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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

The significance of fractional crystallization in the petrogenesis of Apollo 17 Type A and B high-Ti basalts

Whole-rock and mineral analyses of 26 'new' type A and B Apollo 17 basalts are reported. The petrography and mineral chemistry of these basalts are similar to previously reported Apollo 17 basalts. However, these 'new' whole-rock data extend the compositional ranges of previously reported type A and B basalts and require the division of the type B basalts into type B1 and B2 varieties. These three types display similar trends when both major and trace elements are plotted against a fractionation index of Cr/La ratio. Major element compositions of basalts from all three types fall on olivine + Ti oxide control lines. This study demonstrates that Apollo 17 type A, B1, and B2 basalts have a relatively simple petrogenesis, with the only postmagma-generation process being fractional crystallization.

Neal, Clive R.↗
NASA NTRS1990

An eight-month sample of marine stratocumulus cloud fraction, albedo, and integrated liquid water

Surface-meteorology and shortwave/longwave irradiance measurements taken on the northwest tip of San Nicolas Island off the coast of Southern California from March through October 1987 are analyzed. Experimental details are summarized, and shortwave cloud-radiation parameterization is outlined with emphasis on a shortwave algorithm. Frequency distributions indicate the stratocumulus clouds at the island have a cloud base on the order of 400 m, an integrated liquid water content of 75 g/sq m, and an albedo of 0.55 with substantial diurnal variations. The longwave parameterization for cloud fraction is also considered, and it is noted that using these models for downward longwave and shortwave irradiances, cloud fraction, integrated liquid water content, and albedo are deduced from the data.

Fairall, C. W.↗
NASA NTRS1990

Fractional cloudiness in shallow cumulus layers

Fractional cloudiness influences the planetary boundary layer (PBL) by controlling the cloud-top radiative cooling rate, and regulating the buoyant production and consumption of turbulence kinetic energy. Betts, Hanson, and Albrecht have modeled partly cloudy PBLs by assuming a single family of convective circulations. The same idealized model has been used in observational studies, based on conditional sampling and/or joint distribution functions, by Lenschow, Albrecht, and others. This approach is extended. None of these authors has proposed a method to determine the fractional area covered by rising motion; finding such a method was a key objective of the present study.

Randall, David A.↗
NASA NTRS1990

Development of a time-dependent incompressible Navier-Stokes solver based on a fractional-step method

The development, validation and application of a fractional step solution method of the time-dependent incompressible Navier-Stokes equations in generalized coordinate systems are discussed. A solution method that combines a finite-volume discretization with a novel choice of the dependent variables and a fractional step splitting to obtain accurate solutions in arbitrary geometries was previously developed for fixed-grids. In the present research effort, this solution method is extended to include more general situations, including cases with moving grids. The numerical techniques are enhanced to gain efficiency and generality.

Rosenfeld, Moshe↗
NASA NTRS1990

Development of a time-dependent incompressible Navier-Stokes solver based on a fractional-step method

The main goals are the development, validation, and application of a fractional step solution method of the time-dependent incompressible Navier-Stokes equations in generalized coordinate systems. A solution method that combines a finite volume discretization with a novel choice of the dependent variables and a fractional step splitting to obtain accurate solutions in arbitrary geometries is extended to include more general situations, including cases with moving grids. The numerical techniques are enhanced to gain efficiency and generality.

Rosenfeld, Moshe↗
NASA NTRS1991

Solar-wind krypton and solid/gas fractionation in the early solar nebula

The solar-system Kr abundance is calculated from solar-wind noble-gas ratios, determined previously by low-temperature oxidations of lunar ilmenite grains, normalized to Si by spacecraft solar-wind measurements. The estimated Kr-83 abundance of 4.1 + or - 1.5 per million Si atoms is within uncertainty of estimates assuming no fractionation, determined from CI-chondrite abundances of surrounding elements. This is significant because it is the first such constraint on solid/gas fractionation, though the large uncertainty only confines it to somewhat less than a factor of two.

Wiens, Roger C.↗
NASA NTRS1991

Reflectance spectra of some fractions of Migei and Murchison SM chondrites in the range of 0.3-2.6 microns

The reddening observed in CM chondrites is not understood. Johnson and Fanale observed that, as CM chondrites are more finely powdered, their spectra become more reddened. In the process of meteorite crushing, the chondrules are broken up. Those authors suggested that in this case the silicate components of the chondrules (mainly olivine), which have higher IR reflectivities, were able to contribute more to the overall spectrum. Gaffey and McCord proposed two possible physical mechanisms which could produce such an effect. But it is also possible that the presence of the organic polymers in the matrix material results in the reddening of the CM spectra. To test these two hypotheses, the matrix material and the material enriched in olivines were separated from Migei and Murchison CM chondrites using a binocular microscope. The spectra of these fractions were compared with the spectra of the bulk samples of Migei and Murchison. The spectra of the most 'clean' Migei matrix fractions indicate that the reddish slopes of CM spectral curves in the near infrared are due to the enhanced olivine feature, rather than to organic matter. The authors propose that the red slope of the spectra of some C-type asteroids may indirectly suggest the presence of olivine in the surface material. At the same time, the red sloped spectra of more distant D-type asteroids seem to be due to the presence of organic materials. If so, then the chemical or physical form of this organic matter must be quite different from the organic materials which would be on the surfaces of the parent bodies of CM chondrites (most likely C-type asteroids). It is also possible that the content of organic components in the CM matrix is too low to change the slope of the spectra.

