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At least 235 records · Page 13

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NASA NTRS1981

Geophysical study of the structure and processes of the continental convergence zones: Alpine-Himalayan Belt

The seismic wave velocity structure in the crust and upper mantle region beneath the Tibetan plateau was studied in detail. Also, a preliminary study of the uppermost mantle P wave velocity beneath Iran and Turkey was carried out, and the results are compared with those for the Tibetan plateau. These two studies compose the bulk of the efforts on the observational aspects of continental collision zones in addition to satellite derived data. On the theoretical aspects the thermal evolution of converging plate boundaries was explored using a finite difference scheme.

Toksoez, M. N.↗

NASA NTRS1985

Nonlinear quasi-fracture behavior of polymers

This paper deals with the nonlinear quasi-fracture behavior of polymers. Using the finite element method, the quasi-fracture model composed of an isolated craze opening with a nonlinear boundary envelope forces represented by oriented fibril domains is analyzed in simply-stressed rectangular plate. The stress distribution, the displacement profile, as well as the yield or plastic or flow zone adjacent to the craze tips are obtained. It is found that the variation of the stress distribution near the craze tip is not as sharp as that for an elastic crack. The initial load required for yielding or flowing is greater for the quasifracture as compared with that for a classical fracture. Under the same load the yield or flow zone is somewhat smaller as compared with that obtained on the basis of linear elastic-plastic fracture mechanics theory.

Sun, B. N.↗

NASA NTRS1986

Weld Repair of Thin Aluminum Sheet

Weld repairing of thin aluminum sheets now possible, using niobium shield and copper heat sinks. Refractory niobium shield protects aluminum adjacent to hole, while copper heat sinks help conduct heat away from repair site. Technique limits tungsten/inert-gas (TIG) welding bombardment zone to melt area, leaving surrounding areas around weld unaffected. Used successfully to repair aluminum cold plates on Space Shuttle, Commercial applications, especially in sealing fractures, dents, and holes in thin aluminum face sheets or clad brazing sheet in cold plates, heat exchangers, coolers, and Solar panels. While particularly suited to thin aluminum sheet, this process also used in thicker aluminum material to prevent surface damage near weld area.

Beuyukian, C. S.↗

NASA NTRS1985

The importance of GRM gravity observations in continental regions

Altimetric satellites showed the existence of gravity anomalies with wavelengths between 1500 km and 3000 km which correlated with residual depth anomalies. It was suggested that there is also an even smaller scale convective circulation with a wavelength of less than 500 km. Numerical experiments and SEASAT profiles demonstrate that similar small scale circulation must also exist near fracture zones. The mantle therefore must contain at least three scales of circulation: (1) comparable to the plate dimensions, the large scale circulation, which returns material from the Island arcs to the ridges; (2) an intermediate scale comparable to the depth of the upper mantle; and (3) a small scale comparable to the thickness of the asthenosphere. This picture was established by observations of gravity and bathymetry in oceanic regions.

Mckenzie, D. P.↗

NASA NTRS1992

Tectonics and volcanism of Eastern Aphrodite Terra: No subduction, no spreading

Eastern Aphrodite Terra is approximately equal in size to the western North American Cordillera, from Mexico to Alaska. Its size and unique landforms make it an important area for understanding the tectonics of Venus, yet models for its formation are diametrically opposed. This region is part of the Equatorial Highlands, which was proposed as a region of lithospheric thinning, isostatic uplift, and attendant volcanism. Eastern Aphrodite Terra is dominated by circular structures within which deformation and volcanism are intimately related. These structures are marked by radial and concentric fractures, and volcanic flows that emanate from a central vent, as well as from concentric fracture sets. Cross-cutting relations between flows and concentric fracture sets indicate that outer concentric fracture sets are younger than inner fracture sets. The circular structures are joined by regional northeast- to east-trending fractures that dominantly postdate formation of the circular structures. We propose that the circular structures 'grow' outward with time. Although these structures probably represent addition of crust to the lithosphere, they do not represent significant lithospheric spreading or convergence, and the region does not mark the boundary between two distinct tectonic plates. This region is not easily explained by analogy with either terrestrial midocean rifts or subduction zones. It is perhaps best explained by upwelling of magma diapirs that blister the surface, but do not cause significant lithospheric spreading. Further study of the structural and volcanic evolution of this region using Magellan altimetry and SAR data should lead to better understanding of the tectonic evolution of this region.

