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At least 307 records · Page 17

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NASA NTRS1983

Magnetospheric currents

After outlining the constituent parts of the magnetospheric system, a historical review is presented of studies of the earth's magnetic field. It is noted that a connection between the aurora and variations in the magnetic field was first suggested by Halley in 1716. In discussing the magnetosphere, it is pointed out that the geomagnetic field can be thought of as being produced by a huge bar magnet embedded in the earth, with the axis of the magnet tilted away slightly from the earth's rotational axis. Attention is also given to the interplanetary magnetic field, to the relationship between the interplanetary magnetic field and the geomagnetic field, to convective flow, to field-aligned currents, and to Birkeland currents and auroral emissions. Various questions concerning the Birkeland currents are summarized.

Potemra, T. A.↗

NASA NTRS1986

Laser Schlieren Crystal-Growth Imager

Crystal observed as it grows from melt with aid of laser schlieren imaging. Observation method allows entire perimeter of growing crystal to be inspected. Isolated crystal facets examined, convection flows and temperature and concentration gradients revealed. Method does not require contact with, or proximity to, crystal.

Owen, R. B.↗

NASA NTRS1984

Transitions in shallow convection - An explanation for lateral cell expansion

The generalized, seven-coefficient model presented for two-dimensional Rayleigh-Benard convection successfully simulates one way in which lateral cell expansion can occur as the value of the imposed vertical temperature difference is changed. The triad of interacting wavenumbers is varied as the vertical heating rate is varied and only the energetically active components are retained, so that transitional behavior within two-dimensional convective flow can be both simulated and physically interpreted as the representation of the cell expansion process via successive secondary branching. It is found that while a two-dimensional cell broadening mechanism is likely to operate for small Prandtl number values, a three-dimensional mechanism is expected at values greater than about 0.7.

Chang, H.-R.↗

NASA NTRS1985

Lower mantle heterogeneity, dynamic topography and the geoid

Density contrasts in the lower mantle, recently imaged using seismic tomography, drive convective flow which results in kilometers of dynamically maintained topography at the core-mantle boundary and at the earth's surface. The total gravity field due to interior density constrasts and boundary topography predicts the largest wavelength components of the geoid remarkably well. Neglecting dynamic surface deformation leads to geoid anomalies of opposite sign than are observed.

Hager, B. H.↗

NASA NTRS1985

The source of the Earth's long wavelength geoid anomalies: Implications for mantle and core dynamics

The long wavelength components of the Earth's gravity field result mainly from density contrasts associated with convection in the mantle. Direct interpretation of the geoid for mantle convection is complicated by the fact that convective flow results in dynamically maintained deformation of the surface of the Earth, the core mantle boundary (CMB), and any interior chemical boundaries which might exist. These boundary deformations effect the geoid opposite in sign and are comparable in magnitude to those of the interior density contrasts driving the flow. The total difference of two relatively large quantities.

Hager, B. H.↗

NASA NTRS1985

Geodynamical basis for crustal deformation under the Tibetan Plateau

Plate tectonics and satellite-derived gravity data are used to examine crustal deformation under the Tibetan Plateau. A spherical harmonic analysis is given for the global plate boundary system, and the crustal stresses in Tibet are calculated from satellite gravity data. A superimposed stress system is constructed. The stress patterns reveal that the cold downwelling mantle convection flow beneath southern Tibet pulls the Indian plate down but applies a bending moment on the end of the plate to uplift and support the mass of the Himalayas.

Liu, H.-S.↗

NASA NTRS1985

An experimental investigation on flame interaction and the existence of negative flame speeds

Downstream interaction between two counterflow premixed flames of different stoichiometries are investigated. Various flame configurations are observed and quantified; these include the binary system of two lean or rich flames, the triplet system of a lean and a rich flame separated by a diffusion flame, and single diffusion flames with some degree of premixedness. Extinction limits are determined for methane/air and butane/air mixtures over the entire range of mixture concentrations. Results show that these extinction limits can be significantly modified in the presence of interaction such that a mixture much beyond the flammability limit can still burn if it is supported by a stronger flame. The experiment also demonstrates the existence of negative flames whose propagation velocity is in the same general direction as that of the bulk convective flow. Implications of the present results on the flammability of stratified mixtures and on the modeling of turbulent flames are discussed.