Moroz, L. V.↗
NASA NTRS1991

Chemical fractionation of siderophile elements in impactites from Australian meteorite craters

The abundance pattern of siderophile elements in terrestrial and lunar impact melt rocks was used extensively to infer the nature of the impacting projectiles. An implicit assumption made is that the siderophile abundance ratios of the projectiles are approximately preserved during mixing of the projectile constituents with the impact melts. As this mixture occurs during flow of strongly shocked materials at high temperatures, however there are grounds for suspecting that the underlying assumption is not always valid. In particular, fractionation of the melted and partly vaporized material of the projectile might be expected because of differences in volatility, solubility in silicate melts, and other characteristics of the constituent elements. Impactites from craters with associated meteorites offer special opportunities to test the assumptions on which projectile identifications are based and to study chemical fractionation that occurred during the impact process.

Attrep, A., Jr.↗
NASA NTRS1991

Optimum combinations of visible and near-infrared reflectances for estimating the fraction of photosynthetically available radiation absorbed by plants

A useful parameter to estimate terrestrial primary productivity, that can be sensed from space, is the daily averaged fraction of Photosynthetically Available Radiation (PAR) absorbed by plants. To evaluate this parameter, investigators have relied on the fact that the relative amount of radiation reflected by a vegetated surface in the visible and near infrared depends on the fraction of the surface covered by the vegetation and therefore, correlates with absorbed PAR. They have used vegetation indices, namely normalized difference and simple ratio, to derive absorbed PAR. The problem with normalized difference and simple ratio is first, they are non linear functions of radiance or reflectance and therefore, cannot be readily applied to heterogeneous targets, second, they are used in generally nonlinear relationships, which make time integrals of the indices not proportional to primary productivity, and third, the relationships depend strongly on the type of canopy and background. To remove these limitations, linear combinations of visible and near infrared reflectances at optimum (one or two) viewing zenith angles are proposed.

Podaire, Alain↗
NASA NTRS1992

Imperfect fractional crystallization of the lunar magma ocean and formation of the lunar mantle: A realistic chemical approach

It is generally considered that lunar mare basalts were generated by the melting of a cumulate mantle formed in an early Moon-wide magma ocean or magmasphere. However, the nature and chemistry of this cumulate mantle and the logistics of its origin have remained elusive. Extensive studies of terrestrial layered mafic intrusions over the past sixty years have emphasized the imperfection of fractional crystallization and attendant crystal-crystal and crystal-liquid separation in a convecting magma chamber. These separations were similarly inefficient during evolution of the lunar magmasphere, allowing for the trapping of interstitial melt and entrainment of a small proportion of less-dense plagioclase into the more-dense mafic cumulate mush. Indeed, petrography of lunar highlands samples demonstrates this well for anorthosites (with 1-10 percent olivine). Therefore, we propose a 'realistic' model for the evolution of the lunar mantle, which takes these observations into consideration, by the imperfect fractional crystallization of an early lunar magma ocean.

Snyder, Gregory A.↗
NASA NTRS1991

Review and statistical analysis of the use of ultrasonic velocity for estimating the porosity fraction in polycrystalline materials

A review and statistical analysis of the ultrasonic velocity method for estimating the porosity fraction in polycrystalline materials is presented. Initially, a semiempirical model is developed showing the origin of the linear relationship between ultrasonic velocity and porosity fraction. Then, from a compilation of data produced by many researchers, scatter plots of velocity versus percent porosity data are shown for Al2O3, MgO, porcelain-based ceramics, PZT, SiC, Si3N4, steel, tungsten, UO2,(U0.30Pu0.70)C, and YBa2Cu3O(7-x). Linear regression analysis produces predicted slope, intercept, correlation coefficient, level of significance, and confidence interval statistics for the data. Velocity values predicted from regression analysis of fully-dense materials are in good agreement with those calculated from elastic properties.

Roth, D. J.↗
NASA NTRS1991

Rock fraction effects on the interpretation of microwave emission from soils

The effects of rock fraction on the relationship between microwave emission and surface soil moisture were investigated through a combination of laboratory dielectric measurements and field observation of emissivity. Field measurements were conducted which included soils with and without rocks. Microwave brightness temperature was measured at L and C band using a truck-mounted radiometer. Dielectric measurements were made at L band. Field observations of emissivity showed that the presence of rocks decreases the range of the measurements. At 21 cm this decrease was relatively small; however, at 6 cm almost all sensitivity to soil moisture was eliminated by the presence of 35 percent rocks. Comparisons between observed and predicted data showed that the effects of rock fraction are not significant in estimating the sample soil moisture for the tested conditions.