Hansen, Vicki L.↗

NASA NTRS1979

Analyses of earth radiation budget data from unrestricted broadband radiometers on the ESSA 7 satellite

Six months of data from the wide-field-of-view low resolution infrared radiometers on the Environmental Science Services Administration (ESSA) 7 satellite were analyzed. Earth emitted and earth reflected irradiances were computed at satellite altitude using data from a new in-flight calibration technique. Flux densitites and albedos were computed for the top of the earth's atmosphere. Monthly averages of these quantities over 100 latitude zones, each hemisphere, and the globe are presented for each month analyzed, and global distributions are presented for typical months. Emitted flux densities are generally lower and albedos higher than those of previous studies. This may be due, in part, to the fact that the ESSA 7 satellite was in a 3 p.m. Sun-synchronous orbit and some of the comparison data were obtained from satellites in 12 noon sun-synchronous orbits. The ESSA 7 detectors seem to smooth out spatial flux density variations more than scanning radiometers or wide-field-of-view fixed-plate detectors. Significant longitudinal and latitudinal variations of emitted flux density and albedo were identified in the tropics in a zone extending about + or - 25 deg in latitude.

Weaver, W. L.↗

NASA NTRS1984

Geoid anomalies and fracture zones in the Pacific Ocean

The high degree and order geoid field in the Pacific is a superposition of fracture zone anomalies and hot-spot swell anomalies. A two-dimensional spectral analysis of this field reveals a very strong north-south wavenumber contribution with a dominant wavelength of about 2000 km, a much smaller contribution from east-west wavenumbers, and negligible contributions from other directions. One dimensional profiles were taken in order to appreciate the magnitudes of the north-south and east-west components. A calculated geoid anomaly using an idealized fracture zone model contains just about the same amount of power in the 2350 km band wavelength as does the north-south profile of the SEASAT geoid field. In an attempt to correlate plate age with geoid anomalies, a digitized age map of the Pacific was used to generate a synthetic geoid, which was subtracted from SEASAT. This procedure produces a residual geoid in which the fracture zone anomalies appear to be diminished, if not removed.

Source record↗

NASA NTRS1978

Thermal structure and kinematics of mid-oceanic ridge axis - Some implications to basaltic volcanism

Based on studies of ophiolitic complexes, a characterization of basaltic magmas at midocean ridges is proposed. A thin molten region about 3 km wide probably exists at the top of the magma chamber; primitive material enters the uppermost part of the chamber from below, and fractionated material is removed by the upward extrusion of dikes and settling of crystals to form a mush which probably fills the bulk of the chamber. The roof of the magma chamber is probably cooler than the melting temperature of primitive basalt. Secondary melts and the volume of the uppermost molten region are considered. Weaknesses along the line of dike intrusion and limited plating of material onto the roof of the chamber may restrict dike intrusion to the exceeding narrow zone inferred from ophiolitic complexes.

Sleep, N. H.↗

NASA NTRS1982

The earthquake cycle in subduction zones

A simplified model of a subduction zone is presented, which incorporates the mechanical asymmetry induced by the subducted slab to anchor the subducting plate during post-seismic rebound and thus throw most of the coseismic stream release into the overthrust plate. The model predicts that the trench moves with respect to the deep mantle toward the subducting plate at a velocity equal to one-half of the convergence rate. A strong extensional pulse is propagated into the overthrust plate shortly after the earthquake, and although this extension changes into compression before the next earthquake in the cycle, the period of strong extension following the earthquake may be responsible for extensional tectonic features in the back-arc region.

Melosh, H. J.↗

NASA NTRS1986

Manual for obscuration code with space station applications

The Obscuration Code, referred to as SHADOW, is a user-oriented computer code to determine the case shadow of an antenna in a complex environment onto the far zone sphere. The surrounding structure can be composed of multiple composite cone frustums and multiply sided flat plates. These structural pieces are ideal for modeling space station configurations. The means of describing the geometry input is compatible with the NEC-BASIC Scattering Code. In addition, an interactive mode of operation has been provided for DEC VAX computers. The first part of this document is a user's manual designed to give a description of the method used to obtain the shadow map, to provide an overall view of the operation of the computer code, to instruct a user in how to model structures, and to give examples of inputs and outputs. The second part is a code manual that details how to set up the interactive and non-interactive modes of the code and provides a listing and brief description of each of the subroutines.

