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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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

A study of spectrum fatigue crack propagation in two aluminum alloys. I - Spectrum simplification. II - Influence of microstructures

An investigation of the fatigue crack propagation FCP behavior of two aluminum alloys is performed to simulate spectrum loading conditions found at critical locations in high performance fighter aircraft. Negative loads are shown to be eliminated for the tension-compression spectrum for low to intermediate maximum stress intensities, and load interactions are found to be more significant at higher stress intensities and with more plasticity at the crack tip. In the second part, the influence of microstructural features including grain size, inclusions, and dispersoids on constant amplitude and spectrum crack growth behavior in aluminum alloys is studied. At low stress intensities the I/M alloy demonstrated better FCP resistance than the P/M 7091 alloy for both constant amplitude and spectrum testing, and the inhomogeneous planar slip and large grain size of 7050 limit dislocation interactions, thereby improving FCP performance.

Telesman, J.↗
NASA NTRS1985

A study of spectrum fatigue crack propagation in two aluminum alloys. 1: Spectrum simplification

The fatigue crack propagation behavior of two commercial Al alloys was studied using spectrum loading conditions characteristics of those encountered at critical locations in high performance fighter aircraft. A tension dominated (TD) and tension compression (TC) spectrum were employed for each alloy. Using a mechanics-based analysis, it was suggested that negative loads could be eliminated for the TC spectrum for low to intermediate maximum stress intensities. The suggestion was verified by subsequent testing. Using fractographic evidence, it was suggested that a further similification in the spectra could be accomplished by eliminating low and intermediate peak load points resulting in near or below threshold maximum peak stress intensity values. It is concluded that load interactions become more important at higher stress intensities and more plasticity at the crack tip. These results suggest that a combined mechanics/fractographic mechanisms approach can be used to simplify other complex spectra.

Telesman, J.↗
NASA NTRS1992

The complete UV spectrum of SO2 by electron impact. I - The vacuum ultraviolet spectrum

Measurements of the VUV emission cross sections of SO2 from 40 to 200 nm in the laboratory in a crossed beam experiment are reported. The wavelength range comprised the EUV spectrum, from 40 to 120 nm, and the FUV spectrum, from 120 to 200 nm. The studies included the measurement of excitation functions of the strongest multiplets from 0 to 1 keV impact energy and the identification of dissociation processes from the analysis of the threshold excitation function. It is shown that the FUV emission cross sections for dissociative excitation of atomic multiplets are generally much larger in the FUV than in the EUV. The emission cross sections measured are important for the preparation and data analysis of spacecraft observations of Io to be obtained for the 1990s.

Ajello, Joseph M.↗
NASA NTRS1992

The complete UV spectrum of SO2 and electron impact. II - The middle ultraviolet spectrum

Measurements of the middle ultraviolet (MUV) electron impact induced fluorescence spectrum of SO2 from 200 to 430 nm in a crossed beam experiment are reported. The spectrum is dominated by two features at the experimental resolution of 0.5 nm; these two features are referred to as MUV1 and MUV2. MUV1 is the SO band system extending from 240 to 265 nm produced by dissociative excitation. MUV2 is a blend of the SO2 and SO2(+) molecular band systems in the range 264-430 nm. The excitation function measurements of MUV2 indicate that low-energy electrons effectively prepare SO2 in one or more electronically excited triplet states that involve the SO2 state by direct excitation and/or cascading. The peak cross section for MUV1 occurs at 20 eV with a value of 25.0 +/-5.5 x 10 exp -19 sq cm. The peak cross section for MUV2 arises at 9 eV with a value of 368 +/-81 x 10 exp -19 sq cm. The laboratory results provide a plausible explanation of the Io auroral hot spots observed by Voyager 2.

