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  • 1
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    Longitudinal vortex control - Techniques and applications (The 32nd Lanchester Lecture) (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A summary is presented of vortex control applications and current techniques for the control of longitudinal vortices produced by bodies, leading edges, tips and intersections. Vortex control has up till now been performed by many approaches in an empirical fashion, assisted by the essentially inviscid nature of much of longitudinal vortex behavior. Attention is given to Reynolds number sensitivities, vortex breakdown and interactions, vortex control on highly swept wings, and vortex control in juncture flows.
    Schlagwort(e): AERODYNAMICS
    Materialart: Aeronautical Journal (ISSN 0001-9240); 96; 958; p. 293-312.
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  • 2
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    Supernovae as sources of interstellar diamonds (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Small hydrocarbon grains in the vicinity of a supernova could be annealed by the absorption of several far-ultraviolet photons to produce the tiny diamonds found in meteorites. These freshly-synthesized diamond grains would be bombarded by the heavy ions and neutrals in the supernovae outflow and would thereby acquire the distinctive noble-gas isotopic signature by which they were first isolated. Only diamonds formed relatively close to supernovae would acquire such a signature, since grains formed farther out would be subjected to a much diluted and less energetic plasma environment.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysics and Space Science (ISSN 0004-640X); 196; 1; p. 117-123.
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  • 3
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    UV spectroscopy of Z Chamaeleontis. II - The 1988 January normal outburst (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: IUE observations taken during the 1988 January normal outburst of Z Cha are presented and a detailed comparison with the 1987 April superoutburst is made. The most important difference from the superoutburst is that the normal outburst continuum flux shows less than 10 percent orbital variation away from the eclipse, implying that there is no 'cool' bulge on the disk to occult the brighter inner disk periodically. The implications for the outburst mechanism in the types of outburst are discussed. The evolution of the continuum flux distribution and emission-line fluxes, the modulation of the continuum and line fluxes with orbital phase, and the behavior of the mideclipse spectral during normal outburst are investigated.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 259; 4; p. 593-603.
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  • 4
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    Search for periodic gamma-ray emission from Cygnus X-3 by the EGRET telescope on the Compton Gamma-Ray Observatory (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory observed the Cygnus region of the Galaxy during the periods 1991 May 30-June 8 and 1991 August 8-15. We report on a periodicity analysis of the gamma rays originating from the direction of Cygnus X-3. This analysis showed no evidence of periodic modulation during these observations of the gamma-ray emission at the approximately 4.8 hr period observed at X-ray wavelengths.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 401; 2; p. 724-727.
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  • 5
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    Unsteady two- and three-dimensional Navier-Stokes simulations of multistage turbomachinery flows (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The flow-field within an axial flow turbomachine, such as a turbine or compressor, is extremely complex because of three-dimensional features such as hub-corner stall, tip-leakage flows, and airfoil wakes. These flow features interact with each other and with rotor and stator airfoils inducing time-varying forces on the airfoils. These complicated rotor-stator interactions must be understood in order to design turbomachines that are light and compact as well as reliable and efficient. Two codes, STAGE-2 and STAGE-3, have been developed to compute these unsteady rotor-stator interaction flows in multistage turbomachines. An implicit, thin-layer Euler/Navier-Stokes zonal algorithm is used to compute the unsteady flow-field within both turbine and compressor configurations. Results include surface pressures and wake profiles for two-dimensional turbine and compressor configurations and surface pressures for a three-dimensional single-stage turbine configuration. The results compare well with experimental data and other unsteady computations.
    Schlagwort(e): AERODYNAMICS
    Materialart: Computing Systems in Engineering (ISSN 0956-0521); 3; 1-4; p. 231-240.
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  • 6
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    X-ray flares from runaway pair production in active galactic nuclei (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The hard X-ray spectrum of AGNs is nonthermal, probably arising from an electron-positron pair cascade, with some emission reflected off relatively cold matter. There has been interest in models on which protons are accelerated and create relativistic electrons on interaction with a local radiation field. It is shown here that a sufficient column density of protons can lead to runaway pair production: photons generated by the relativistic pairs are the targets for the protons to produce more pairs. This can produce X-ray fluxes with the characteristics observed in AGN. The model predicts the maximum ratio of luminosity to source size as well as their spectrum in the early phases. The same mechanism may also be able to create the knots of synchrotron-radiating pair plasma seen in sources such as 3C273.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Nature (ISSN 0028-0836); 360; 6400; p. 135-137.
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  • 7
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    Probabilistic structural analysis of adaptive/smart/intelligent space structures (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A three-bay, space, cantilever truss is probabilistically evaluated for adaptive/smart/intelligent behavior. For each behavior, the scatter (ranges) in buckling loads, vibration frequencies, and member axial forces are probabilistically determined. Sensitivities associated with uncertainties in the structure, material and load variables that describe the truss are determined for different probabilities. The relative magnitude for these sensitivities are used to identify significant truss variables that control/classify its behavior to respond as an adaptive/smart/intelligent structure. Results show that the probabilistic buckling loads and vibration frequencies increase for each truss classification, with a substantial increase for intelligent trusses. Similarly, the probabilistic member axial forces reduce for adaptive and intelligent trusses and increase for smart trusses.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: Journal of Intelligent Material Systems and Structures (ISSN 1045-389X); 3; 4; p. 600-616.
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  • 8
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    Scattering of infrared radiation by dust in NGC 7023 and NGC 2023 (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The contribution of scattered light to the total nebular emission is determined on the basis of linear polarization measurements at 1.25, 1.65, and 2.2 microns of the visual reflection nebulae NGC 7023 and NGC 2023. The percentage polarization of NGC 7023 slowly increases from 0.3 to 1 micron, with peak polarizations of up to 26 percent at 1.25 micron, then rapidly decreases, with values of 4-7 percent at 2.2 microns. This is interpreted as implying that scattered starlight contributes most to the SW emission, while unpolarized emission from small grains or large molecules dominates at longer wavelengths. IR polarization and surface brightness measurements are combined to derive the intensity of scattered light, which is then compared with scattering models. While the near-IR emission of both NGC 2023 and NGC 7023 is dominated by small-grain or large-molecule emission, IR scattered light plays a larger role in NGC 2023 than in NGC 7023.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 400; 1; p. 238-247.
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  • 9
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    The X-ray variability of NGC 6814 - Power spectrum (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The existence of the periodic component seen in NGC 6814 with Exosat at 12,000 +/- 100 s is confirmed by a power spectrum and folded light curve analysis of unevenly sampled Ginga data. A comparison of the power spectra produced from simulated light curves with that observed enables the intrinsic shape of the power spectrum of the source to be determined despite the distortions introduced by the window function. The best estimate for the period is 12,132 +/- 3 s, where the error is that derived from simulations. An upper limit to the rate of change of period of about 10 exp -9 is inferred if the light curves are truly phase-coherent, but as this is not required by the data, the conservative upper limit is not greater than 5 x 10 exp -7. The large amount of power in the periodic component and its stability both suggest occultation of the source as its origin.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 400; 1; p. 138-152.
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  • 10
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    Cold dissipationless collapse of spherical systems - Sensitivity to the initial density law (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The collapse of cold, initially spherical systems with varying degrees of central condensation is investigated. The way in which the final shape of a collapsing system depends on the initial density law is examined. For an initial stellar number density rho varies as r exp -n, where n is in the range 0-2.5, the final, nearly prolate shape is given by a/c is approximately equal to 1.28(1 + 0.16 n), where a/c is the ratio of long to short axes of the inertia ellipsoid computed from the moment of inertia tensor of the most tightly bound 80 percent of the mass. The properties associated with the final states in the present computations are also studied. The collapsing systems develop an anisotropic halo dominated by radial orbits surrounding an isotropic core as predicted by Burkert (1990).
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 400; 1; p. 58-64.
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  • 11
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    Pulsed high-energy gamma-rays from the radio pulsar PSR1706-44 (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Gamma radiation above 100 MeV in energy has been detected from the radio pulsar PSR1706-44. The gamma emission forms a single broad peak within the pulsar period of 102 ms, in contrast to the two narrow peaks seen in the other three known high-energy gamma-ray pulsars. The emission mechanism in all cases is probably the same, the differences arising from the geometry of the magnetic and rotation axes and the line of sight. Gamma-ray emission accounts for as much as 1 percent of the total neutron star spindown energy in these pulsars, much more than emerges at optical or radio frequencies. Thus, study of this emission is important in understanding pulsar emission and evolution.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Nature (ISSN 0028-0836); 359; 6396; p. 615, 616.
