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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,153 records · Page 64
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NASA NTRS1990

Pitch angle diffusion of newborn ions due to intrinsic turbulence in the solar wind

The objective of the present study is to understand the interaction of the solar wind with newborn ions in far upstream regions of a comet where the level of intrinsic turbulence is moderately low. Based on the assumption that quasi-linear theory is adequate and applicable, the pitch angle diffusion process and the time evolution of the newborn ion distribution function are investigated. Numerical solutions to the quasi-linear diffusion equation, including the effect of resonance broadening and that of continuous creation of newborn ions due to the ionization process, are obtained under several assumptions and approximations. It is found that theoretical results are consistent with the Giotto observations recently reported by Neugebauer et al. (1989).

Ziebell, L. F.↗
NASA NTRS1990

Diffusive shock acceleration of energetic electrons and the method of generations

The diffusive shock acceleration of energetic electrons at a plane shock is studied by means of the method of generations developed by Petukhov et al. (1986), where the particles are binned according to the number of particle acceleration cycles. The method emphasizes the cyclic character of the shock acceleration process, and takes into account the particle energy increment per cycle, the energy loss per cycle, and the escape probability of particles downstream of the shock. In the limit of a small fractional momentum gain per cycle, the method reproduces the known steady-state semianalytic solution for constant diffusion coefficients on both sides of the shock. It is demonstrated that the method may also be applied to time-dependent shock-acceleration problems.

Webb, G. M.↗
NASA NTRS1990

The fast diffusion of Au IN Pb

A treatment of the phenomenon of fast diffusion in lead is presented. The model used is based upon the fast diffusion of free solute interstitials. The very large negative enhancement coefficients found in the Pb-(Au, Ag) systems is explained by the formation of first and second order clusters of vacancies and substitutional solute atoms.

Mclellan, R. B.↗
NASA NTRS1990

A parallel algorithm for nonlinear convection-diffusion equations

A parallel algorithm for the efficient solution of nonlinear time-dependent convection-diffusion equations with small parameter on the diffusion term is presented. The method is based on a physically motivated domain decomposition that is dictated by singular perturbation analysis. The analysis is used to determine regions where certain reduced equations may be solved in place of the full equation. The method is suitable for the solution of problems arising in the simulation of fluid dynamics. Experimental results for a nonlinear equation in two-dimensions are presented.

Scroggs, Jeffrey S.↗
NASA NTRS1990

Validation of parameterization scheme for eddy diffusion from satellite data

The eddy diffusion coefficient K(yy) has been calculated usign LIMS for the months of December 1978 and January and February 1979. Two methods have been used. The first implements the suggestion made by Tung (1987) to parameterize the eddy transport as a diffusive process along isentropes. The second method integrates the equation relating the parcel displacements to the eddy velocity fields. The latter method uses a filtering on both space and time domains to isolate transients and is referred to as the 'spectral method'. Results from the first method are shown to be reliable only for quiescent periods, breaking down when the meridional gradient of potential vorticity is negligible. Results from the two methods are in agreement only for very disturbed conditions, when transience is readily isolated. It is concluded that the parameterizations suggested for eddy transport and calculated in this paper may be meaningful for quiet periods, but are not reliable for unsteady and very large amplitude disturbances.

Sassi, F.↗
NASA NTRS1991

Plasma transport in the Io torus - The importance of microscopic diffusion

This paper considers the question of whether the distribution of mass in the Io plasma torus is consistent with the concept of interchange eddy transport. Specifically, the flux tube content exhibits a gradual decrease with increasing radial distance from the source near Io without any evidence for substantial density irregularity associated with the plasma source or loss. Using a simple one-dimensional numerical model to simulate macroscopic interchange eddy transport, it is demonstrated that this smooth equilibrium distribution of mass can occur but only with the inclusion of a minimal level of small scale microscopic mixing at a rate approaching Bohm diffusion. Otherwise, the system exhibits a chaotic appearance which never approaches an equilibrium distribution. Various physical mechanisms for the microscopic diffusion process which is required to provide a sufficiently rapid mixing of material between the macroscopic eddies are discussed.