Ajello, Joseph M.↗
NASA NTRS1988

The ultraviolet spectrum of noncoronal late-type stars - The Gamma Crucis (M3.4 III) reference spectrum

A guide is presented to the UV spectrum of M-type giants and supergiants whose outer atmospheres contain warm chromospheres but not coronae. The M3 giant Gamma Crucis is taken as the archetype of the cooler, oxygen-rich, noncoronal stars. Line identifications and integrated line flux measurements of the chromospheric emission features seen in the 1200-3200 A range of IUE high-resolution spectra are presented. The major fluorescence processes operating in the outer atmosphere of Gamma Crucis, including eight previously unknown pumping processes and 21 new fluorescent line products, are summarized, and the enhancements of selected line strengths by 'line leakage' is discussed. A set of absorption features toward the longer wavelength end of this range is identified which can be used to characterize the radial velocity of the stellar photospheres. The applicability of the results to the spectra of noncoronal stars with different effective temperatures and gravities is discussed.

Carpenter, Kenneth G.↗
NASA NTRS1989

A new calculation of the cosmic-ray antiproton spectrum in the Galaxy and heliospheric modulation effects on this spectrum using a drift plus wavy current sheet model

The expected interstellar antiproton spectrum arising from cosmic-ray interactions in the Galaxy is recalculated, and the modulation of both antiprotons and protons is calculated using a two-dimensional modulation model incorporating gradient and curvature drifts and a wavy current sheet as well as the usual diffusion, convection, and energy-loss effects. Significant differences in the antiproton/proton ratio for different solar magnetic field polarities are predicted as well as a 'low-energy' component for antiprotons below about 1 GeV.

Webber, W. R.↗
NASA NTRS1991

Direct sequence spread spectrum CDMA in shared spectrum applications

Personal Communication Network (PCN) is an entirely wireless communication system with the capability of assessing the wired telephone system to reach anyone processing only a wired telephone. It is expected to compete with the existing mobile cellular system which connects directly to the wired telephone system. While many PCN systems employ TDMA technology, the PCN system described here uses Broadband CDMA (BCDMA(sup SM)) which is capable of sharing the spectrum with other users and which is extremely resistant to fading caused by multipath.

Schilling, Donald L.↗
NASA NTRS1994

The BL LAC phenomenon: X-ray observations of transition objects and determination of the x-ray spectrum of a complete sample of flat-spectrum radio sources

This report summarizes the activities related to two ROSAT investigations: (1) x-ray properties of radio galaxies thought to contain BL Lac type nuclei; and (2) x-ray spectra of a complete sample of flat-spectrum radio sources. The following papers describing the research are provided as attachments: Multiple X-ray Emission Components in Low Power Radio Galaxies; New X-ray Results on Radio Galaxies; Analysis Techniques for a Multiwavelength Study of Radio Galaxies; Separation of X-ray Emission Components in Radio Galaxies; X-ray Emission in Powerful Radio Galaxies and Quasars; Extended and Compact X-ray Emission in Powerful Radio Galaxies; and X-ray Spectra of a Complete Sample of Extragalactic Core-dominated Radio Sources.

Worrall, Diana M.↗
NASA NTRS2000

Estimating the Crustal Power Spectrum From Vector Magsat Data: Crustal Power Spectrum

The Earth's magnetic field can be subdivided into core and crustal components and we seek to characterize the crustal part through its spatial power spectrum (R(sub l)). We process vector Magsat data to isolate the crustal field and then invert power spectral densities of flight-local components along-track for R(sub l) following O'Brien et al. [1999]. Our model (LPPC) is accurate up to approximately degree 45 (lambda=900 km) - this is the resolution limit of our data and suggests that global crustal anomaly maps constructed from vector Magsat data should not contain features with wavelengths less than 900 km. We find continental power spectra to be greater than oceanic ones and attribute this to the relative thicknesses of continental and oceanic crust.

Lowe, David A. J.↗
NASA NTRS1982

Analysis of the ultraviolet spectrum of RWT 152 - A subluminous O star with a main-sequence visual spectrum

Observations of luminous early-type stars provide one of the most sensitive ways to detect the low density gas of the interstellar medium. As part of an ongoing program studying the gas in the galactic halo, stars believed to be outside the plane of the Milky Way have been observed. In the interest of studying the halo gas in additional directions, a search has been conducted for candidate stars which are bright enough to be observable with the International Ultraviolet Explorer Satellite (IUE) high dispersion mode and far enough from the galactic plane to provide a long line of sight through the halo gas. One such star is star 152 in the catalog of Rubin, Westpfahl, and Tuve (1974), which is called RWT 152 in the investigation. The IUE spectra of RWT 152 were obtained and analyzed. It appears that RWT 152 is not a young massive star, but rather a low mass object in an advanced stage of evolution.