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  • 12
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    Major outburst of periodic Comet Halley at a heliocentric distance of 14 AU (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The properties and evolution of the major outburst experienced by Comet Halley at 14 AU during February-March 1991 are studied. It is found that the observed halo is a segment of a conical surface populated by solid particles ejected from a localized, temporarily activated region on the sunlit hemisphere of the spinning nucleus. The total mass of the ejecta was at least 10 exp 12 g and CO was probably the prime driver, accelerating the smallest grains to a terminal velocity of about 45 m/s. The mass loading of the gas flow by particulate matter is enormous, with the mass production rate exceeding the expected production rate of CO by a factor of several tens.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 263; 1-2; p. 367-386.
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  • 13
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    Stratification of chemical elements in the atmosphere of the Ap star 53 Camelopardalis (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A detailed abundance analysis of Cr, Fe, Mn, Ca, and Ti in the Ap star 53 Cam from IUE high-resolution spectra has been performed. It is found that UV lines give significantly lower abundance than visible lines for Cr and Fe. Theoretical spectra of Cr II line calculated with the stratified chromium abundance distributions predicted a priori by the diffusion-mass loss model match well both visible and UV lines. The spectral shape of the Ca II K line and its variation are well accounted for by the diffusion model with mass loss. A spectrum synthesis with a stratified abundance well reproduces the observations. These findings provide convincing evidence for stratification in the photosphere of 53 Cam.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 263; 1-2; p. 232-240.
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  • 14
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    Modelling of IR emission of interstellar clouds. I - Emission of isolated clouds and dust abundance variations (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A model for the IR emission and extinction properties of small dust particles is used here to compute the IR emissions in the IRAS photometric bands for a set of isolated, spherical, and nonhomogeneous clouds heated by the local interstellar radiation field. It is predicted that the IR limb brightening (LB) in the IRAS photometric bands caused by selective absorption of UV photons of different energies in the cloud generally happens for central extinctions over 4 mag at 12 microns and greater than 10 mag at 100 microns and intermediate extinctions for the other IRAS bands. The surface brightness in the four IRAS bands is limited to about 0.4, 0.6, 3, and 10 MJy/sr at 12, 25, 60, and 100 microns respectively when no strong density discontinuity is present at the cloud edge.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 263; 1-2; p. 258-274.
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  • 15
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    Design of highly damage-tolerant sandwich panels (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The effects of different fabrication procedures to increase the damage tolerance of sandwich panels were studied. Baseline panels consisted of a 25.4 mm premolded core, surfaced with 177 C cure film adhesive and carbon-bismaleimide prepreg which were subsequently cocured onto the core. It was found that panels with a prefabricated skin, which was subsequently bonded onto the core with room temperature cure adhesive, showed greatly increased damage tolerance.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: In: International SAMPE Symposium and Exhibition, 37th, Anaheim, CA, Mar. 9-12, 1992, Proceedings (A93-15726 04-23); p. 1228-1242.
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  • 16
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    A spatial domain Stokes flow model for the gravity and topography of the middle latitudes of Venus (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A novel modeling technique is developed and applied to the long-wavelength gravity, topography, and internal density structure of Venus. The focus of this study is to employ data on the gravity and topography of Venus, which are obtained from the Pioneer Venus Orbiter mission, to enhance understanding some key aspects of the internal structure of the planet. A modeling strategy utilizing 'stokeslets' or basic units of slow viscous flow (governed by the Stokes equations) to duplicate the observed gravity and topography is implemented.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 97; E11; p. 18,285-18,294.
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  • 17
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    Numerical simulation of jet noise (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Jet noise and jet-induced structural loads have become key issues in the design of commercial and military aircraft. Computational Fluid Dynamics (CFD) can be of use in predicting the underlying jet shear-layer instabilities and, in conjunction with classical acoustic theory, jet noise. The computational issues involved in the resolution of high Reynolds number unsteady jet flows are addressed in this paper. Once these jet flows can be accurately resolved, it should be possible to use acoustic theory to extract, for example, the far-field jet noise. An assessment of future work and computational resources required for directly computing far-field jet noise is also presented.
    Schlagwort(e): AERODYNAMICS
    Materialart: Computing Systems in Engineering (ISSN 0956-0521); 3; 1-4; p. 169-179.
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  • 18
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    First-order inflation (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: I discuss the most recent model of inflation. In first-order inflation the inflationary epoch is associated with a first-order phase transition, with the most likely candidate being GUT symmetry breaking. The transition from the false-vacuum inflationary phase to the true-vacuum radiation-dominated phase proceeds through the nucleation and percolation of true-vacuum bubbles. The first successful and simplest model of first-order inflation, extended inflation, is discussed in some detail: evolution of the cosmic-scale factor, reheating, density perturbations, and the production of gravitational waves both from quantum fluctuations and bubble collisions. Particular attention is paid to the most critical issue in any model of first-order inflation: the requirements on the nucleation rate to ensure a graceful transition from the inflationary phase to the radiation-dominated phase.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: The infrared and submillimetre sky after COBE; Proceedings of the NATO Advanced Study Institute, Les Houches, France, Mar. 20-30, 1991 (A93-51701 22-90); p. 35-74.
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  • 19
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    An analysis of the focusing performance of Magellan SAR image data (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Synthetic aperture radar (SAR) focusing can be achieved either based on accurate ephemeris data or on an autofocusing process. For the Magellan project, such a decision must be made in the early phase of Magellan SAR system design. The analysis of the emphemeris requirement is complicated. The analysis given by the author leads to the conclusion that empheris data obtained from the Magellan navigation system provide sufficient accuracy to meet the Magellan image resolution requirement.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: IGARSS '92; Proceedings of the 12th Annual International Geoscience and Remote Sensing Symposium, Houston, TX, May 26-29, 1992. Vol. 1 (A93-47551 20-43); p. 610-613.
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  • 20
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    Local gravitomagnetic perturbations of the lunar orbit (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Using the metric in the local inertial frame of the Earth, we calculate relativistic effects on the lunar orbit with the synodic month period. It is shown that such perturbations arise entirely from the gravitomagnetic components of the local metric which exist because of the relative motion of the sun with respect to the Earth. In the case of general relativity, the net perturbation has an amplitude of 3 cm for the lunar range.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Physical Review D - Particles and Fields, 3rd Series (ISSN 0556-2821); 46; 12; p. 5404-5406.
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  • 21
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    Accelerated complete-linearization method for calculating NLTE model stellar atmospheres (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Two approaches to accelerating the method of complete linearization for calculating NLTE model stellar atmospheres are suggested. The first one, the so-called Kantorovich variant of the Newton-Raphson method, consists of keeping the Jacobi matrix of the system fixed, which allows us to calculate the costly matrix inversions only a few times and then keep them fixed during the subsequent computations. The second method is an application of the Ng acceleration. Both methods are extremely easy to implement with any model atmosphere code based on complete linearization. It is demonstrated that both methods, and especially their combination, yield a rapidly and globally convergent algorithm, which takes 2 to 5 times less computer time, depending on the model at hand and the required accuracy, than the ordinary complete linearization. Generally, the time gain is more significant for more complicated models. The methods were tested for a broad range of atmospheric parameters, and in all cases they exhibited similar behavior. Ng acceleration applied on the Kantorovich variant thus offers a significant improvement of the standard complete-linearization method, and may now be used for calculating relatively involved NLTE model stellar atmospheres.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 262; 2; p. 501-514.
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  • 22
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    An IRAS study of pre-main sequence stars in B209 (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: We have constructed high resolution IRAS maps of the B209 star forming region in the Taurus complex. By combining survey scans with pointed observations using advanced techniques specially developed to handle IRAS data in crowded fields, we have constructed images that reveal two new premain-sequence (PMS) objects. One of these, IRAS 04114+2757G, has FIR colors close to those of typical T Tauri stars and is probably associated with a faint stellar object visible on the POSS prints. The other new PMS source, IRAS 04111+2800G, is a deeply embedded object not detected at 12 microns and not optically visible. The object is projected close to the center of an ammonia core. Our detection of compact blue and red shifted (C-12)O emission peaks around the position of IRAS 04111+2800G confirms the PMS nature and youth of this object. We have examined the reasons for the exclusion of IRAS 04111+2800G and IRAS 04114+2757G from the IRAS Point Source Catalog and estimated the incompleteness of the currently known sample of embedded PMS stars in Taurus. The implications for the determination of evolutionary time scales in low mass star formation are discussed.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 259; 2; p. 537-542.
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  • 23
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    A different view of plasma flow inside P/Halley (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The Giotto spacecraft carried two different instruments - the JPA and the IMS - for the observation of hot ions in the coma of P/Halley. Although there are many similarities in the time and distance profiles of the plasma flow parameters (bulk velocity, number density, and temperature) computed from the two data sets, there are also some significant differences, especially at cometocentric distances less than 500,000 km. The principal discrepancies between the JPA results presented by Formisano et al. (1990) and the IMS observations are: (1) the IMS did not detect the levelling off of the speed and temperature profiles that Formisano et al. interpreted as flow stabilization; (2) the IMS detected differential north-south flow between the solar wind and cometary ions for only a brief interval when the magnetic field was oriented nearly southward, whereas Formisano et al. reported more extensive differential north-south flow that was independent of the direction of the field; (3) the JPA ion densities were factors of 2 to 4 higher than the IMS ion densities which, in turn, were an order of magnitude greater than theoretical values.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 258; 2; p. 549-554.
