imager, and possibly one or two other instruments.

Moss, C. E.↗
NASA NTRS1992

Combined Gamma Ray/neutron Spectroscopy for Mapping Lunar Resources

Some elements in the Moon can be resources, such as hydrogen and oxygen. Other elements, like Ti or the minerals in which they occur, such as ilmenite, could be used in processing lunar materials. Certain elements can also be used as tracers for other elements or lunar processes, such as hydrogen for mature regoliths with other solar-wind-implanted elements like helium, carbon, and nitrogen. A complete knowledge of the elemental composition of a lunar region is desirable both in identifying lunar resources and in lunar geochemical studies, which also helps in identifying and using lunar resources. The use of gamma ray and neutron spectroscopy together to determine abundances of many elements in the top few tens of centimeters of the lunar surface is discussed. To date, very few discussions of elemental mapping of planetary surfaces considered measurements of both gamma rays and the full range of neutron energies. The theories for gamma ray and neutron spectroscopy of the Moon and calculations of leakage fluxes are presented here with emphasis on why combined gamma ray/neutron spectroscopy is much more powerful than measuring either radiation alone.

Reedy, R. C.↗
NASA NTRS1993

Gamma Ray/neutron Spectrometers for Planetary Elemental Mapping

Los Alamos has designed gamma ray and neutron spectrometers for Lunar Scout, two robotic missions to map the Moon from 100 km polar orbits. Knowledge of the elemental composition is desirable in identifying resources and for geochemical studies and can be obtained using gamma ray and neutron spectrometers. Measurements with gamma ray and neutron spectrometers complement each other in determining elemental abundances in a planet's surface. Various aspects of the instruments are discussed.

Reedy, R. C.↗
NASA NTRS1993

A Team Approach to the Development of Gamma Ray and x Ray Remote Sensing and in Situ Spectroscopy for Planetary Exploration Missions

An important part of the investigation of planetary origin and evolution is the determination of the surface composition of planets, comets, and asteroids. Measurements of discrete line X-ray and gamma ray emissions from condensed bodies in space can be used to obtain both qualitative and quantitative elemental composition information. The Planetary Instrumentation Definition and Development Program (PIDDP) X-Ray/Gamma Ray Team has been established to develop remote sensing and in situ technologies for future planetary exploration missions.

Trombka, J. I.↗
NASA NTRS1993

Radioactivities induced in some LDEF samples

Radioactivities induced in several Long Duration Exposure Facilities (LDEF) samples were measured by low-level counting at Los Alamos and elsewhere. These radionuclides have activities similar to those observed in meteorites and lunar samples. Some trends were observed in these measurements in terms of profiles in trunnion layers and as a function of radionuclide half-life. Several existing computer codes were used to model the production by the protons trapped in the Earth's radiation belts and by the galactic cosmic rays of some of these radionuclides, Mn-54 and Co-57 in steel, Sc-46 in titanium, and Na-22 in alloys of titanium and aluminum. Production rates were also calculated for radionuclides possibly implanted in LDEF, Be-7, Be-10, and C-14. Enhanced concentrations of induced isotopes in the surfaces of trunnion sections relative to their concentrations in the center are caused by the lower-energy protons in the trapped radiation. Secondary neutrons made by high-energy trapped protons and by galactic cosmic rays produce much of the observed radioactivities, especially deep in an object. Comparisons of the observed to calculated activities of several radionuclides with different half-lives indicate that the flux of trapped protons at LDEF decreased significantly at the end of the mission.

Reedy, Robert C.↗
NASA NTRS1993

Near wake structure for a generic ASTV configuration

Results of a numerical study are presented for hypersonic low-density flow about a 70-deg blunt cone using the direct simulation Monte Carlo method. Particular emphasis is given to the near wake flow and its sensitivity to rarefaction and other parametric variations. The flow conditions simulated are attainable in existing low-density hypersonic wind tunnels; that is, Mach 20 nitrogen flow encompassing freestream Knudsen numbers of 0.03 to 0.001. A stable vortex forms in the near wake at and below a freestream Knudsen number of 0.01 and the size of the vortex increases with decreasing freestream Knudsen number. The base region of the flow remains in thermal nonequilibrium for all cases. There is no formation of a lip separation shock or a distinct wake shock at these rarefied conditions.

Dogra, Virendra K.↗
NASA NTRS1993

An H-alpha survey of cluster spirals - Comparison of star formation in clusters and the field

In an objective prism survey of eight nearby Abell clusters, we have detected H-alpha emission from 77 out of a total of 201 CGCG spiral galaxies. We find that detection of H alpha emission is approximately independent of galaxy absolute magnitude, distance to the cluster center, and the presence of a bar. However, tidally distorted spirals are much more likely to be detected than undistorted spirals. Furthermore, there is a strong tendency for tidally distorted spirals to have compact nuclear emission rather than more extended disk-wide emission. When compared to field spirals, we find that late-type (Sc and Sc-Irr) cluster spirals have less H alpha emission, while early-type (Sa and Sab) cluster spirals can have significantly enhanced emission. The enhanced emission is most likely to be due to tidally induced star formation from galaxy-galaxy interactions.

Moss, C.↗