Jackson, Thomas J.↗
NASA NTRS1992

The transition from columnar to equiaxed dendritic growth in proeutectic, low-volume fraction copper, Pb-Cu alloys

Lead, 17.1, 11.2, and 5 volume fraction copper (14, 9, and 4 wt pct Cu) alloys have been directionally solidified at constant growth velocities ranging from 1 to 100 microns/s. Serially increasing the growth velocity within this range results in a graded microstructural transition from fully columnar, albeit segregated, copper dendrites in a lead matrix to one consisting only of equiaxed grains. The imposed velocity necessary to effect fully equiaxed growth is found to drop rapidly as the volume fraction of copper is decreased. Factors which complicate the controlled, directional solidification of these alloys are discussed and the experimental results are interpreted in view of, and seen to be in qualitative agreement with, Hunt's theory on the transition from columnar to equiaxed growth of dendrites.

Kim, Shinwoo↗
NASA NTRS1992

Development of a fractional-step method for the unsteady incompressible Navier-Stokes equations in generalized coordinate systems

A fractional step method is developed for solving the time-dependent three-dimensional incompressible Navier-Stokes equations in generalized coordinate systems. The primitive variable formulation uses the pressure, defined at the center of the computational cell, and the volume fluxes across the faces of the cells as the dependent variables, instead of the Cartesian components of the velocity. This choice is equivalent to using the contravariant velocity components in a staggered grid multiplied by the volume of the computational cell. The governing equations are discretized by finite volumes using a staggered mesh system. The solution of the continuity equation is decoupled from the momentum equations by a fractional step method which enforces mass conservation by solving a Poisson equation. This procedure, combined with the consistent approximations of the geometric quantities, is done to satisfy the discretized mass conservation equation to machine accuracy, as well as to gain the favorable convergence properties of the Poisson solver. The momentum equations are solved by an approximate factorization method, and a novel ZEBRA scheme with four-color ordering is devised for the efficient solution of the Poisson equation. Several two- and three-dimensional laminar test cases are computed and compared with other numerical and experimental results to validate the solution method. Good agreement is obtained in all cases.

Rosenfeld, Moshe↗
NASA NTRS1992

Calculations of ion-molecule deuterium fractionation reactions involving HD

Gas-phase chemical models of deuterium fractionation in dense interstellar clouds utilize a small number of exothermic reactions to achieve fractionation. Although HD is a major repository of deuterium, it appears not to exchange deuterium with many molecular ions. Useful semiquantitative reasons have been given for the unusual lack of reactivity of exothermic ion-HD deuterium exchange systems, but quantum chemical studies are needed to understand these ideas in more detail and to determine if the lack of reactivity pertains at very low temperatures not studied in the laboratory, or whether tunneling can drive the reactions. Accordingly, the potential energy surfaces of three representative ion-molecule exchange reactions involving protonated ions and HD have been investigated with ab initio quantum chemical techniques. Our results generally confirm the semiquantitative picture as to which reactions are likely to occur and show that tunneling at low temperatures is unlikely to alter this picture.

Maluendes, Sergio A.↗
NASA NTRS1993

Isotopic fractionation of hydrogen in planetary exospheres due to ionosphere-exosphere coupling - Implications for Venus

The paper considers isotopic hydrogen fractionation processes in the Venusian exosphere due to ionosphere-exosphere coupling by addressing two deficiencies in the present theory of differential escape. First, a set of D/H isotopic fractionation curves is derived for the ion-neutral interactions of charge and collisional momentum transfer, and these are compared with the results of Gurwell and Yung (1993) for hot O collisional ejection. Then, the question of the relative importance of collisional ejection in atmospheric escape is reexamined using two simple exosphere models. It is shown that O-O collisions suppress the high energy component of the hot O distribution by more than a factor of 10. Moreover, the ballistic trajectories of fast O atoms that reach the nighttime reservoir of exospheric hydrogen favor downward scatter of D and H rather than their escape. It is concluded that, due to severe limits placed on the effectiveness of collisional ejection, the differential escape of D and H from Venus is determined by charge exchange interactions rather than the collisional ejection.

Hodges, R. R., Jr.↗
NASA NTRS1950

Determination of aromatics and olefins in wide-boiling petroleum fractions

A chromatographic method is described herein for the analysis of aromatics and olefins in wide boiling petroleum fractions. The fuel is split into four fractions: nonaromatic, intermediate, pure aromatic, and wash. The analysis, which need be run only on the intermediate cut to determine aromatics in the fuel, is based on specific dispersion. With analysis times of less than 8 hours, accuracies of 1 percent were attained.

FUELS - TURBINE ENGINES, RAM JETS, AND PULSE JETS↗
NASA NTRS1994

Heat-Pulse Measurements Reveal Fiber Volume Fractions

Measurements of thermal diffusivities by heat-pulse method constitutes basis of noncontact, nondestructive method of determining fiber volume fractions (FVFs) of samples of composite (matrix/fiber) materials. Modulated radiant heat applied to sample, while resulting modulated infrared emitted is monitored. Phase shift between two modulations indicative of thermal diffusivity and fiber volume fraction of sample. Testing method takes less time, provides data to characterize sample through its thickness, and amenable to scanning for global determination of gradual spatial variations in FVF along sample.

Winfree, William P.↗