Sohrab, S. H.↗

NASA NTRS1985

Structure in the DC and AC electric fields associated with the dayside cusp region

The cusp region as seen in the AC and DC electric fields is one of intense variation. The intensity peaks within the soft particle precipitation. The only AC signal that appears to be unique to the cusp is broadband ULF-ELF magnetic noise. Other types of emissions are also found at other local times at high latitudes. The pattern of these signals, especially that of ULF-ELF broadband electrostatic noise (BEN), distinguishes the cusp region from other regions. BEN signatures are indicators of magnetosheath-like soft particle precipitation but not necessarily of open field lines. In addition, large spike-like features in the DC electric field are seen near local magnetic noon which appear to be related to the large convective electric fields that have been observed at the magnetopause. These features are not necessarily tied to convection reversals, but may appear within broader regions of zonal convective flow.

Maynard, N. C.↗

NASA NTRS1985

Space plasma turbulent dissipation - Reality or myth?

A prevalent approach to understanding magnetospheric dynamics is to combine a hydromagnetic description of the large scale magnetic structure and convection flows with a locally determined anomalous dissipation which develops in boundary layers. Three problems (nose and tail reconnection, auroral field-aligned currents, and diffuse auroral precipitation) are critically examined to test the validity of this theoretical philosophy. Although the expected plasma wave turbulence is observed for each case, the concept of local anomalous dissipation fails to provide an adequate or complete description of the phenomenae.

Coroniti, F. V.↗

NASA NTRS1986

Velocity filter mechanism for ion bowl disributions (Bimodal conics)

The 'bowl-shaped' ion distributions in the high altitude auroral region observed by Klumpar et al. (1984) were originally interpreted as being due to a two-stage acceleration involving transverse ion heating and upward-aligned electric field acceleration. In this paper, it is shown that qualitatively similar bowl-shaped distributions can also be formed by transverse heating in a region of finite horizontal extent, followed by essentially adiabatic convective flow to the observation location. The latter stage contributes a velocity-filtering effect which produces some characteristics of the distributions observed. It is suggested it may be possible to use ion species observations to distinguish the mechanism proposed by Klumpar et al. from the alternative mechanism outlined in the present study.

Horwitz, J. L.↗

NASA NTRS1987

Optical strain measurement system development, phase 1

A laser speckle, differential strain measurement system was built and tested for future applications in hostile environments. One dimensional electronic correlation of speckle pattern movement allows a quasi-real time measure of strain. The system was used successfully to measure uniaxial strain reaching into plastic deformation of a test specimen, at temperatures ranging to 450 C. A resolution of 16 microstrain is given by the photodiode array sensor pitch and the specimen to sensor separation. The strain measurement limit of the gauge is determined by air density perturbations causing decorrelation of the reference and shifted speckle patterns, and may be improved by limiting convective flow in the immediate vicinity of the test specimen.

Lant, Christian T.↗

NASA NTRS1987

Optical strain measurement system development

A laser speckle, differential strain measurement system has been built and tested for future applications in hostile environments. One-dimensional electronic correlation of speckle pattern movement allows a quasi-real time measure of strain. The system has been used successfully to measure uniaxial strain reaching into plastic deformation of a test specimen, at temperatures ranging to 450 C. A resolution of 16 microstrain is given by the photodiode array sensor pitch and the specimen to sensor separation. The strain measurement error is estimated to be +/-18 microstrain +/-3 percent of the strain reading. The upper temperature limit of the gauge is determined by air density perturbations causing decorrelation of the reference and shifted speckle patterns, and may be improved by limiting convective flow in the immediate vicinity of the test specimen.

Lant, Christian T.↗

NASA NTRS1986

A mechanism for tectonic deformation on Venus

In the absence of identifiable physiographic features directly associated with plate tectonics, alternate mechanisms are sought for the intense tectonic deformation observed in radar images of Venus. One possible mechanism is direct coupling into an elastic lithosphere of the stresses associated with convective flow in the interior. Spectral Green's function solutions have been obtained for stresses in an elastic lithosphere overlying a Newtonian interior with an exponential depth dependence of viscosity, and a specified surface-density distribution driving the flow. At long wavelengths and for a rigid elastic/fluid boundary condition, horizontal normal stresses in the elastic lid are controlled by the vertical shear stress gradient and are directly proportional to the depth of the density disturbance in the underlying fluid. The depth and strength of density anomalies in the Venusian interior inferred by analyses of long wavelength gravity data suggest that stresses in excess of 100 MPa would be generated in a 10 km thick elastic lid unless a low viscosity channel occurring beneath the lid or a positive viscosity gradient uncouples the flow stresses. The great apparent depth of compensation of topographic features argues against this, however, thus supporting the importance of the coupling mechanism. If there is no elastic lid, stresses will also be very high near the surface, providing also that the viscosity gradient is negative.