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  • 24
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    Theoretical profiles of Lyman-alpha satellites and application to synthetic spectra of DA white dwarfs (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: We present new theoretical calculations for the red wing of the Lyman-alpha profile. Close collisions with neutral and ionized hydrogen lead to the formation of the pseudomolecules H-H and H-H(+) with the appearance of satellite features near 1600 and 1400 A. The calculations include multiperturber effects, which are responsible for the formation of H3 and H3 with features near 1950 and 2600 A. The theoretical absorption profiles are included in stellar atmosphere codes and used to predict synthetic spectra for DA white dwarfs of intermediate temperatures (20,000 to 8000 K). These new calculations offer a unique opportunity to determine accurate effective temperatures and surface gravities for the variable ZZ Ceti stars.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 258; 2; p. 464-468.
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  • 25
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    Mass loss in main-sequence A-type stars? (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: We present new observations of the H-alpha line profile for five main-sequence A-type stars, where very high SNRs were achieved. A search for weak asymmetries was carried out to detect a stellar wind, but the many telluric absorption lines in this range prevent us from taking the full benefit of the high SNRs. This situation was improved by modeling the telluric absorptions to remove them from the observed spectra, but the H-alpha profiles were nevertheless found to be quite symmetric. We calculated the profile of the H-alpha line for a grid of model atmospheres of an A-type star including a weak wind, in order to assess an upper limit on the mass loss rate. The asymmetry is found to be sensitive to the velocity law and to the turbulent velocity of the wind, but its first moment depends much less on them. The upper limits deduced on the mass loss rate are between 1 and 2 x 10 exp -10 solar mass/yr. An improvement up to a factor 10 could be anticipated if new observations could be secured from a much drier location, achieving similar SNRs.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 257; 2; p. 663-670.
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  • 26
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    A study of the optical and ultraviolet spectral characteristics and of the circumstellar material around the new luminous blue variable WRA 751 (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The LBV characteristics of the new LBV candidate WRA 751 suggested by Hu et al. (1990) are studied in greater detail. A comparison is made with observational characteristics of the well-known LBVs Ag Car and HR Car using photometric and spectroscopic data of Hu et al. supplemented with an IUE spectrum and direct CCD images in H-alpha, blue, and red. It is found that the spectra of WRA 751 and HR Car discussed in this paper are similar to the minimum-phase spectrum of AG Car and therefore resemble the Of/WN 9 spectral type. The characteristics of the optical, near- and far-UV (IUE) spectra of the Fe II spectrum in these wavelength regions is due to continuum fluorescence. The H, He I, Fe II and forbidden Fe II emission lines are formed in different parts of the stars' envelopes, corresponding to their expanding velocities. It is concluded that WRA 751 has properties which fit well the LBV properties at minimum-brightness phase, and possesses circumstellar material with observational properties similar to those of the known LBVs AG Car and HR Car.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 257; 2; p. 632-640.
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  • 27
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    Supersonic laminar flow control (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: A development status evaluation is presented for the theoretical understanding and design conceptualization of boundary layer control (BLC) systems applicable to supersonic transports, such as the currently envisioned NASA High Speed Civil Transport. By reducing fuel burned, supersonic BLC techniques could expand ranges to Pacific-crossing scales, while lowering sonic boom effects and upper-atmosphere pollution and even reducing skin friction temperature. The critical consideration for supersonic BLC is the presence of wave effects.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: Natural laminar flow and laminar flow control (A93-41776 17-02); p. 233-245.
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  • 28
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    Wave interaction theory and LFC (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Attention is drawn to the influence of preexisting finite-amplitude instabilities on the growth of other disturbances; current design tools for LFC take no notice of this kind of interaction. When a rational accounting is accomplished for the evolution of incoming disturbances in finite-amplitude solutions of the equations of motion, future transition-prediction methods will need to take these wave interactions into account. Attention is given here to interactions in the presence of crossflow vortices and interactions involving Goertler vortices.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: Natural laminar flow and laminar flow control (A93-41776 17-02); p. 223-232.
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  • 29
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    Laminar flow control - Introduction and overview (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: A development history and a development-trends evaluation are presented for laminar flow controlled airfoil technologies and design concepts, including the search for 'natural' laminar flow and actively controlled flow via suction through small pores on the airfoil surface. While most NASA activities in this field have been concerned with subsonic aircraft, it has been projected that the control of boundary layer turbulence may be even more critical to the aerodynamic efficiency of supersonic aircraft. Developmental programs for these techniques have been conducted with several modified conventional aircraft.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: Natural laminar flow and laminar flow control (A93-41776 17-02); p. 1-21.
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  • 30
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    Subsonic natural-laminar-flow airfoils (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: An account is given of the development history of natural laminar-flow (NLF) airfoil profiles under guidance of an experimentally well-verified theoretical method for the design of airfoils suited to virtually all subcritical applications. This method, the Eppler Airfoil Design and Analysis Program, contains a conformal-mapping method for airfoils having prescribed velocity-distribution characteristics, as well as a panel method for the analysis of potential flow about given airfoils and a boundary-layer method. Several of the NLF airfoils thus obtained are discussed.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: Natural laminar flow and laminar flow control (A93-41776 17-02); p. 143-176.
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  • 31
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    Condensation of cosmic analog material in microgravity conditions - Preliminary analysis of a first set of flights (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The first results of the STARDUST project, aimed at producing and analyzing cosmic-dust analog materials in microgravity conditions, are summarized. The discussion covers the purpose of the investigation, cosmic-dust formation and properties, previous simulations of cosmic-dust formation, the current approach, the microgravity experimental apparatus, and potential advantages of studying dust formation under microgravity conditions.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Nuovo Cimento C, Serie 1 (ISSN 0390-5551); 15 C; 6; p. 1071-1076.
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  • 32
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    Observations of the 18.3-GHz transition of C3H2 towards Sgr B2 (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Three molecular clouds of Sgr B2 have been mapped in the 1(10) - 1(01) orthotransition of C3H2 at 18.3 GHz. The distribution of C3H2 and the C3H2 optical depth of the three clouds, centered near 50, 65, and 80 km/s, are reported.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomical Society of Australia, Proceedings (ISSN 0066-9997); 10; 2; p. 113-117.
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  • 33
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    Of narrow lines and beaming in the strong-field SSC emission model (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The synchrotron self-Compton (SSC) emission model for gamma-ray bursts is capable of producing narrow line features for a variety of modelled field strengths, primary electron injection distributions and burst luminosities. Multiple resonant scattering with synchrotron continuum photons efficiently traps and cools pairs in the ground state to an average energy where the Compton energy loss rate is zero. Annihilation between pairs in these cooled distributions can be very efficient. For isotropic injection of primary electrons, there is an anti-correlation of the intensity of the angular-binned emission between the portions of the continuum spectrum below the cyclotron peak and above the annihilation line feature. Small-angle emission dominates the continuum above the annihilation line feature and is smooth through the line up to the pair production cut-off, which can be above several MeV for small enough angles. The angle-averaged annihilation line is made of components which can have peak emission centered at energies away from 511 keV, due to Doppler shifting. For beamed injection,the annihilation line breaks up into relatively narrow components in the angular emission, which sum into a broad feature in the angle-averaged emission.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Gamma-ray bursts; Proceedings of the Workshop, Univ. of Alabama, Huntsville, Oct. 16-18, 1991 (A93-40051 16-93); p. 272-276.
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  • 34
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    Physical processes and diagnostics of gamma-ray burst emission (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: With improved data from BATSE and other instruments, it is important to develop a range of diagnostic tools to link gamma-ray burst observations with theory. I will review some of the physical processes which may take place to form the spectrum of gamma-ray burst sources, assuming that the bursts originate on strongly magnetized neutron stars. The important diagnostics that these processes provide to probe the emission region and how they might be used to interpret observed spectra will also be discussed.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Gamma-ray bursts; Proceedings of the Workshop, Univ. of Alabama, Huntsville, Oct. 16-18, 1991 (A93-40051 16-93); p. 231-240.
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  • 35
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    Pickup ions observed at Comet Halley (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Data on the velocity distributions of pickup ions in the vicinity of Comet Halley that were obtained by the Giotto spacecraft are presented. Combination of data from two different instruments allows a comparative study of the pitch-angle scattering rates and velocity diffusion rates of protons and water-group ions.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Particle acceleration in cosmic plasmas; Proceedings of the Workshop, Bartol Research Inst., Newark, DE, Dec. 4-6, 1991 (A93-39976 16-93); p. 273-278.
