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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

The reality of the secular change in solar wind nitrogen isotopes

Evidence concerning the isotopic composition of N in the early solar system currently seems to favor a picture of isotopic inhomogeneity rather than of a unique primordial composition modified by local processes. Certainly the range of N-15/N-14 ratios found in meteorites points to the existence of more than one nucleogenetic N component, though mass dependent modification of them explain the isotopic variation observed for N in the lunar regolith. The observational evidence is addressed which can be used to discriminate between such a model and one invoking a secular change in the composition of the solar wind. Three tests of this model were considered, starting with a search for light planetary N surviving in lunar rocks. Results so far are negative. The results of these tests may not eliminate the two component model for regolith N but they seriously weaken it. The alternate view, involving a secular change in solar wind N composition, has its problems but continues to survive by default.

Kerridge, J. F.↗
NASA NTRS1986

Applications of a Venus thermospheric circulation model

A variety of Pioneer Venus observations suggest a global scale, day-to-night Venus thermospheric circulation. Model studies of the dynamics and energetics of the Venus thermosphere are presented in order to address new driving, mixing and cooling mechanisms for an improved model simulation. The adopted approach was to reexamine the circulation by first using a previous two dimensional code to quantify those physical processes which can be inferred from the Pioneer Venus observations. Specifically, the model was used to perform sensitivity studies to determine the degree to which eddy cooling, eddy or wave drag, eddy diffusion and 15 micrometer radiational cooling are necessary to bring the model temperature and composition fields into agreement with observations. Three EUV heating cases were isolated for study. Global temperature and composition fields in good agreement with Pioneer data were obtained. Large scale horizontal winds 220 m/s were found to be consistent with the observed cold nightside temperatures and dayside bulges of O, CO and CO2. Observed dayside temperatures were obtained by using a 7 to 19% EUV heating efficiency profile. The enhanced 15 micrometer cooling needed for thermal balance is obtained using the best rate coefficient available for atomic O collisional excitation of CO2(0,1,0). Eddy conduction was not found to be a viable cooling mechanism due to the weakened global circulation. The strong 15 micrometer damping and low EUV efficiency imply a very weak dependence of the general circulation to solar cycle variability. The NCAR terrestrial thermospheric general circulation model was adapted for Venus inputs using the above two dimensional model parameters, to give a three dimensional benchmark for future Venus modelling work.

Bougher, S. W.↗
NASA NTRS1985

Numerical simulation of transonic flow over porous airfoils

A numerical study was made to examine the effect of a porous surface on the aerodynamic performance of a transonic airfoil. The pressure jump across the normal shock wave on the upper surface of the airfoil was reduced by making the surface below the shock porous. The weakened shock is preceded by an oblique shock at the upstream end of the porous surface where air is blown out of the cavity. The lambda shock structure shown in the numerical result qualitatively agrees with that observed in the wind tunnel. According to the present analysis, the porous airfoil has a smaller drag and a higher lift than the solid airfoil.

Chow, C.-Y.↗
NASA NTRS1985

The orbit of Lageos and solar eclipses

An eclipse of the sun by the moon as seen by the Lageos satellite can affect the orbital semimajor axis at the centimeter level. The weakened radiation pressure acting on Lageos perturbs the orbit differently from that due to full sunlight. This difference amounted to less than 2 mm in the semimajor axis for 23 of the 30 eclipses Lageos experienced between launch in 1976 and the end of 1983. However, it was 17.6 mm for the eclipses on 28 March 1979 and 11.2 mm for the one on 15 December 1982. Differences such as these generate large enough along-track errors to make it worthwhile to include eclipses in complex orbit determination programs such as GEODYN which integrate the orbit. Eclipses cannot explain the presently unmolded variations in along-track acceleration which have a magnitude of about 3 x 10(-12) ms(-2).

Rubincam, D. P.↗
NASA NTRS1986

Search for ultraviolet emission lines from a hot gaseous halo in the edge-on galaxy NGC 4244

Short and long wavelength IUE spectra of the halo region in the edge-on galaxy NGC 4244 are analyzed in order to identify evidence of line emission at the level of 0.000001 ergs per cu cm sr/s. Features are found at 1245 A and 1402 A, having peaks four times greater than the rms intensity fluctuations of nearby spectra. The spectral features are identified with semi-forbidden N V, semi-forbidden S IV at 1240 A, and Si IV and semi-forbidden O IV multiplets at 1400 A, respectively. The appearance of high-peak features and the lack of astrophysically important lines in the sample are evidence of a gas near T = 10 exp 5.2 and emission measure (EM) equal to about 0.000001 pc. However, the case for the existence of such a gas is weakened due to the existence of two other similarly sized features with no identifiable astrophysical origin and the extremely faint nature of the candidate features. The assumed upper limit for the line intensities in NGC 4244 leads to the conclusion that at T less than 100,000 K any emitting gas is either highly clumped or has a p/k value of less than 1000 per cu cm K. It is suggested that if the observed low level features in the short wavelength spectrum are real, the temperature and emission measures allow for a single component gas in the halo of NGC 4244, and are in agreement with those derived by Paresce et al. (1983).

