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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 595 records · Page 33
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NASA NTRS1981

The ultraviolet spectrum of the peculiar A star HD 51418

High-dispersion observations of the peculiar A star HD 51418 have been carried out in the ultraviolet by the IUE satellite. Atomic resonance lines observed in the regions 1800-2000 and 2500-3000 A include those of Mg II, Ti II, Cr I, Cr II, Mn II, Fe II and Zn II, and possibly Si II, S I, V II, Fe I, Co II, Y II, Ce II, Ce III, Eu II, Eu III and Tb III. The Mg II 2796 and 2803 A resonance lines exhibit definite variations in equivalent width with orbital phase, which can be attributed to changes in the local atmospheric structure due to the patchy distribution of other elements. In addition, the observations confirm the well known trend for a light-heavy asymmetry in iron peak spectra similar to that observed in the rare earths.

Adelman, S. J.↗
NASA NTRS1982

Total ozone-solar activity relationship

Approximately six years of global ozone monthly mean data from the Nimbus 4 BUV instrument are compared with monthly values of solar activity using 10.7-cm flux, F(10.7), as a parameter. Several techniques are explored in calculating the correlation between the two data sets, and all are found to yield relatively high correlations, ranging from R = 0.68 for 'raw' monthly means to 0.94 using a six-month running mean for each data set. It is shown, however, that the bulk of the correlation derives from the long-term decreasing trends in both data sets. When the long-term trends are removed, a cross-correlation analysis produces a maximum with no phase shift or with the ozone variations leading the solar variations by one month, thereby reducing the likelihood of a cause and effect relationship on time scales of this order. In view of the current uncertainty in the long-term stability of the BUV instrument and the resulting uncertainty in any long-term trend derived from its data, it is considered unrealistic to draw firm conclusions about a solar cycle influence on total ozone from this satellite data set.

Reber, C. A.↗
NASA NTRS1982

Image sampling, reconstruction, and the effect of sample-scene phasing

This paper is a 1-D analysis of the degradation caused by image sampling and interpolative reconstruction. The analysis includes the sample-scene phase as an explicit random parameter and provides a complete characterization of this image degradation as the sum of two terms: one term accounts for the mean effect of undersampling (aliasing) and nonideal reconstruction averaged over all sample-scene phases; the other term accounts for variations about this mean. The results of this paper have application to the design and performance analysis of image scanning, sampling, and reconstruction systems.

Park, S. K.↗
NASA NTRS1984

HEAO 3 observations of the Crab pulsar

The Crab pulsar (PSR 0531+21) is the only pulsar which has been observed throughout almost the entire electromagnetic spectrum from 0.5 keV to over 2000 GeV. In general, the emission from the Crab nebula and its pulsar has been remarkably constant in time (except for the pulsations). However, several recent observations, especially of gamma-ray line emission, indicate possible time variations. The present investigation is concerned with the data obtained with the HEAO C-1 experiment. The data have been studied with the hope of finding information regarding a number of questions, taking into account the existence of line emission from the pulsar, the variation of the shape of the light curve with time, and the spectral variations as a function of pulsar phase. The spectrum of the total phase-averaged pulsed emission is found to be consistent with a single power-law spectrum from 50 keV to 10 MeV.

Mahoney, W. A.↗
NASA NTRS1986

Microstructure-property relationships in directionally solidified single crystal nickel-base superalloys

Some of the microstructural features which influence the creep properties of directionally solidified and single crystal nickel-base superalloys are discussed. Gamma precipitate size and morphology, gamma-gamma lattice mismatch, phase instability, alloy composition, and processing variations are among the factors considered. Recent experimental results are reviewed and related to the operative deformation mechanisms and to the corresponding mechanical properties. Special emphasis is placed on the creep behavior of single crystal superalloys at high temperatures, where directional gamma coarsening is prominent, and at lower temperatures, where gamma coarsening rates are significantly reduced. It can be seen that very subtle changes in microstructural features can have profound effects on the subsequent properties of these materials.