Marhefka, R. J.↗

NASA NTRS1993

Users manual for the NASA Lewis three-dimensional ice accretion code (LEWICE 3D)

A description of the methodology, the algorithms, and the input and output data along with an example case for the NASA Lewis 3D ice accretion code (LEWICE3D) has been produced. The manual has been designed to help the user understand the capabilities, the methodologies, and the use of the code. The LEWICE3D code is a conglomeration of several codes for the purpose of calculating ice shapes on three-dimensional external surfaces. A three-dimensional external flow panel code is incorporated which has the capability of calculating flow about arbitrary 3D lifting and nonlifting bodies with external flow. A fourth order Runge-Kutta integration scheme is used to calculate arbitrary streamlines. An Adams type predictor-corrector trajectory integration scheme has been included to calculate arbitrary trajectories. Schemes for calculating tangent trajectories, collection efficiencies, and concentration factors for arbitrary regions of interest for single droplets or droplet distributions have been incorporated. A LEWICE 2D based heat transfer algorithm can be used to calculate ice accretions along surface streamlines. A geometry modification scheme is incorporated which calculates the new geometry based on the ice accretions generated at each section of interest. The three-dimensional ice accretion calculation is based on the LEWICE 2D calculation. Both codes calculate the flow, pressure distribution, and collection efficiency distribution along surface streamlines. For both codes the heat transfer calculation is divided into two regions, one above the stagnation point and one below the stagnation point, and solved for each region assuming a flat plate with pressure distribution. Water is assumed to follow the surface streamlines, hence starting at the stagnation zone any water that is not frozen out at a control volume is assumed to run back into the next control volume. After the amount of frozen water at each control volume has been calculated the geometry is modified by adding the ice at each control volume in the surface normal direction.

Bidwell, Colin S.↗

NASA NTRS1984

Photographic surveys of the southern sky

Parameters of the UK 1.2 meter Schmidt telescope are described. Plates taken with this instrument are in two categories, those for systematic sky surveys and those taken at the request of research users. A collaborative project with the European Southern Observatory was undertaken to obtain a two-color survey of the sky south of -20 deg declination to complement the Palomar survey. A near infrared survey of the Galactic Plane and the Megallanic Clouds is being done. The area south of -20 deg and the zone between 0 deg and -15 deg are also being surveyed. Pending a decision on survey parameters, all available A quality prism plates are being retained to form a basis for systematic survey. Nearly half the plates taken on a service basis for the UK astronomical community are to fulfill nonsurvey requests. Plates taken for surveys which are not of A grade quality are also made available for research purposes.

Sim, M. E.↗

NASA NTRS1985

Sulfur transformations at the hydrogen sulfide/oxygen interface in stratified waters and in cyanobacterial mats

Stratified water bodies allow the development of several microbial plates along the water column. The microbial plates develop in relation to nutrient availability, light penetration, and the distribution of oxygen and sulfide. Sulfide is initially produced in the sediment by sulfate-reducing bacteria. It diffuses along the water column creating a zone of hydrogen sulfide/oxygen interface. In the chemocline of Solar Lake oxygen and sulfide coexist in a 0 to 10 cm layer that moves up and down during a diurnal cycle. The microbial plate at the chemocline is exposed to oxygen and hydrogen sulfide, alternating on a diurnal basis. The cyanobacteria occupying the interface switch from anoxygenic photosynthesis in the morning to oxygenic photosynthesis during the rest of the day which results in a temporal build up of elemental sulfur during the day and disappears at night due to both oxidation to thiosulfate and sulfate by thiobacilli, and reduction to hydrogen sulfide by Desulfuromonas sp. and anaerobically respiring cyanobacteria. Sulfate reduction was enhanced in the light at the surface of the cyanobacterial mats. Microsulfate reduction measurements showed enhanced activity of sulfate reduction even under high oxygen concentrations of 300 to 800 micrometer. Apparent aerobic SO sub 4 reduction activity is explained by the co-occurrence of H sub 2. The physiology of this apparent sulfate reduction activity is studied.

Cohen, Y.↗

NASA NTRS1984

The role of an intracrustal asthenosphere on the behavior of major strike-slip faults

Strain accumulation measurements adjacent to the San Andreas fault have indicated that the strain accumulation zone extends only a few tens of kms away from the fault. While the restricted zone of cyclic accumulation and release of elastic energy adjacent to major strike-slip faults has been attributed to a viscoelastic asthenosphere's damping effect, the narrowness of the San Andreas zone implies a thickness of the lithosphere that, at 10-20 km, may not be consistent with the relatively low surface heat flow measurements obtained. It is presently proposed that an upper elastic plate extends to a depth of 15 km, and that beneath this upper elastic plate is a soft, intracrustal asthenosphere exhibiting a viscoelastic behavior. A second elastic layer lies under this, followed by the asthenosphere. It is shown that the damping due to the intracrustal asthenosphere can explain the observed narrow zone of cyclic strain accumulation and release.