Deharveng, J.-M.↗
NASA NTRS1986

Catching the whispers from Uranus

Sophisticated telecommunications techniques are described that were used to acquire images of Uranus, its 14 moons and ten narrow rings darker than coal. The images, equal in quality to those transmitted from Saturn several years earlier despite the signal being weaker by 6 dB due to the increased distance, were received from Voyager 2 during its January 24, 1986 flyby of Uranus. Solutions to the problem of the weakening signal were found in modifications to Voyager's image processing system and NASA's ground tracking network. In April 1985, Voyager's prime flight data computer was reconfigured to accept only nonimaging science data, and its backup, only imaging data; the latter was reprogrammed to determine only arithmetic differences between adjacent pixel intensities rather than absolute intensities. By image compression, equivalent imaging information could be sent at lower bit rates. Instead of Golay coding, Reed-Solomon onboard encoding was used. These techniques gained the equivalent of 4-dB in imaging yield. Additional improvements were gained by using earth station antennas in pairs (the Parkes radio telescope and the Canberra ground station antenna). Moves under way to prepare for the Voyager encounter with Neptune in 1989 are described (using additional antennas and arrays, scaling up the Deep Space Network antennas from 64 m to 70 m, etc.) to assure almost Saturn-equivalent pictures despite a further 3.5-dB drop in signal strength.

Bartok, C. D.↗
NASA NTRS1986

Mechanics of shear rupture applied to earthquake zones

The mechanics of shear slippage and rupture in rock masses are reviewed. The essential ideas in fracture mechanics are summarized emphasizing the interpretation and relation among the fracture parameters in shear cracks. The slip-weakening model is described. The general formulation of the problem of nonuniform slip distribution in a continuum is covered.

Li, Victor C.↗
NASA NTRS1986

The discrete one-sided Lipschitz condition for convex scalar conservation laws

Physical solutions to convex scalar conservation laws satisfy a one-sided Lipschitz condition (OSLC) that enforces both the entropy condition and their variation boundedness. Consistency with this condition is therefore desirable for a numerical scheme and was proved for both the Godunov and the Lax-Friedrichs scheme--also, in a weakened version, for the Roe scheme, all of them being only first order accurate. A new, fully second order scheme is introduced here, which is consistent with the OSLC. The modified equation is considered and shows interesting features. Another second order scheme is then considered and numerical results are discussed.

Brenier, Yann↗
NASA NTRS1986

Long-time creep behavior of Nb-1Zr alloy containing carbon

Creep tests were conducted on the Nb-1Zr base alloy with and without carbon. Testing was performed at 10 to the -6 MPa in the 1350 to 1400 K range. Creep times, to 1 percent strain, ranged from 60 to 6000 hr. All 1 percent creep data were filled by linear regression to a temperature compensating rate equation. The Nb-1Zr-0.06C alloy, tested in a weakened aged condition, appears to be four times as strong as the Nb-1Zr alloy.

Titran, R. H.↗
NASA NTRS1986

Spectra of plages on the sun and stars. I - Ca II H and K lines. II - The H I H-alpha line

Spectra of solar plages are used to define indices of plage properties in the Ca II H and K line region. Chromospheric and photospheric emission is examined and the details of height structure are noted. It is found that: (1) all spectral lines are weakened in plages except those of H I, He I, and some ionized metals; (2) the irradiation contribution from a plage is roughly constant throughout its disk passage; and (3) plage emission distorts spectral and photometric measures of stellar gravity, metallicity, and temperature.

Labonte, B. J.↗
NASA NTRS1986

Further evidence for lightning on Venus

Impulsive electrical signals at all frequencies recorded by the plasma wave instrument on Pioneer Venus are found to cluster around periapsis. The signals are strongest at 100 Hz and have a secondary peak at 5.4 kHz. These characteristics are consistent with generation by lightning in the Venus atmosphere and the direct propagation of whistler mode signals below the electron gyro frequency and the partial transmission of electromagnetic waves in the presence of electron density inhomogeneities above the electron gyro frequency. The scattering by electron density inhomogeneities may also weaken any correlation with surface topographical features. Thus, the source of these signals may either be intercloud lightning at high altitudes or lightning in volcanic plumes near the ground, despite the large scatter in inferred source locations.

Singh, R. N.↗
NASA NTRS1986

Interannual variability of global dust storms on Mars

Under the assumption that the cross-equatorial Hadley circulation plays a key role in the onset of Martian global dust storms, numerical simulations indicate that a northern hemisphere dust haze weakens its intensity and, thereby, its contribution to the surface stress in the southern hemisphere. Since this in turn reduces the possibility of global dust storm development, the interannual variability observed is the result of either a competition between circulations in opposite hemispheres, in which case the variability has a random component, or it is the result of dust cycling between hemispheres, so that the variability is related to the characteristics of the global dust storms themselves.