Ebbets, D. C.↗
NASA NTRS1994

Power spectrum, correlation function, and tests for luminosity bias in the CfA redshift survey

We describe and apply a method for directly computing the power spectrum for the galaxy distribution in the extension of the Center for Astrophysics Redshift Survey. Tests show that our technique accurately reproduces the true power spectrum for k greater than 0.03 h Mpc(exp -1). The dense sampling and large spatial coverage of this survey allow accurate measurement of the redshift-space power spectrum on scales from 5 to approximately 200 h(exp -1) Mpc. The power spectrum has slope n approximately equal -2.1 on small scales (lambda less than or equal 25 h(exp -1) Mpc) and n approximately -1.1 on scales 30 less than lambda less than 120 h(exp -1) Mpc. On larger scales the power spectrum flattens somewhat, but we do not detect a turnover. Comparison with N-body simulations of cosmological models shows that an unbiased, open universe CDM model (OMEGA h = 0.2) and a nonzero cosmological constant (CDM) model (OMEGA h = 0.24, lambda(sub zero) = 0.6, b = 1.3) match the CfA power spectrum over the wavelength range we explore. The standard biased CDM model (OMEGA h = 0.5, b = 1.5) fails (99% significance level) because it has insufficient power on scales lambda greater than 30 h(exp -1) Mpc. Biased CDM with a normalization that matches the Cosmic Microwave Background (CMB) anisotropy (OMEGA h = 0.5, b = 1.4, sigma(sub 8) (mass) = 1) has too much power on small scales to match the observed galaxy power spectrum. This model with b = 1 matches both Cosmic Background Explorer Satellite (COBE) and the small-scale power spect rum but has insufficient power on scales lambda approximately 100 h(exp -1) Mpc. We derive a formula for the effect of small-scale peculiar velocities on the power spectrum and combine this formula with the linear-regime amplification described by Kaiser to compute an estimate of the real-space power spectrum. Two tests reveal luminosity bias in the galaxy distribution: First, the amplitude of the pwer spectrum is approximately 40% larger for the brightest 50% of galaxies in volume-limited samples that have M(sub lim) greater than M*. This bias in the power spectrum is independent of scale, consistent with the peaks-bias paradigm for galaxy formation. Second, the distribution of local density around galaxies shows that regions of moderate and high density contain both very bright (M less than M* = -19.2 + 5 log h) and fainter galaxies, but that voids preferentially harbor fainter galaxies (approximately 2 sigma significance level).

Park, Changbom↗
NASA NTRS2003

First Year Wilkinson Microwave Anisotropy Probe(WMAP)Observations: The Angular Power Spectrum

We present the angular power spectrum derived from the first-year Wilkinson Microwave Anisotropy Probe (WMAP) sky maps. We study a variety of power spectrum estimation methods and data combinations and demonstrate that the results are robust. The data are modestly contaminated by diffuse Galactic foreground emission, but we show that a simple Galactic template model is sufficient to remove the signal. Point sources produce a modest contamination in the low frequency data. After masking approximately 700 known bright sources from the maps, we estimate residual sources contribute approximately 3500 mu sq Kappa at 41 GHz, and approximately 130 mu sq Kappa at 94 GHz, to the power spectrum [iota(iota + 1)C(sub iota)/2pi] at iota = 1000. Systematic errors are negligible compared to the (modest) level of foreground emission. Our best estimate of the power spectrum is derived from 28 cross-power spectra of statistically independent channels. The final spectrum is essentially independent of the noise properties of an individual radiometer. The resulting spectrum provides a definitive measurement of the CMB power spectrum, with uncertainties limited by cosmic variance, up to iota approximately 350. The spectrum clearly exhibits a first acoustic peak at iota = 220 and a second acoustic peak at iota approximately 540, and it provides strong support for adiabatic initial conditions. Researchers have analyzed the CT(sup Epsilon) power spectrum, and present evidence for a relatively high optical depth, and an early period of cosmic reionization. Among other things, this implies that the temperature power spectrum has been suppressed by approximately 30% on degree angular scales, due to secondary scattering.

Hinshaw, G.↗