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  • 36
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    Buckling of symmetrically laminated plates with compression, shear, and in-plane bending (1992)
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: AIAA Journal (ISSN 0001-1452); 30; 12; p. 2959-2965.
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  • 37
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    The contribution of stars to the AGN line emission (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: It has been suggested that the presence of red giants in the vicinity of an active nucleus can contribute significantly to the line emission by the reprocessing of the continuum radiation onto the red giant's surface or in the stellar wind that usually accompanies these stars. We present herein the line profiles and the corresponding transfer functions for this model for the line emission, using a realistic phase space distribution function for the stars in the vicinity of the active nucleus.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 560-563.
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  • 38
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    High energy continuum measurements of active galactic nuclei (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: We review recent hard X-ray and gamma-ray observations of Cen A, NGC 4151, 3C273, and 3C279. The new data showed remarkable variability of these objects in the high-energy gamma-ray regime. Recent observations by GRANAT show an unexpected sharp break at about 50 keV in the 1990 spectrum of NGC 4151. Recent observations by the Compton Observatory show intense GeV emission from 3C279 in June 1991. We present composite energy spectra of these objects from the radio to gamma-ray energies.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 348-355.
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  • 39
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    The high energy AGN continuum - Models without e+e- -pairs (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The general properties of the high energy AGN continuum are reviewed with particular emphasis on the connection between radio loud and radio quiet AGN in the broad context of the dynamics of accretion onto a black hole. Arguments are provided indicating the possibility that the emission from radio loud AGN originates at distances much larger than a few Schwarzschild radii from the black hole and hence the role of pairs in defining the observed spectrum is limited. A connection is made between the radio and high energy continuum of AGN.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 301-310.
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  • 40
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    The benefit of amateur observations for research in dwarf novae (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Contributions of amateur astronomers to research on dwarf novae, which are based on carefully monitoring the outburst behavior of these objects, are reviewed. These contributions range from scheduling of observations to the observational basis for research on the dwarf nova outburst mechanism. It is suggested, that, with better equipment, observations of orbital light variations in dwarf novae might be performed by amateur astronomers.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Variable star research: An international perspective (A93-29397 10-89); p. 279-289.
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  • 41
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    Origin and use of Martian nomenclature (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The history and construction of Martian place names are examined. The 24 specific descriptor terms in use for Mars are defined. Informal names of individual rocks are discussed: the human fondness for informality is evident in the names attached to individual rocks at the Viking Lander sites.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 1305-1314.
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  • 42
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    SAR data processing for the Magellan prime mission (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The Magellan prime mission involves mapping the planet of Venus once around its rotational axis. The Magellan synthetic aperture radar (SAR) data processing facility processes all SAR data collected by the Magellan spacecraft into image data on an orbit-by-orbit basis. About 1700 million bits of radar data were collected. A complete description of the Magellan SAR Data Processing Facility is provided with emphasis on key design features of the data processors that satisfied the project data processing requirements. A summary of the attained data processing performance is included, as well as a brief discussion of some of the constraints and considerations regarding the applicability of the processors to meeting the data processing goals anticipated for the follow-on mission phases (i.e., cycles II, III, and beyond).
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: IGARSS '92; Proceedings of the 12th Annual International Geoscience and Remote Sensing Symposium, Houston, TX, May 26-29, 1992. Vol. 1 (A93-47551 20-43); p. 606-609.
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  • 43
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    The (C-12)O/(C-13)O ratio toward Zeta Ophiuchi (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: A tentative detection of the J = 1 - 0 emission line of (C-13)O has been obtained with SEST from a 24.4 hour integration. The velocity resolution used was 0.23 km/s and the FWHP beamwidth was 45 arcsec. If the (C-13)O line data are conservatively interpreted as an upper limit, the (C-12)O/(C-13)O ratio is not less than 60. Our result supports the previous determination of a large value of the isotope ratio in this cloud, made using radio emission lines with a 1.6-arcmin beam, and extends the ratio based on emission lines to a smaller region. When interpreted as a lower limit, our data is consistent with the ratio obtained from UV absorption line data for (C-12)O and (C-13)O.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 262; 1; p. 248-250.
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  • 44
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    The integrated radio continuum spectrum of M33 - Evidence for free-free absorption by cool ionized gas (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: We present measurements of the integrated radio continuum flux density of M33 at frequencies between 22 and 610 MHz and discuss the radio continuum spectrum of M33 between 22 MHz and 10 GHz. This spectrum has a turnover between 500 and 900 MHz, depending on the steepness of the high frequency radio spectrum of M33. Below 500 MHz the spectrum is relatively flat. We discuss possible mechanisms to explain this spectral shape and consider efficient free-free absorption of nonthermal emission by a cool (not greater than 1000 K) ionized gas to be a very likely possibility. The surface filling factor of both the nonthermal and the thermal material appears to be small (of order 0.001), which could be explained by magnetic field/density fluctuations in the M 33 interstellar medium. We briefly speculate on the possible presence of a nuclear radio source with a steep spectrum.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 261; 1; p. 47-56.
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  • 45
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    Warm neutral halos around molecular clouds. IV - H I and continuum: Aperture synthesis observations towards the molecular cloud B5 (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: We present aperture synthesis observations of H I (21 cm) line radiation and continuum emission at 408 and 1420 MHz towards a field centered on the molecular cloud B5. The H I emission shows an extended atomic halo around the molecular cloud. The opacity of the halo is derived using H I absorption toward several background sources and a simple source model is presented. The model indicates that the halo is not gravitationally bound to the molecular cloud and that it is in fact expanding away from it. Approximately 350 solar masses are contained in the H I halo. Flux densities and spectral indices for the sources detected in both of the continuum bands are given.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 260; 1-2; p. 355-369.
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  • 46
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    Interferometric observations of Mira (Omicron Ceti) (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The long-period variable Mira Omicron Ceti has been observed at 800 nm wavelength with the Mk III Optical Interferometer at photometric phases 0 = 0.96, 0.05, and 0.14 in 1990; some additional data were taken in 1989. The star is not spherically symmetric, and temporal variations of the size and the position angle of the asymmetry are detected. The visibility data can be represented by two uniform elliptical disks, which could correspond to different layers in Mira's atmosphere. If the size variations are interpreted in terms of physical motion of the emitting material, an infall velocity of about 15 km/s around phi = 0 is derived.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 259; 1; p. L19-L22.
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  • 47
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    Search for H2D(+) at 372 GHz in dense interstellar clouds (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The 1(10)-1(11) transition of ortho-H2D(+) at 372 GHz has been sought in several dark clouds. The transition was not detected; the best upper limits obtained are about 0.3 K (3 sigma). We derive upper limits for the ortho-H2D(+) column density and briefly discuss their meaning in comparison with a simple chemical model we have developed (Pagani et al., 1992).
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 258; 2; p. 472-478.
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  • 48
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    The variable Herbig Ae Star HR 5999. X - Its photometric 'pulse-shaped' variability (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: We present additional photometric observations (Stromgren y filter) of the Herbig Ae Star HR 5999. This new set of data, composed of 282 data points covering April 1983 to August 1989, was used in conjunction with the 362 data points compiled by Baade and Stahl (1989). Our aim was to detect single or multiple periods in the integrated set of data spanning nearly 20 yr. Through the use of series analysis techniques, and by removing any linear component present in the raw data, we were able to detect weak peaks in the power spectra (in the order of intensity the strongest peaks are at 301 and 113 d) which were not self-evident in the phase diagrams. The overall picture of the data shows a small linear component, which is more noticeable in the new set of data, indicating that the recent maxima appear brighter. By a detailed analysis of the full light curve we were able to detect the presence of well-defined 'pulses' or bursts. Further Gaussian fits of these bursts indicated that the pulses are relatively rapid, of the order of 10 d, with some of them being closely spaced. No periodicity was found for the pulses. A possible explanation of these aperiodic outbursts is that they detect a flow of matter accompanied by magnetic field disturbances originated from the interior of the star, unlike those originating externally such as binary perturbations, which tend to be periodic.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 257; 1; p. 209-217.
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  • 49
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    EGRET detection of high energy gamma rays from the gamma-ray burst of 3 May 1991 (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: On May 3, 1991, the Energetic Gamma Ray Experiment Telescope on the Compton Gamma Ray Observatory detected a gamma-ray burst both in the energy measurement subsystem and independently in the spark chamber assembly. Six individual photons were detected in the spark chamber, allowing a determination of the burst arrival direction which was l(II) = 171.9 deg +/- 1.3 deg, b(II) = 5.3 deg +/- 1.1 deg. Three energy spectra were measured from 1 to 200 MeV; they were measured during the first second after the Burst and Transient Sources Experiment trigger, the next two seconds, and the subsequent four seconds. The first two spectra exhibit a similar differential spectra index of about -2.2 with no apparent high-energy cut-off. By the time of the third spectrum, an additional soft component is evident.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 255; 1-2; p. L13-L16.