Haberle, Robert M.↗
NASA NTRS1987

Trajectory options for the Comet Rendezvous Asteroid Flyby mission

The Comet Rendezvous Asteroid Flyby mission is strongly supported by the scientific community. Two major events during the past year have forced a re-analysis of the mission options, with the goal of selecting a baseline mission for launch in 1993. Venus and earth gravity assists can be used to allow rendezvous with comet Tempel 2 in 1996; the scientific potential of the resulting mission is excellent. Although backup mission opportunities have been identified, the significantly longer flight times weaken the scientific appeal of these missions.

Miller, S. L.↗
NASA NTRS1987

Development of a two-dimensional zonally averaged statistical-dynamical model. I - The parameterization of moist convection and its role in the general circulation

The moist convection parameterization used in the GISS 3-D GCM is adapted for use in a two-dimensional (2-D) zonally averaged statistical-dynamical model. Experiments with different versions of the parameterization show that its impact on the general circulation in the 2-D model does not parallel its impact in the 3-D model unless the effect of zonal variations is parameterized in the moist convection calculations. A parameterization of the variations in moist static energy is introduced in which the temperature variations are calculated from baroclinic stability theory, and the relative humidity is assumed to be constant. Inclusion of the zonal variations of moist static energy in the 2-D moist convection parameterization allows just a fraction of a latitude circle to be unstable and enhances the amount of deep convection. This leads to a 2-D simulation of the general circulation very similar to that in the 3-D model. The experiments show that the general circulation is sensitive to the parameterized amount of deep convection in the subsident branch of the Hadley cell. The more there is, the weaker are the Hadley cell circulations and the westerly jets. The experiments also confirm the effects of momentum mixing associated with moist convection found by earlier investigators and, in addition, show that the momentum mixing weakens the Ferrel cell. An experiment in which the moist convection was removed while the hydrological cycle was retained and the eddy forcing was held fixed shows that moist convection by itself stabilizes the tropics, reduces the Hadley circulation, and reduces the maximum speeds in the westerly jets.

Yao, Mao-Sung↗
NASA NTRS1987

A new computational model for the prediction of mass loading phenomena for solar wind interactions with cometary and planetary ionospheres

The modified gasdynamic convected-magnetic-field MHD model developed by Spreiter and Stahara (1980) to simulate the supersonic flow of the solar wind past planetary magnetoionospheres is extended to account for cases (such as Venus and comets) in which significant numbers of neutral atmospheric atoms become ionized in the surrounding flow and add to its momentum, energy, and mass. The mathematical model and the solution procedures for the nose and tail regions are explained; typical computational grids are shown; and numerical results for a comet and for the Venus ionosheath and bow shock are presented graphically. It is found that the bow shock weakens and moves further upstream of the obstacle as mass loading is increased and the flow upstream of the bow shock becomes more compressed.

Stahara, Stephen S.↗
NASA NTRS1987

Solar cycle modulation of Galactic protons and electrons

It is demonstrated here, through a detailed application of the model of Perko and Fisk (1983), that the cyclic variation in the cosmic ray spectra, the gradient, and the hysteresis, all in the ecliptic plane, can be caused by the accumulated effect of strong scattering regions moving out from the sun. The proton data in the form of spectral ratios, considered to be signatures of drifts, can be simply explained in terms of hysteresis effects, although a charge-dependent effect cannot be ruled out. It is also shown that cosmic ray recovery in this model can occur immediately after a sudden intensity decrease without requiring the disturbance to weaken as it propagates away; however, it is an energy-dependent recovery, a result of diffusive propagation inward, which partially contradicts data gathered at 1 AU. The cosmic ray intensity profiles at 1 AU may result from mechanisms different from those that exist beyond several AU.

Perko, John S.↗
NASA NTRS1988

Self-Protecting Heat Exchanger

Double-wall heat-exchanger tube protects from overloads by changing thermal resistance automatically. When temperature at any location on tube increases above prescribed limit, thermal resistance through walls of tube increases at location. Prevents local excess heat load from overheating tube or boosting internal pressure and weakening tube, shortening life expectancy, or destroying it. When heat load falls to normal level, tube automatically lowers thermal resistance and resumes heat exhange at rated capacity.

Wagner, William R.↗
NASA NTRS1987

Numerical and analytical study of fluid dynamic forces in seals and bearings

A numerical model based on a transformed, conservative form of the three dimensional Navier-Stokes equation and an analytical model based on lumped fluid parameters are presented and compared with studies of modeled rotor bearing seal systems. The rotor destabilizing factors are related to the rotative character of the flow field. It is shown that these destabilizing factors can be reduced through a decrease in the fluid average circumferential velocity. However, the rotative character of the flow field is a complex three dimensional system with bifurcated secondary flow patterns that significantly alter the fluid circumferential velocity. By transforming the Navier-Stokes equations to those for a rotating observer and using the numerical code PHOENICS-84 with a nonorthogonal body fitted grid, several numerical experiments were carried out to demonstrate the character of this complex flow field. In general, fluid injection and/or preswirl of the flow field opposing the shaft rotation significantly intensified these secondary recirculation zones and thus reduced the average circumferential velocity; injection or preswirl in the direction of rotation significantly weakened these zones.

Tam, L. T.↗