Turcotte, D. L.↗

NASA NTRS1974

Film cooling in a combustor operating at fuel-rich exit conditions

Data were taken with a film-cooled test plate placed in the exhaust stream of a rectangular combustor. Results showed that in a fuel-rich zone, fuel entrained into the film-cooling air would burn at the conditions tested. Test conditions were cooling-gas flow rates, 9.5 and 5 percent of total gas flow; cooling-gas velocities, 23 and 12 m/sec; ambient-temperature cooling gas; hot-gas velocity, nominally 215 m/sec; fuel-air ratio to stoichiometric fuel-air ratio values of 0.1, 0.8, 1.0, 1.2, and 1.35, resulting in hot-gas temperatures from 590 to 2100 K; and atmospheric pressure. Analytical prediction of wall temperatures agreed reasonably well with experimental results.

Tacina, R. R.↗

NASA NTRS1991

Regional tectonic analysis of Venus as part of the Pioneer Venus guest investigator project

Over the past year, much of the tectonic analysis of Venus we have done has centered on global properties of the planet, in order to understand fundamental aspects of the dynamics of the mantle and lithosphere of Venus. We have developed convection models of the Earth and Venus. These models assume whole mantle internally-heated convection. The viscosity is temperature, volatile-content, and stress dependent. An initial temperature and volatile content is assumed, and the thermal evolution is tracked for 4.6 billion years. During this time, heating occurs by decay of radiogenic elements in the mantle, and degassing and regassing of volatiles takes place at the surface. For a model assuming plate tectonics as the primary heat loss mechanism, representing the Earth through most of it's history and perhaps Venus' earlier history, degassing of the mantle was found to occur rapidly (approximately 200 My) over a large range of parameters. Even for parameters chosen to represent extreme cases of an initially cool planet, low radiogenic heating, and large initial volatile complement, the mantle water content was degassed to an equilibrium value in about 2 By. These values may be applicable to the early Venus, if a large, Moon-forming impact on Earth resulted in efficient heating and loss of water, leaving Venus with a comparably greater volatile budget and less vigorous early convection. It may therefore be impossible to retain large amounts of water in the interior of Venus until the planet cools down enough for the 'cold-trap' effect to take place. This effect traps crust forming melts within the mantle due to a cusp in the solidus, causing these melts to refreeze at depth into a dense eclogite phase, which will inhibit ascent of this material to the surface. This effect, however, requires a hydrous mantle, so early loss of water might prevent it from taking place. Since without plate tectonics there is no mechanism for regassing volatiles into the mantle, as occurs on Earth at subduction zones, this means the interior of Venus would at present be almost completely dry. We have also calculated argon degassing, and mantle flow velocities. viscosities, and cooling rates in these models, and these values can provide constraints on present day mantle dynamics.

Williams, David R.↗

NASA NTRS2018

Analysis of Progressive Damage in Cross-Ply and Quasi-Isotropic Panels Subjected to Quasi-Static Indentation

The performance of a three-dimensional continuum damage mechanics (CDM) model for intralaminar damage, coupled with a cohesive zone model for delamination, is examined for damage prediction in cross-ply and quasi-isotropic laminated plates subjected to quasi-static transverse indentation. Details of the CDM model based on the deformation gradient decomposition (DGD) method and the finite element (FE) modeling strategy are presented and discussed. Critical features of the damage model and FE modeling approach are assessed by comparing analysis predictions to experimental data for cross-ply and quasi-isotropic panels in which damage is dominated by complex interactions between matrix cracks and delaminations.

Song, Kyongchan↗

NASA NTRS1991

The interpretation of crustal dynamics data in terms of plate motions and regional deformation near plate boundaries

The focus of the research was in two broad areas: (1) the nature and dynamics of time dependent deformation and stress along major seismic zones; and (2) the nature of long wavelength oceanic geoid anomalies in terms of lateral variations in upper mantle temperature and composition. The principle findings of the research are described in the accompanying appendices. The first two and the fourth appendices are reprints of papers recently submitted for publication, and the third is the abstract of a recently completed thesis supported by this project.

Soloman, Sean C.↗