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  • 50
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    Energetics of gas-surface interactions in transitional flows at entry velocities (1992)
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): AERODYNAMICS
    Materialart: Journal of Spacecraft and Rockets (ISSN 0022-4650); 29; 6; p. 786-793.
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  • 51
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    Shock interference prediction using direct simulation Monte Carlo (1992)
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): AERODYNAMICS
    Materialart: Journal of Spacecraft and Rockets (ISSN 0022-4650); 29; 6; p. 780-785.
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  • 52
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    On the use of the hole-drilling technique for residual stress measurements in thin plates (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The strain gage blind hole-drilling technique may be used to determine residual stresses at and below the surface of components. In this paper, the hole-drilling analysis methodology for thick plates is reviewed, and experimental data are used to evaluate the methodology and to assess its applicability to thin plates. Data on the effects of gage pattern, surface preparation, hole spacing, hole eccentricity, and stress level are also presented.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: ASME, Transactions, Journal of Solar Energy Engineering (ISSN 0199-6231); p. 292-299.
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  • 53
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    Scaling of energy absorbing composite plates (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The energy absorption response and crushing characteristics of geometrically scaled graphite-Kevlar epoxy composite plates were investigated. Three different trigger mechanisms including chamfer, notch, and steeple geometries were incorporated into the plate specimens to initiate crushing. Sustained crushing was achieved with a simple test fixture which provided lateral support to prevent global buckling. Values of specific sustained crushing stress (SSCS) were obtained which were comparable to values reported for tube specimens from previously published data. Two sizes of hybrid plates were fabricated; a baseline or model plate, and a full-scale plate with in-plane dimensions scaled by a factor of two. The thickness dimension of the full-scale plates was increased using two different techniques; the ply-level method in which each ply orientation in the baseline laminate stacking sequence is doubled, and the sublaminate technique in which the baseline laminate stacking sequence is repeated as a group. Results indicated that the SSCS is independent of trigger mechanism geometry. However, a reduction in the SSCS of 10-25 percent was observed for the full-scale plates as compared with the baseline specimens, indicating a scaling effect in the crushing response.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: In: AHS, Annual Forum, 48th, Washington, June 3-5, 1992, Proceedings. Vol. 2 (A93-35901 14-01); p. 1431-1440.
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    Hover performance analysis of advanced rotor blades (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: This is an effort aimed at validating recent hover prediction methods. The experimental basis for this validation work is an extensive set of loads, wake and performance data, which were obtained from a pressure instrumented model UH-60 rotor tested at the Sikorsky hover test facility and at Duits-Nederlandse Windtunnel (DNW). This model was equipped with replaceable tips - including a tapered and a BERP-type tip - which permitted studies of the effects of rotor geometry. The central prediction method studied is a free-wake, vortex embedded, full-potential CFD method - called HELIX-I. It is found that the HELIX-I code produces very good comparisons with the data including wake, surface pressure and performance. Comparisons with the measured radial load distributions have permitted an improved understanding of the wake resolution modelling requirements of CFD methods. Since HELIX-I is a combined Eulerian/Lagrangian method, limited comparisons are also made with a Lagrangian boundary element code (called EHPIC) and an Eulerian Navier-Stokes code (called TURNS). In most cases all methods produce good comparisons with the data. It is found that the HELIX-I code provides a good compromise between the speed of boundary integral methods and the comprehensive nature of Navier-Stokes methods.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: AHS, Annual Forum, 48th, Washington, June 3-5, 1992, Proceedings. Vol. 2 (A93-35901 14-01); p. 1367-1384.
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    Shadowgraph flow visualization of isolated tiltrotor and rotor/wing wakes (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Shadowgraph flow visualization images were acquired from a 0.184-scale tiltrotor and wing in hover. Measurements and details of the vortex core structure were examined as a function of thrust condition and wake age. Experimental data for the isolated rotor wake geometry and rotor wake interactions with a semi-span wing and image plane were acquired. Quantitative measurements and comparisons of wake geometry and distortion were made for three configurations: the isolated rotor, rotor/wing, and rotor/wing/image plane. Comparisons between tiltrotor and helicopter rotor wake geometry measurements were made. Experimental wake geometry data were also compared with two wake models. Suggestions for improvements to existing prescribed-wake and free-wake models are proposed.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: AHS, Annual Forum, 48th, Washington, June 3-5, 1992, Proceedings. Vol. 2 (A93-35901 14-01); p. 1323-1344.
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    A study of the rotor wake of a small-scale rotor model in forward flight using laser light sheet flow visualization with comparisons to analytical models (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: An experimental investigation was conducted in the 14 by 22 ft subsonic tunnel at NASA Langley Research Center to quantify the rotor wake behind a scale model helicopter rotor in forward flight (mu = 0.15 and 0.23) at one thrust level (C sub T = 0.0064). The rotor system used in the present test consisted of a four-bladed, fully articulated hub and utilized blades of rectangular planform with a NACA-0012 airfoil section. A laser light sheet, seeded with propylene glycol smoke, was used to visualize the flow in planes parallel and perpendicular to the freestream flow. Quantitative measurements of vortex location, vertical skew angle, and vortex particle void radius were obtained for vortices in the flow; convective velocities were obtained for blade tip vortices. Comparisons were made between the experimental results and the wake geometry generated by computational predictions. The results of these comparisons show that the interaction between wake vortex structures is an important consideration for correctly predicting the wake geometry.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: AHS, Annual Forum, 48th, Washington, June 3-5, 1992, Proceedings. Vol. 1 (A93-35901 14-01); p. 697-719.
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    Numerical simulation of a hovering rotor using embedded grids (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The flow field for a rotor blade in hover was computed by numerically solving the compressible thin-layer Navier-Stokes equations on embedded grids. In this work, three embedded grids were used to discretize the flow field - one for the rotor blade and two to convect the rotor wake. The computations were performed at two hovering test conditions, for a two-bladed rectangular rotor of aspect ratio six. The results compare fairly with experiment and illustrates the use of embedded grids in solving helicopter type flow fields.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: AHS, Annual Forum, 48th, Washington, June 3-5, 1992, Proceedings. Vol. 1 (A93-35901 14-01); p. 429-445.
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    A finite-volume Euler solver for computing rotary-wing aerodynamics on unstructured meshes (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: An unstructured-grid solver for the unsteady Euler equations has been developed for predicting the aerodynamics of helicopter rotor blades. This flow solver is a finite-volume scheme that computes flow quantities at the vertices of the mesh. Special treatments are used for the flux differencing and boundary conditions in order to compute rotary-wing flowfields, and these are detailed in the paper. The unstructured-grid solver permits adaptive grid refinement in order to improve the resolution of flow features such as shocks, rotor wakes and acoustic waves. These capabilities are demonstrated in the paper. Example calculations are presented for two hovering rotors. In both cases, adaptive-grid refinement is used to resolve high gradients near the rotor surface and also to capture the vortical regions in the rotor wake. The computed results show good agreement with experimental results for surface airloads and wake geometry.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: AHS, Annual Forum, 48th, Washington, June 3-5, 1992, Proceedings. Vol. 1 (A93-35901 14-01); p. 419-428.
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    Control of resonant frequencies in adaptive structures by prestressing (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The natural vibration frequencies of a structure can be affected by inducing stress in the structure. The success of this kind of control of the resonant frequencies of a truss structure depends on the geometry of the structure. It is shown that in adaptive truss structures the method is effective for vibrations in less stiff directions, such as the normal direction of the plane containing all of the bars of a node, suggesting its applicability for cable, membrane, and thin plate and shell structures.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: In: Joint Japan(U.S. Conference on Adaptive Structures, 2nd, Nagoya, Japan, Nov. 12-14, 1991, Collection of Papers (A93-31226 11-18); p. 297-314.
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    Geodesy and cartography of the Martian satellites (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The difficulties connected with conventional maps of Phobos and Deimos are largely overcome by producing maps in digital forms, i.e., by projecting Viking Orbiter images onto a global topographic model made from collections of radii derived by photogrammetry. The resulting digital mosaics are then formatted as arrays of body-centered latitudes, longitudes, radii, and brightness values of Viking Orbiter images. The Phobos mapping described was done with Viking Orbiter data. Significant new coverage was obtained by the Soviet Phobos mission. The mapping of Deimos is in progress, using the techniques developed for Phobos.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 1249-1256.