Mei, YI↗
NASA NTRS1991

The albedo and scattering phase function of interstellar dust and the diffuse background at far-ultraviolet wavelengths

The diffuse FUV background in a variety of targets has been observed with a nebular spectrometer designed specifically to eliminate potential sources of contamination. The intensity of the background near the Galactic plane is found to be much lower than previously published. The data are analyzed using a radiative transfer model with a limited set of free parameters, including those which describe the scattering properties of the interstellar dust. Strong evidence is presented that scattering of starlight by Galactic dust associated with neutral hydrogen produces the major part of the FUV diffuse background. Few assumptions are necessary to conclude that the albedo of the grains is low in the FUV. The grains, at least at high latitude, scatter fairly isotropically in the FUV. The low FUV albedo and the usual phase factor g are not consistent with predictions of standard models for the size distribution and composition of interstellar grains.

Hurwitz, Mark↗
NASA NTRS1991

NASA/DOD Aerospace Knowledge Diffusion Research Project. Paper 11: The Voice of the User: How US Aerospace Engineers and Scientists View DoD Technical Reports

The project examines how the results of NASA/DOD research diffuse into the aerospace R&D process, and empirically analyzes the implications of the aerospace knowledge diffusion process. Specific issues considered are the roles played by government technical reports, the recognition of the value of scientific and technical information (STI), and the optimization of the STI aerospace transfer system. Information-seeking habits are assessed for the U.S. aerospace community, the general community, the academic sector, and the international community. U.S. aerospace engineers and scientists use 65 percent of working time to communicate STI, and prefer 'internal' STI over 'external' STI. The isolation from 'external' information is found to be detrimental to U.S. aerospace R&D in general.

Pinelli, Thomas E.↗
NASA NTRS1991

Pressure flux-split viscous solutions for subsonic diffusers

A method for predicting flow in diffusers with inlet swirl has been developed. Solutions of the pressure flux-split Reduced Navier Stokes (RNS) equations are obtained for flows in axisymmetric diffusers. Viscous-Inviscid interactions and flow fields with toroidal recirculation regions are efficiently captured. The computational model is verified by comparision with experimental data and other computations. The extreme sensitivity to grid, turbulent closure model and inlet profiles is discussed.

Pordal, H. S.↗
NASA NTRS1991

Sputtered Ta-Si-N diffusion barriers in Cu metallizations for Si

Electrical measurements on shallow Si n+-p junction diodes with a 30-nm TiSi2 contacting layer demonstrate that an 80-nm-thick amorphous Ta36Si14N50 film prepared by reactive RF sputtering of a Ta5Si3 target in an Ar/N2 plasma very effectively prevents the interaction between the Si substrate with the TiSi2 contacting layer and a 500-nm Cu overlayer. The Ta36Si14N50 diffusion barrier maintains the integrity of the I-V characteristics up to 900 C for 30-min annealing in vacuum. It is concluded that the amorphous Ta36Si14N50 alloy is not only a material with a very low reactivity for copper, titanium, and silicon, but must have a small diffusivity for copper as well.

Kolawa, E.↗
NASA NTRS1990

The SHEAL Diffuse X-ray Spectrometer experiment

The paper describes the Diffuse X-ray Spectrometer (DXS) experiment which will be carried on board the NASA's SHEAL 2 mission, scheduled to be launched as an attached Shuttle payload in 1992. The SHEAL DXS is designed to measure the spectrum of the low-energy (0.15 to 0.28 keV) diffuse X-ray background with the energy resolution better than 0.01 keV. The results of calculations of the anticipated data are presented together with diagrams of the DXS assembly.

Sanders, W. T.↗
NASA NTRS1992

Measuring Thermal Diffusivity Of A High-Tc Superconductor

Technique for measuring thermal diffusivity of superconductor of high critical temperature based on Angstrom's temperature-wave method. Peltier junction generates temperature oscillations, which propagate with attenuation up specimen. Thermal diffusivity of specimen calculated from distance between thermocouples and amplitudes and phases of oscillatory components of thermocouple readings.

Powers, Charles E.↗
NASA NTRS1991

On the origin of the diffuse x ray background

The first measurement is reported of the intensity and spectrum of the diffuse x ray background in the 0.16 to 3.5 keV band which is free from contamination by sources with fluxes greater than approx. 6 x 10(exp -14) erg/sq cm/s. This result was made possible by the development of a number of techniques for reducing cosmic ray contamination and instrumental artifacts in the data collected by the Einstein Observatory imaging proportional counter. The analysis of the background data reveals a mean absolute intensity for the emission I sub x(0.16 to 3.5 keV) approx. = 5.6 x 10(exp -8) erg/sq cm/s/sr. The intensity is dependent on galactic longitude even when only high galactic data are used, allowing the setting of a lower limit of 20 pct. on the galactic contribution to the mean emission band. The spectrum of the total background is consistent with a power law of slope similar to 0.7 between 0.16 and 3.5 keV with evidence for a steep rise toward lower energies. Preliminary evidence is also reported for the association of faint radio sources with peaks in the arcminute scale fluctuations of the diffuse x ray surface brightness distribution and then some implications are considered of these results.