Mackay, R. A.↗
NASA NTRS1987

Submillimeter diagnostics of the response of the solar chromosphere to compressional waves

The NASA Infrared Telescope Facility has been used to observe local continuum brightness variations of the quiet sun in 350 and 800 micron radiation simultaneously. Variations of order 5 K are found at both wavelengths, with a strong correlation between the two. The 800 micron variations lag behind the 350 micron variations by 25-35 deg in phase. This is similar to unexpected phase shifts reported by Lites and Chipman in 1979 and Lites, Chipman, and White in 1982 between velocity and intensity for certain chromospheric lines. It is proposed that the phase lags observed are the result of nonadiabatic response of the chromosphere to compression, which could be an important mechanism for dissipation of mechanical energy.

Lindsey, C.↗
NASA NTRS1988

Studies of the vestibulo-ocular reflex on STS 4, 5 and 6

The vestibulo-ocular reflex (VOR) may be altered by weightlessness. Since this reflex plays a large role in visual stabilization, it was important to document any changes caused by space flight. This is a report on findings on STS-4 through 6 and is part of a larger study of neurosensory adaptation done on STS-4 through 8. Voluntary horizontal head oscillations at 1/3 Hz with amplitude of 30 deg right and left of center were recorded by a potentiometer and compared to eye position recorded by electroculography under the following conditions: eyes open, head fixed, tracking horizontal targets switched 0, 15, and 30 degrees right and left (optokinetic reflex - OKR - and calibration); eyes open and fixed on static external target with oscillation, (vestibulo ocular reflex, eyes closed - VOR EC); eyes open and wearing opaque goggles with target fixed in imagination (vestibulo-ocular reflex, eyes shaded - VOR ES); and eyes open and fixed on a head synchronized target with head oscillation (VOR suppression). No significant changes were found in voluntary head oscillation frequency or amplitude in those with (n=5), and without (n=3), space motion sickness (SMS), with phase of flight or test condition. Variations in head oscillation were too small to have produced detectable changes in test results.

Thornton, William E.↗
NASA NTRS1988

Microstructure-property relationships in directionally solidified single-crystal nickel-base superalloys

This paper discusses some of the microstructural features which influence the creep properties of directionally solidified and single-crystal nickel-base superalloys. Gamma prime precipitate size and morphology, gamma-gamma (prime) lattice mismatch, phase instability, alloy composition, and processing variations are among the factors considered. Recent experimental results are reviewed and related to the operative deformation mechanisms and to the corresponding mechanical properties. Special emphasis is placed on the creep behavior of single-crystal superalloys at high temperatures, where directional gamma (prime) coarsening is prominent, and at lower temperatures, where gamma (prime) coarsening rates are significantly reduced. It can be seen that very subtle changes in microstructural features can have profound effects on the subsequent properties of these materials.

Mackay, Rebecca A.↗
NASA NTRS1990

Microstructure and magnetization of doped Y-Ba-Ca-O materials prepared by melt quench and post annealing method

Y-Ba-Cu-O bulk materials prepared using the melt quench and post annealing method were shown to have very high maximum as well as remanent magnetization. Studies were carried out on materials prepared using this method which deviate from the Y:Ba:Cu = 1:2:3 stoichiometry. In one series of materials, only the stoichiometry was changed, in particular by introducing an excess of yttrium. In other cases, dopants including several rare earths were introduced. Effects of variations in composition on microstructure and phase evolution are discussed, as well as effects on the magnetic susceptibility and on the magnetization. The results show that doped materials can exhibit improvements in magnetic properties. Furthermore, the use of dopants sheds light on the role of defect sites in flux pinning.

Hojaji, H.↗
NASA NTRS1990

Far-infrared intensity variations caused by 5 minute oscillations

Observations of solar IR intensity variations at 50, 100, and 200 microns were made simultaneously and cospatially with Doppler measurements in the sodium D1 line at 5896 A. Brightness temperature variations of several K in amplitude are highly correlated with five minute Doppler oscillations. The brightness variations are attributed to work done on the chromospheric medium by compression, driven by the five minute oscillations. The Doppler oscillations lead the brightness variations by about 47 deg in phase at 50 and 100 microns and by about 72 deg in phase at 200 microns.