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    Physical and chemical weathering (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Physical and chemical weathering processes that might be important on Mars are reviewed, and the limited observations, including relevant Viking results and laboratory simulations, are summarized. Physical weathering may have included rock splitting through growth of ice, salt or secondary silicate crystals in voids. Chemical weathering probably involved reactions of minerals with water, oxygen, and carbon dioxide, although predicted products vary sensitively with the abundance and physical form postulated for the water. On the basis of kinetics data for hydration of rock glass on earth, the fate of weathering-rind formation on glass-bearing Martian volcanic rocks is tentatively estimated to have been on the order of 0.1 to 4.5 cm/Gyr; lower rates would be expected for crystalline rocks.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 626-651.
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    Long-term orbital and spin dynamics of Mars (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: This review of the long-term dynamical behavior of Mars covers secular variations of the orbit, oscillations of the obliquity, and polar wandering. Calculations of the large-scale obliquity oscillations of Mars are updated using the most recent orbit theory and contrasted with the earth. The motion for Mars is characterized by about 100,000-yr oscillations driven by differential spin axis and orbit plane precession rates during which the obliquity may change by as much as about 20 deg. The possible role of spin-orbit secular resonances to the spin axis histories of the earth and Mars is also considered. Numerical integrations of the equations of motion indicate that Mars may have passed through resonance as little as 5 Myr ago and that obliquities approaching about 45 deg could have been achieved during such an event.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 298-320.
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    Stress and tectonics on Mars (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Deformation of the Martian lithosphere and the various theoretical formulations used to model its behavior on local, regional, and global scales are studied. An overview of the various classes of tectonic features found on Mars is presented. The tectonic record on Mars, which provides the basic framework for interpreting theoretical thermomechanical models in terms of major tectonic events and provinces is summarized. The thickness of the elastic lithosphere is estimated using its response to local surface loads, and significant spatial variability is found, with values ranging from 20 to 300 km. A comparison of the major radial graben systems with theoretical stress models shows that more than one mechanism of lithospheric deformation is required to produce its enormous extent.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 249-297.
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    Gravity and topography (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The paper summarizes the fundamental gravity field constants for Mars and a brief historical review of early determinations and current-day accurate estimates. These include the planetary gravitational constant, global figure, dynamical oblateness, mean density, and rotational period. Topographic results from data acquired from the 1967 opposition to the most recent, 1988, opposition are presented. Both global and selected local topographic variations and features are discussed. The inertia tensor and the nonhydrostatic component of Mars are examined in detail. The dimensionless moment of inertia about the rotational axis is 0.4 for a body of uniform density and 0.37621 if Mars were in hydrostatic equilibrium. By comparing models of both gravity and topography, inferences are made about the degree and depth of compensation in the interior and stresses in the lithosphere.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 209-248.
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    Spacecraft exploration of Mars (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Soviet and American spacecraft exploration of Mars over the past quarter century is reviewed. Data on the earliest Soviet attempts to send spacecraft to observe the planet are presented. Of the series of spacecraft that were announced (designated Mars 1 to Mars 7), none fulfilled all its scientific goals, but some good photographs and other important data were obtained. Of the six spacecraft in the Mariner series, two failed, but Mariner 4 first revealed the cratered surface of Mars, and Mariner 9 discovered all the major geologic features. The Viking mission, with its two Orbiters, two Landers, and its 6-yr duration, surpassed in quantity and variety of data all other missions combined. The Phobos mission ended in two failures, but the second of the two spacecraft acquired significant new data about Mars and Phobos. An appendix listing special issues of journals containing collections of papers about Mars is provided.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 71-119.
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    The planet Mars - From antiquity to the present (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Telescopic observations of Mars, from those of Galileo in 1610 to the late 19th century, were summarized by Flammarion (1892, 1909). Major compilations of knowledge of Mars were produced by Antoniadi (1930) and de Vaucouleurs (1954). Polar cap composition was debated until the discovery by the Viking mission that the north and south perennial polar caps are composed of different materials. Spacecraft data indicate a rich and diverse geologic history as well as many unsolved puzzles. An annotated list of books about Mars is provided. Basic physical and chemical data on Mars are summarized in tabular form, and a guide to the Martian seasons is given.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 1-33.
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    Scale effects in the response and failure of fiber reinforced composite laminates loaded in tension and in flexure (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The feasibility of using scale model testing for predicting the full-scale behavior of flat composite coupons loaded in tension and beam-columns loaded in flexure is examined. Classical laws of similitude are applied to fabricate and test replica model specimens to identify scaling effects in the load response, strength, and mode of failure. Experiments were performed on graphite-epoxy composite specimens having different laminate stacking sequences and a range of scaled sizes. From the experiments it was deduced that the elastic response of scaled composite specimens was independent of size. However, a significant scale effect in strength was observed. In addition, a transition in failure mode was observed among scaled specimens of certain laminate stacking sequences. A Weibull statistical model and a fracture mechanics based model were applied to predict the strength scale effect since standard failure criteria cannot account for the influence of absolute specimen size on strength.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: Journal of Composite Materials (ISSN 0021-9983); 26; 18; p. 2674-2705.
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    SAR data processing adaptation for Magellan Cycle II (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The authors describe the effort made at the Magellan SAR Data Processing Facility in adapting existing data processors to handle data collected from various imaging experiments. The different data processing requirements imposed by the various types of data are discussed, the tradeoffs and compromises made to work within the constraint of the existing systems are explained, and summary processing results obtained to date are given.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: IGARSS '92; Proceedings of the 12th Annual International Geoscience and Remote Sensing Symposium, Houston, TX, May 26-29, 1992. Vol. 1 (A93-47551 20-43); p. 100-103.
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    Remote radio observations of solar wind parameters upstream of planetary bow shocks (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Radio emission is frequently produced at twice the electron plasma frequency 2fp in the foreshock region upstream of the terrestrial bow shock. Observations of this emission provide a remote diagnostic of solar wind parameters in the foreshock. Using ISEE-3 radio data, we present the first evidence that the radio intensity is proportional to the kinetic energy flux and to other parameters correlated with solar wind density. We provide a qualitative explanation of this intensity behavior and predict the detection of similar emission at Jupiter by the Ulysses spacecraft.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Solar Wind Seven; Proceedings of the 3rd COSPAR Colloquium, Goslar, Germany, Sept. 16-20, 1991 (A93-33554 13-92); p. 647-650.
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    Eigenvalue error analysis of viscously damped structures using a Ritz reduction method (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The efficient solution of the eigenvalue problem that results from inserting passive dampers with variable stiffness and damping coefficients into a structure is addressed. Eigenanalysis of reduced models obtained by retaining a number of normal modes augmented with Ritz vectors corresponding to the static solutions resulting from the load patterns introduced by the dampers has been empirically shown to yield excellent approximations to the full eigenvalue problem. An analysis of this technique in the case of a single damper is presented. A priori and a posteriori error estimates are generated and tested on numerical examples. Comparison theorems with modally truncated models and a Markov parameter matching reduced-order model are derived. These theorems corroborate the heuristic that residual flexibility methods improve low-frequency approximation of the system. The analysis leads to other techniques for eigenvalue approximation. Approximate closed-form solutions are derived that include a refinement to eigenvalue derivative methods for approximation. An efficient Newton scheme is also developed. A numerical example is presented demonstrating the effectiveness of each of these methods.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: AIAA Journal (ISSN 0001-1452); 30; 12; p. 2935-2944.
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    Development of the wake of an airfoil with riblets (1992)
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): AERODYNAMICS
    Materialart: AIAA Journal (ISSN 0001-1452); 30; 12; p. 2817, 2818. Abrid
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    Acoustic testing of high-temperature panels (1992)
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: Journal of Aircraft (ISSN 0021-8669); 29; 6; p. 1130-1136.
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    Observational evidence for thin AGN disks (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: AGN spectrum and spectral features, polarization, inclination, and X-ray line and continuum reflection features are discussed in a critical way in order to determine the ones that are the least model-dependent. The sign and strength of absorption and emission edges are found to be model-dependent, and relativistic broadening and shifting makes them hard to detect. The presence or absence of the predicted Lyman edge polarization feature may be used as a decisive test for thin, bare AGN disks. Other good model-independent tests are several inclination-related line and continuum correlations in big AGN samples. It is shown that electron temperature near the surface of the disk can greatly exceed the disk equilibrium temperature, which causes deviations from LTE. This effect must be incorporated into realistic disk models.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 146-154.
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    Astrophysical bags - A new paradigm for active galactic nuclei? (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Active galaxies are believed to consist of a compact nucleus, the standard model for which is a massive black hole or a cluster of black holes. A different paradigm is considered here, deriving from quark confinement theory in QCD. It is an 'astrophysical bag', modelled after the 'hadron bags' of particle physics which have already been studied in astrophysics as quark stars. Another interpretation of the cosmological constant in general relativity, and possibly a new quasar redshift formula, are introduced. As a highly-energetic object, this model may resolve the baryonic matter problem for fuelling AGN accretion processes which black hole paradigms cannot account for. Here, baryons, cosmic rays, and neutrinos are free.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 109-112.