Helfand, David J.↗
NASA NTRS1991

Laser Raman diagnostics in subsonic and supersonic turbulent jet diffusion flames

Ultraviolet (UV) spontaneous vibrational Raman scattering combined with laser-induced predissociative fluorescence (LIPF) is developed for temperature and multi-species concentration measurements. Simultaneous measurements of temperature, major species (H2, O2, N2, H2O), and minor species (OH) concentrations are made with a 'single' narrow band KrF excimer laser in subsonic and supersonic lifted turbulent hydrogen-air diffusion flames. The UV Raman system is calibrated with a flat-flame diffusion burner operated at several known equivalence ratios from fuel-lean to fuel-rich. Temperature measurements made by the ratio of Stokes/anti-Stokes signal and by the ideal gas law are compared. The single shot measurement precision for concentration and temperature measurement is 5 to 10 pct. Calibration constants and bandwidth factors are determined from the flat burner measurements and used in a data reduction program to arrive at temperature and species concentration measurements. These simultaneous measurements of temperature and multi-species concentrations allow a better understanding of the complex turbulence-chemistry interactions and provide information for the input and validation of CFD models.

Cheng, T. S.↗
NASA NTRS1991

A technique to measure the thermal diffusivity of high Tc superconductors

High T(sub c) superconducting electrical current leads and ground straps will be used in cryogenic coolers in future NASA Goddard Space Flight Center missions. These superconducting samples are long, thin leads with a typical diameter of two millimeters. A longitudinal method is developed to measure the thermal diffusivity of candidate materials for this application. This technique uses a peltier junction to supply an oscillatory heat wave into one end of a sample and will use low mass thermocouples to follow the heat wave along the sample. The thermal diffusivity is calculated using both the exponential decay of the heat wave and the phase shift to the wave. Measurements are performed in a cryostat between 10 K and room temperature.

Powers, Charles E.↗
NASA NTRS1991

The role of thermal vapor diffusion in the subsurface hydrologic evolution of Mars

The hydrologic response of groundwater to the thermal evolution of the early martian crust is considered. When a temperature gradient is present in a moist porous medium, it gives rise to a vapor-pressure gradient that drives the diffusion of water vapor from regions of high to low temperature. By this process, a geothermal gradient as small as 15 K/km could drive the vertical transport of 1 km of water to the freezing front at the base of the martian crysophere every 10 exp 6-10 exp 7 years, or the equivalent of about 100-1000 km of water over the course of martian geologic history. Models of the thermal history of Mars suggest that this thermally-driven vapor flux may have been as much as 3-5 times greater in the past. The magnitude of this transport suggests that the process of geothermally-induced vapor diffusion may have played a critical role in the initial emplacement of ground ice and the subsequent geomorphic and geochemical evolution of the martian crust.

Clifford, Stephen M.↗
NASA NTRS1990

Quantitative thermal diffusivity imaging of disbonds in thermal protective coatings using inductive heating

An inductive heating technique for making thermal diffusivity images of disbonds between thermal protective coatings and their substrates is presented. Any flaw in the bonding of the coating and the substrate shows as an area of lowered values in the diffusivity image. The benefits of the inductive heating approach lie in its ability to heat the conductive substrate without directly heating the dielectric coating. Results are provided for a series of samples with fabricated disbonds, for a range of coating thicknesses.

Heath, D. M.↗
NASA NTRS1991

Constraints on the optical properties of interstellar dust in the far-ultraviolet - Voyager observations of the diffuse sky background

Four regions of the sky have been observed at various Galactic latitudes and H I column densities, with the ultraviolet spectrometer aboard Voyager 2 finding no evidence for the presence of a diffuse radiation field between 912 and 1100 A at a level of 100-200 photons per sq cm s sr A. As starlight scattered by dust grains may be expected to be a major contributor to the diffuse background, especially at low Galactic latitudes, it is possible to place a very stringent upper limit of less than 0.1 on the reflectivity of the dust in this spectral range. This albedo is much lower than the canonical value of about 0.5 for standard graphite-silicate dust models.

Murthy, Jayant↗