Lindsey, C.↗
NASA NTRS1990

The ultraviolet and visible light variability of BP Tauri - Possible clues for the origin of T Tauri star activity

BP Tau is a moderate-strength T Tauri star, which has previously been studied photometrically by Vrba et al. (1986, 1989) and shown to have a 7.6-day light cycle. Here, BP Tau was reobserved in order to monitor this star with the IUE satellite. In conjunction with the UV spectroscopy, nearly simultaneous ground-based optical photometry was also obtained. At optical wavelengths, the star was initially found to be varying with a period of 7.6 days. This variability suddenly vanished and was followed by a quiescent state that lasted about 11 days. The star then resumed its variability with a period of about 6.1 days. The UV continuum and most of the UV emission lines appeared to vary in phase with the optical flux. No variations were observed on the very short timescales ranging from several hours to half a day, expected on the basis of axisymmetric, quasi-steady-state accretion disk models of T Tauri stars. However, the nearly week-long variations that were observed may be consistent with a magnetically buffered accretion model, which has been proposed by Uchida and Shibata.

Simon, Theodore↗
NASA NTRS1991

First epoch geodetic measurements with the Global Positioning System across the northern Caribbean plate boundary zone

The first geodetic survey across the northern Caribbean plate boundary zone with GPS was conducted in June 1986. Baseline vectors defined by the six-station regional GPS network ranged from 170 to 1260 km in length. Repeatability of independent daily baseline estimates was better than 8 mm plus 1.3 parts in 10 to the 8th of baseline length for horizontal components. The wet tropospheric path delay during the experiment was both high, sometimes exceeding 30 cm at zenith, and variable, sometimes exceeding 5 cm variation over several hours. Successful carrier phase cycle ambiguity resolution (bias fixing) could not be achieved prior to construction of a regional troposphere model. With optimum troposphere treatment and single-day orbital arcs, most biases on baselines were resolved up to about 550 km in length. With multiday orbital arcs most biases in the network were resolved regardless of baseline length. The results suggest that constraints on plate-boundary zone deformation in the Greater Antilles, and on the North America-Caribbean relative plate motion vector, can be obtained with a series of GPS experiments spanning less than 10 and 15 years, respectively.

Dixon, T. H.↗
NASA NTRS1991

Instantaneous heat transfer coefficient based upon two-dimensional analyses of Stirling space engine components

Twelve different cases of multidimensional models of Stirling engine components for space applications have been numerically investigated for oscillating, incompressible laminar flow with heat transfer. The cases studied covered wide ranges of Valensi number (from 44 to 700), Re(max) number (from 8250 to 60,000), and relative amplitude of fluid motion of 0.686 and 1.32. The Nusselt numbers obtained from the present study indicate a very complex shape with respect to time and axial location in the channel. The results indicate that three parameters can be used to define the local Nusselt number variation, namely: time average, amplitude, and phase angle. These parameters could be correlated respectively using: Re(max), Va and Re(max), and the relative amplitude of fluid motion.

Ibrahim, Mounir↗
NASA NTRS1993

Analysis of the IUE spectra of the strongly interacting binary beta Lyrae

The six-band ultraviolet light curves of beta Lyrae obtained with the Orbiting Astronomical Observatory A-2 in 1970 exhibited a very unusual behavior. The secondary minimum deepened at shorter wavelength, indicating that one was not observing light variations caused primarily by the eclipses of two stars having a roughly Planckian energy distribution. It was then suggested that the light variations were caused by a viewing angle effect of an optically-thick, ellipsoidal circumbinary gas cloud. Since 1978 beta Lyrae has been observed with the International Ultraviolet Explorer (IUE) satellite. We have constructed ultraviolet light curves from the IUE archival data for comparison with the OAO-A2 results. We find that they are in substantial agreement with each other. The Voyager ultraviolet spectrometer was also used to observe this binary during a period covered by IUE observations. The Voyager results agree with those of the two other satellite observatories at wavelengths longer than about 1350 A. However, in the wavelength region shorter than the Lyman-alpha line at 1216 A, the light curves at 1085 A and 965 A show virtually no light variation except an apparent flaring near phase 0.7, which is also in evidence at longer wavelengths. We suggest that the optically-thick circumbinary gas cloud, which envelops the two stars completely, assumes a roughly spherical shape when observed at these shorter wavelengths.