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    Testing the AGN paradigm for our home Galaxy (1992)
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    Beschreibung: Although there are some flaws in each of the methods to constrain the black hole mass, which are described in the above paper, all of them unambiguously indicate that the mass of a putative black hole at the Galactic center is much smaller than a widely quoted value, about 10 exp 6 M(solar). Possibly, the mass of the black hole, if any, does not exceed 100 M(solar) or so. Further work is needed in order to distinguish between such a modest-mass black hole and its potential rival such as a very massive star of about the same mass. Anyway, the Galactic nucleus seems to be a tracer of a past star burst rather than a remnant of a Seyfert galaxy activity.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 44-47.
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    Quasi-periodic climate change on Mars (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The paper examines evidence that the Martian climate undergoes quasi-periodic variations, including the polar layered terrain, differences between the residual polar caps, and the current net southward flow of H2O. The driving functions for these variations are oscillations in the elements of the Martian orbit coupled with precession of the Martian spin axis. These 'astronomic variations' control the distribution of the insolation, which in turn influences the partition of volatiles between atmospheric and surface reservoirs. The major effects anticipated at low obliquity are growth of the polar caps, substantial decrease in surface pressure, cessation of duststorms, release of CO2 from the regolith, and poleward migration of H2O ground ice. At high obliquity, the mass of the perennial polar caps decreases and permanent CO2 frost disappears, CO2 desorbs from the regolith at high latitudes, the surface pressure may increase to several times its current value, and the atmospheric dust load increases.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 1180-1218.
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    Comparative aspects of the climate of Mars - An introduction to the current atmosphere (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The study reviews the historical perspective of the Martian climate, beginning with the early view of inexorable climate change on an older, but otherwise very earthlike, planet and continuing through the period when earth-based spectroscopy and the Mariner 4, 6, and 7 flyby missions portrayed a moonlike body, heavily cratered and almost airless by comparison with earth. The most general features of the earth and Mars are discussed and compared, with consideration given to the standard atmospheres, atmospheric circulation, seasonal cycles of carbon dioxide, water, and dust, and coupling between cycles.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 799-817.
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    Radar determination of Mars surface properties (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Radar studies of Mars have provided measurements of surface texture on scales of centimeters to hundreds of meters and measurements of surface material properties. Texture (rms surface tilts and estimates of small-scale roughness) may be inferred from dispersion and/or polarization of the radar echo; material properties (reflectivity of dielectric constant) are derived from echo strength. Mars is a diverse target; depending on location, rms surface tilts have been found to vary over the range 0.25 to 10 deg while reflectivity covers at least 3 to 13 percent. Plains units are the most variable, having both the smoothest and roughest surfaces, cratered terrain can be considered predictable and 'average' by comparison. Recent data identify scattering by small structures (perhaps rocks on or near the surface) as playing a more important role than previously recognized. Scattering by the residual ice cap near Mars' south pole is particularly unusual. The present state of radar surface studies is summarized.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 652-685.
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    Environmental effects on long term behavior of composite laminates (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Model equations are presented for approximate methods simulating the long-term behavior of composite materials and structures in hot/humid service environments. These equations allow laminate property upgradings with time, and can account for the effects of service environments on creep response. These methodologies are illustrated for various individual and coupled temperature/moisture, longitudinal/transverse, and composite material type cases. Creep deformation is noted to rise dramatically for cases of matrix-borne, but not of fiber-borne, loading in hot, humid environments; the coupled influence of temperature and moisture is greater than a mere combination of their individual influences.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: In: International SAMPE Technical Conference, 24th and International SAMPE Metals and Metals Processing Conference, 3rd, Toronto, Canada, Oct. 20-22, 1992, Proceedings. Vol. 24 (A93-53376 23-23); p. T852-T866.
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    Effect of service environments on adhesively bonded joints in composite structures (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The models employed in the present computational methods for evaluating severe service-environment effects on adhesively bonded joints in composites are based on composite analyses and structural mechanics, encompassing nonlinear environmental degradation. The methods are demonstrated for the case of a butt joint with a single doubler, subjected to the environmental effects as well as static and cyclic loads. The highest joint strength is noted to be required in the case of cyclic loads and hygrothermal service environments; margins of safety for adhesive material stresses decline rapidly in such cases.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: In: International SAMPE Technical Conference, 24th and International SAMPE Metals and Metals Processing Conference, 3rd, Toronto, Canada, Oct. 20-22, 1992, Proceedings. Vol. 24 (A93-53376 23-23); p. T536-T550.
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  • 81
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    Design for inadvertent damage in composite laminates (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Simplified predictive methods and models to computationally simulate durability and damage in polymer matrix composite materials/structures are described. The models include (1) progressive fracture, (2) progressively damaged structural behavior, (3) progressive fracture in aggressive environments, (4) stress concentrations, and (5) impact resistance. Several examples are included to illustrate applications of the models and to identify significant parameters and sensitivities. Comparisons with limited experimental data are made.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: In: International SAMPE Technical Conference, 24th and International SAMPE Metals and Metals Processing Conference, 3rd, Toronto, Canada, Oct. 20-22, 1992, Proceedings. Vol. 24 (A93-53376 23-23); p. T256-T269.
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  • 82
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    Design for cyclic loading endurance of composites (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The present paper describes the application of the computer code IPACS (Integrated Probabilistic Assessment of Composite Structures) to air craft wing type structures. The code performs a complete probabilistic structural analysis for composites taking into account the uncertainties in geometry, boundary conditions, material properties, laminate lay-ups and loads. Results of the analysis are presented in terms of cumulative distribution functions (CDF) and probability density function (PDF) of life of a wing type composite structure under different hygrothermal environments subjected to random pressure. The sensitivity of fatigue life to a number of critical structural/material variables is also computed from the analysis.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: In: International SAMPE Technical Conference, 24th and International SAMPE Metals and Metals Processing Conference, 3rd, Toronto, Canada, Oct. 20-22, 1992, Proceedings. Vol. 24 (A93-53376 23-23); p. T241-T255.
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  • 83
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    Implementation and performance of the Magellan digital correlator subsystem (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The Magellan synthetic aperture radar (SAR) produces Venus surface images from data collected by the SAR carried on board the Magellan spacecraft. The core of the primary Magellan SAR processor is the digital correlator subsystem (DCS). The pipeline DSC architecture enables the Magellan primary SAR processor (PSP) to achieve real-time data processing capability. The implementation and performance of the DSC are described. Hardware (H/W) constraints that influenced the processing algorithm design are highlighted.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: IGARSS '92; Proceedings of the 12th Annual International Geoscience and Remote Sensing Symposium, Houston, TX, May 26-29, 1992. Vol. 2 (A93-47551 20-43); p. 1301-1304.
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    Refinement of Magellan radiometric correction using range centroid estimation (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The Magellan (MGN) synthetic aperture radar (SAR) processing radiometric compensation algorithm is described, and the effective pointing error caused by the terrain is examined. It is shown how the range centroid can be computed from spacecraft ancillary data with an accurate topographic model. In cases where such data are not available, a technique is presented to estimate the range centroid from the coherent radar echoes. This technique is demonstrated using MGN SAR data.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: IGARSS '92; Proceedings of the 12th Annual International Geoscience and Remote Sensing Symposium, Houston, TX, May 26-29, 1992. Vol. 2 (A93-47551 20-43); p. 1167-1170.
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    The magnetic field in the heliosheath (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The interplanetary magnetic field (IMF) behaves in a reasonably well-understood manner between the Sun and the heliospheric termination shock. At the shock, the azimuthal field is amplified by a factor of four (for a strong shock) and undergoes secular amplification in the heliosheath until the flow is fully turned into the downstream direction and has reached its asymptotic state in the distant heliotail. This amplification may lead to important MHD effects that can cause the shock to be closer to the Sun than otherwise expected. Here we further examine whether there are important MHD effects in the heliosheath. We do this by calculating the kinematic compression of the magnetic field in the heliosheath using an analytic incompressible flow model of the dynamics downstream of the shock. We conclude that it is likely that MHD effects are important in the heliosheath in a narrow cone about the upstream direction.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Solar Wind Seven; Proceedings of the 3rd COSPAR Colloquium, Goslar, Germany, Sept. 16-20, 1991 (A93-33554 13-92); p. 281-284.
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    Fracture mechanics parameters for small fatigue cracks (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: This paper presents a review of some common small-crack test specimens, the underlying causes of the small-crack effect, and the fracture-mechanics parameters that have been used to correlate or predict their growth behavior. This review concentrates on continuum mechanics concepts and on the nonlinear behavior of small cracks. The paper reviews some stress-intensity factor solutions for small-crack test specimens and develops some simple elastic-plastic J integral and cyclic J integral expressions that include the influence of crack-closure. These parameters were applied to small-crack growth data on two aluminum alloys, and a fatigue life prediction methodology is demonstrated. For these materials, the crack-closure transient from the plastic wake was found to be the major factor in causing the small-crack effect.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: In: Small-crack test methods (A93-32758 12-39); p. 6-33.