Phillips, Roger J.↗

NASA NTRS1986

Materials processing in space - Review of the early experiments

Basic research performed to study various aspects of fluid behavior in a microgravity environment during the first two decades of spaceflight is described. The uniqueness of space was recognized in the 1950s as a place where buoyancy-driven convective flows and sedimentation would effectively be absent. The conditions were favorable for experiments in crystal growth, alloy solidification, combustion, thermocapillary flows, electroosmotic flows, Stefan flows, Soret diffusion, and solidification processes. Pre-Skylab NASA drop tower experiments and Apollo 14 and 17 experiments made in preparation for Skylab are described. Skylab welding and brazing, crystal growth, metallurgy and liquid float zone trials and Apollo-Soyuz crystal growth, electrophoresis and metallurgical experiments are summarized. SPAR rocket trials along similar lines are also outlined.

Naumann, Robert J.↗

NASA NTRS1987

Double diffusive convection during directional solidification - Nonlinear analysis

The dynamics of doubly diffusive convective flow during the constant-velocity vertical solidification of binary melts to form single-phase solids is examined, summarizing the results of recent experimental and analytical investigations. The effects of horizontal temperature gradients are considered; the nonlinearities of the system and their numerical treatment are discussed; and numerical results are presented graphically for the solidification of a Pb melt containing 0.0015 wt pct Sn, with growth velocity 0.0002 cm/s and a temperature gradient of 200 K/cm in the liquid.

Coriell, S. R.↗

NASA NTRS1987

Numerical observations of dynamic behavior in two-dimensional compressible convection

Using numerical simulations of the anelastic partial differential equations, convective flow in a two-dimensional polytropic atmosphere is investigated. Fixing the aspect ratio at unity and choosing the viscosity as the order parameter, the numerical solutions have indicated a variety of dynamic behavior including (1) oscillatory motion with a period of order the eddy turnover time, (2) quasi-periodic motion with a long period much longer than an eddy turnover time, and (3) at least two branches of solutions in certain regimes.

Ginet, G. P.↗

NASA NTRS1988

MCT crystal growth

Convection and segregation in directional solidification and crystal growth by the Bridgman-Stockbarger technique are traditionally treated by assuming axisymmetric thermal condition on the ampoule wall. It is, however, difficult to achieve such a condition in an experimental setup. Any deviation from an axisymmetric temperature field on the wall of a vertical ampoule represents a horizontal temperature gradient. The horizontal density gradient that results from thermal expansion in the melt under this condition must lead on earth to some buoyance-driven convection, no matter what the axial (vertical) temperature distribution that is imposed on the melt. The magnitude of such convective flows for conditions representative of the MSFC mercury-cadmium-telluride (MCT) Bridgman setup is studied.

Baird, James K.↗

NASA NTRS1988

Spacecraft Fire-Safety Experiments for Space Station: Technology Development Mission

Definitions, overall descriptions, and preliminary apparatus and instrumentation requirements are presented for three spacecraft fire-safety experiments appropriate for inclusion in a growth-version of the Space Station. The experiments are as follows: 1. Combustion and Flame Spread of Typical Spacecraft Materials in a Low Velocity Convective Flow in Low Gravity 2. Fire and Fire-Extinguishant Interactions with Various Fire Scenarios in Low Gravity 3. Smoldering and Deep-Seated Combustion in Low Gravity. The experiments were selected from an assessment of recommendations for advanced technologies for prevention, detection, and control of fires in spacecraft. The experiments and their facilities are intended to constitute a portion of a Spacecraft Fire Safety Technology Development Mission (TDM). Basic requirements for the mission study are that the Space Station is essential for the accomplishment of the experimental objectives and that the technology being developed is appropriate for the growth version of the Space Station. The advantages of these three experiments are that, not only do they explore problems relevant to the Space Station operation and utilization, but also they constitute tests adaptable to two multiuse facilities installed in the Space Station laboratory module, the Combustion Tunnel Facility and the Combustion Facility. Three additional spacecraft fire-safety experiments, essential precursors to the TD M for near-term performance, are also described as follows: 1. Near-Term Testing of Materials for Combustion and Flame Spread 2. Near-Term Extinguishment of Fires in a Low-Gravity Test Bed 3. Post-Fire Recovery of Sensitive Electrical and Electronic Equipment. For all the experiments, the study defines accommodations in terms of first-order space, mass, power, consumables, and crew-time estimates. The study also reviews the key problems to be addressed, namely experiment safety hazards, non-intrusive diagnostics development and waste-products disposal. A preliminary task and time schedule for design, development, and experiment fabrication is proposed.

Youngblood, Wallace W.↗