Mccluskey, George E., Jr.↗
NASA NTRS1994

Experimental and analytical study on fluid whirl and fluid whip modes

Fluid whirl and fluid whip are rotor self-excited, lateral vibrations which occur due to rotor interactions with the surrounding fluid. There exist various modes of fluid whirl and fluid whip. These modes are close to rotor modes corresponding to free vibrations (based on the linear model). Small differences are due to nonlinearities in the system. This paper presents experimental and analytical results on the lowest modes of fluid whirls and fluid whip. Examples of rotors supported in fluid lubricated bearings show the variations of rotor deflection amplitudes and phases in the whirl and whip modes with changes of rotative speeds and/or changes in lumped mass locations along the shaft.

Muszynska, Agnes↗
NASA NTRS1994

Ultraviolet light curves of beta Lyrae: Comparison of OAO A-2, IUE, and Voyager Observations

The six-band ultraviolet light curves of beta Lyrae obtained with the Orbiting Astronomical Observatory (OAO) A-2 in 1970 exhibited a very unusual behavior. The secondary minimum deepened at shorter wavelength, indicating that one was not observing light variations caused primarily by the eclipses of two stars having a roughly Planckian energy distribution. It was then suggested that the light variations were caused by a viewing angle effect of an optically thick, ellipsoidal circumbinary gas cloud. Since 1978 beta Lyrae has been observed with the International Ultraviolet Explorer (IUE) satellite. We have constructed ultraviolet light curves from the IUE archival data for comparison with the OAO A-2 results. We find that they are in substantial agreement with each other. The Voyager ultraviolet spectrometer was also used to observe this binary during a period covered by IUE observations. The Voyager results agree with those of the two other satellite observatories at wavelengths longer than about 1350 A. However, in the wavelength region shorter than the Lyman-alpha line at 1216 A, the light curves at 1085 and 965 A show virtually no light variation except an apparent flaring near phase 0.7, which is also in evidence at longer wavelengths. We suggest that the optically thick circumbinary gas cloud, which envelops the two stars completely, assumes a roughly spherical shape when observed at these shorter wavelengths.

Kondo, Yoji↗
NASA NTRS1994

An outer gap model of high-energy emission from rotation-powered pulsars

We describe a refined calculation of high-energy emission from rotation-powered pulsars based on the outer gap model of Cheng, Ho, & Ruderman (1986 a, b). In this calculation, vacuum gaps form in regions near the speed-of-light cylinder of the pulsar magnetosphere along the boundary between the closed and open field line zones. We have improved upon previous efforts to model the spectra from these pulsars (e.g., Cheng et al. 1986b; Ho 1989) by following the variation in particle production and radiation properties with position in the outer gap. Curvature, synchotron, and inverse-Compton scattering fluxes vary significantly over the gap, and their interactions via photon-photon pair production build up the radiating charge populations at varying rates. We have also incorporated an approximate treatment of the transport particle and photon fluxes between gap emission zones. These effects, along with improved computations of the particle and photon distribution, provide very important modifications of the model gamma-ray flux. In paticular, we attempt to make specific predictions of pulse profile shapes and spectral variations as a function of pulse phase and suggest further extensions to the model which may provide accurate computations of the observed high-energy emissions.

Chiang, James↗
NASA NTRS1994

ROSAT HRI observations of hot stars in the Orion Nebula

ROSAT High Resolution Interferometer (HRI) observations of hot (O6-A5) stars in the Orion Nebula region are presented. Fourteen of 21 O6-B5 stars were detected and all of them appear to have X-ray luminosities and L(sub x)/L(sub bol) ratios similar to field O6-B5 stars. The brightest star in the Trapezium, the O7 V star theta(sup 1) C Ori, has notable variation in its X-ray emission; the variation seems to have the same phase dependence as recently found for the star's hydrogen alpha emission. A maximum of six of 27 B6-A5 stars observed were detected; we argue that the most likely explanation for their X-ray emission is that it arises from unseen, low-mass binary companions.

Caillault, Jean-Pierre↗