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  • 87
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    Effects of interply damping layers on the dynamic characteristics of composite plates (1992)
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: AIAA Journal (ISSN 0001-1452); 30; 12; p. 2906-2913.
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  • 88
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    Unsteady simulation of viscous flowfield around F-18 aircraft at large incidence (1992)
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): AERODYNAMICS
    Materialart: Journal of Aircraft (ISSN 0021-8669); 29; 6; p. 986-992.
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  • 89
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    An inverse method for the calculation of through-thickness fatigue crack closure behavior (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: An inverse technique was used to calculate through-thickness fatigue crack closure behavior. The through-thickness variation in crack opening stress-intensity factor was calculated by considering the variation in the three-dimensional stress-intensity factor, the variation in crack growth rate along the crack front, and a relationship between the crack growth rate and effective stress-intensity factor range (da/dN-Delta-K(eff)). The three-dimensional stress-intensity factor variation was obtained from an elastic finite element analysis of specific crack-front profiles observed experimentally. The variation in crack growth rate along the crack front was obtained experimentally from comparison of observed crack front changes. The da/dN-Delta-K(eff) relationship was estimated from high stress ratio, constant load amplitude, and fatigue crack growth tests. The through-thickness crack opening stress-intensity factor results agreed with crack opening measurements obtained from fatigue striations, near-tip strain gages, and remote strain and displacement gages.
    Schlagwort(e): STRUCTURAL MECHANICS
    Materialart: In: Fracture mechanics; Proceedings of the 22nd National Symposium, Atlanta, GA, June 26-28, 1990. Vol. 2 (A93-31576 11-39); p. 46-57.
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    A new mechanism for viscosity in cloudy accretion disks - Anomalous (turbulent) viscosity (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: We analyze the dynamics of a rotating cloudy gaseous medium relative to 2D perturbations having wavelengths shorter than the disk thickness. The viscosity due to cloud-cloud collisions is confronted with the derived critical viscosity, below which a collective mode of perturbations in the disk is able to grow until going into the saturation regime where fully developed turbulence is established. It is shown that for the assumed parameters of AGN accretion disks consistent with the observational data, the viscosity due to collective modes usually dominates over that due to cloud-cloud collisions.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 280-283.
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    Problems for the standard black hole/accretion disk models in Cygnus X-1? (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Archival EXOSAT and HEAO1-A2 data from Cyg X-1 show the 'high energy excess' above 10 keV seen in X-ray observations of AGN. Using a likelihood ratio test, we are for the first time able to distinguish conclusively in favor of Compton reflection rather than partial covering as the origin of the high energy excess. This supports the idea of an X-ray illuminated accretion disk in Cyg X-1, but the line equivalent width is smaller by a factor of 2-3 than that expected from such a disk. While the larger optical depth required for reflection as opposed to line emission admit the possibility of seeing line without reflection, the converse is not possible. To see a reflection spectrum, including the strong iron absorption edge, implies that strong iron emission must be observed as the line and edge are causally linked.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 196-199.
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    Diagnostics of a putative black hole at the Galactic center (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: A review is given of methods to constrain the mass of a putative black hole at the center of the Milky Way galaxy. These approaches are based on: (1) tidal disruption of stars by a massive black hole; (2) displacement between Sgr A* and either of the IRS 16 components; (3) electron-positron pair production by a black hole, and (4) mass outflow centered on IRS 16/Sgr A* complex.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 40-43.
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  • 93
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    Spectral-luminosity evolution of active galactic nuclei (AGN) (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The origin of the cosmic X-ray and gamma-ray backgrounds is explained via the mechanism of AGN spectral-luminosity evolution. The spectral evolution of precursor active galaxies into AGN, and Newton-Raphson input and output parameters are discussed.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 370-373.
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    A search for microvariability in five OVV quasars (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: We present results of monitoring a sample of five OVV qausars in an effort to determine if these types of AGNs exhibit microvariability similar to that detected for BL Lacertae objects. Three of the five quasars investigated clearly exhibited microvariability. The timescales for the variations for both BL Lacertae objects and OVV quasars are similar which suggests that the radiation is generated in similar spatial volumes. These variations are consistent with the results of models which explain the observed microvariability as the result of excess emission produced by flares or hot spots randomly appearing and disappearing on the accretion disks around supermassive black holes.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 247-250.
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    Using color-color diagrams to test models for the 'blue bump' (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Color-color diagrams are used to study the set of accretion disk plus power-law models and to determine whether they can adequately fit the optical/UV continua of a sample of 34 quasars, and the soft X-ray excesses measured for seven of the sample objects by Masnou et al. (1991). The loci of pure disk models for cos theta is greater than 0 are virtually superimposed on each other regardless of cos theta, demonstrating that a given observed spectrum cannot be described by a unique combination of S and cos theta. The distributions of model and data colors are found to overlap, showing that the disk plus power-law models do fit the distribution of data colors. For six of the seven objects, the soft X-ray excesses may be fit with an accretion disk plus power-law model without requiring the assumption of super-Eddington accretion; this shows the utility of employing Kerr disks.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 208-211.
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    X-ray FE line studies (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: We present the first broadband, moderate-resolution X-ray spectral observations of the Seyfert galaxies NGC 4151 and NGC 1068. The Seyfert 1 galaxy NGC 4151 is shown to have a complex X-ray spectrum with multiple components and a strong, intrinsically narrow Fe K emission line. The characteristics of the scattering medium of the Seyfert 2 galaxy NGC 1068 are determined using the very strong Fe line emission and the lack of features due to oxygen in the X-ray spectrum. The soft X-ray flux from Mrk 3 and Mrk 348 are consistent with that expected from electron scattered X-rays from an obscured Seyfert 1 nucleus. A survey of the X-ray properties of Seyfert 2 galaxies indicates that a substantial fraction may have column densities of about 10 exp 23, suggesting that NGC 1068 has an unusually thick obscuring torus.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 183-191.
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    The X-ray variability of AGN and the anomalous behavior of NGC 6814 (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The power spectra of the X-ray variable AGN are typically scale invariant with no characteristic timescale. The one strong exception to this is NGC 6814, where the EXOSAT data showed evidence for a periodic component at 12200 +/- 100 seconds. The power spectra of a GINGA lightcurve from this source, found using simulation techniques to account for the uneven sampling, also cannot be well fit by a single power law. A folded light curve analysis of GINGA data shows strong evidence for this periodic component. A second GINGA observation of this source taken one year later is consistent with the phenomena being completely periodic and phase coherent for 7 periods in the range of 12,110-12,145 seconds. Including the (optimistic) limits from the folding selects a period of 12130.39 +/- 0.05 seconds. Phase coherence is not maintained between this and the EXOSAT observations, as the structure of the folded light curves is very different. Thus the periodicity is long lived and stable, but phase coherence is only maintained on timescales of about 1 year.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 83-92.
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    Radar mapping of Mercury - Full-disk images and polar anomalies (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A random-code technique has been used at Arecibo to obtain delay-Doppler radar images of the full disk of Mercury. Anomalously bright features were found at the north and south poles. The north polar feature is oblong (4 by 8 deg) and offset from the pole. The smaller south polar feature is mostly confined to the floor of the crater Chao Meng-Fu. The polar locations and radar properties of these features indicate that they may be produced by volume scattering in ice. The images also reveal a variety of more subdued reflectivity features ranging in size from hundreds to thousands of kilometers; some of these appear to have an impact origin.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Science (ISSN 0036-8075); 258; 5082; p. 640-643.
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    Surface chemistry and mineralogy (1992)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The accumulated knowledge on the chemistry and mineralogy of Martian surface materials is reviewed. Pertinent information obtained by direct analyses of the soil on Mars by the Viking Landers, by remote sensing of Mars from flyby and orbiting spacecraft, by telescopic observations from earth, and through detailed analyses of the SNC meteorites presumed to be Martian rocks are summarized and analyzed. A compositional model for Mars soil, giving selected average elemental concentrations of major and trace elements, is suggested. It is proposed that the fine surface materials on Mars are a multicomponent mixture of weathered and nonweathered minerals. Smectite clays, silicate mineraloids similar to palagonite, and scapolite are suggested as possible major candidate components among the weathered minerals.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: In: Mars (A93-27852 09-91); p. 594-625.
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    Study on vortex flow control of inlet distortion (1992)
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): AERODYNAMICS
    Materialart: Journal of Propulsion and Power (ISSN 0748-4658); 8; 6; p. 1266-1272.
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