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At least 955 records · Page 53

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NASA NTRS1985

Short meridional scale anomalies in the lower stratosphere and upper troposphere

Consequences of the discovery of short-meridional scale anomalies (SMSAs) in the lower stratospheric temperature field in the analyses of Microwave Sounding Unit channel 4 data are examined. The time scales of these features, their presence in the u-wind, v-wind, and geopotential, and the heat and momentum fluxes resulting from the SMSAs are investigated. The SMSAs are not regularly propagating features. Latitudinal movement occurs in both northern and southern directions. Both eastward movement and stationarity are observed in the zonal direction. In episodes, SMSAs require a few days to grow, persist from a few days to two weeks, and then decay over a few days. Heat fluxes are small. It is suggested that the SMSAs are possibly caused by secondary effects of baroclinic disturbances.

Newman, P. A.↗

NASA NTRS1985

Red blood cell decreases of microgravity

Postflight decreases in red blood cell mass (RBCM) have regularly been recorded after exposure to microgravity. These 5-25 percent decreases do not relate to the mission duration, workload, caloric intake or to the type of spacecraft used. The decrease is accompanied by normal red cell survivals, increased ferritin levels, normal radioactive iron studies, and increases in mean red blood cell volume. Comparable decreases in red blood cell mass are not found after bed rest, a commonly used simulation of the microgravity state. Inhibited bone marrow erythropoiesis has not been proven to date, although reticulocyte numbers in the peripheral circulation are decreased about 50 percent. To date, the cause of the microgravity induced decreases in RBCM is unknown. Increased splenic trapping of circulating red blood cells seem the most logical way to explain the results obtained.

Johnson, P. C.↗

NASA NTRS1985

Review of methodology and technology available for the detection of extrasolar planetary systems

Four approaches exist for the detection of extrasolar planets. According to the only direct method, the planet is imaged at some wavelength in a manner which makes it possible to differentiate its own feeble luminosity (internal energy source plus reflected starlight) from that of the nearby host star. The three indirect methods involve the detection of a planetary mass companion on the basis of the observable effects it has on the host star. A search is conducted regarding the occurrence of regular, periodic changes in the stellar spatial motion (astrometric method) or the velocity of stellar emission line spectra (spectroscopic method) or in the apparent total stellar luminosity (photometric method). Details regarding the approaches employed for implementing the considered methods are discussed.

Tarter, J. C.↗

NASA NTRS1986

Generalization and extension of the law of acoustic energy conservation in a nonuniform flow

An exact conservation equation is derived which generalizes the familiar acoustic energy equations. The new relation is valid for arbitrary disturbances to a viscous, compressible flow. It is suggested by a development of the acoustic energy equation by means of a regular perturbation expansion of the general energy equation of fluid mechanics. A perturbation energy density and flux are defined and identified as the exact physical quantities whose leading order perturbation representations are the usual acoustic energy density and flux. The conservation equation governing the perturbation energy quantities is shown to yield previously known results for several special cases.

Myers, M. K.↗

NASA NTRS1985

El Nino and atmospheric water vapor - Observations from Nimbus 7 SMMR

Atmospheric water vapor content over the global oceans is measured to study the air-sea interactions associated with El Nino/Southern Oscillation. A scanning multichannel microwave radiometer measured brightness temperatures of the earth-atmosphere system at 6.6, 10.7, 18, 21, and 37 GHz, and from these data water vapor content is derived. The seasonal mean distribution of water vapor for the regular season cycle from 1979-1981 are examined; the minima associated with the subtropical anticyclones in the Pacific and Atlantic Oceans in the Northern and Southern hemispheres, and the Walker circulation in the tropical Pacific Ocean are analyzed. The positive and negative anomalies in water vapor during the 1982-1983 El Nino event, which correspond to convergence and enhanced convective activity, are investigated. It is concluded that during El Nino events a system of convection and subsidence exists; the subsidence slows the supply of water vapor and cloud growth in the convectively active center and the Walker circulation is reversed.

Prabhakara, C.↗

NASA NTRS1986

Experimental study of the influence of wind on Benjamin-Feir sideband instability

A laboratory investigation of the influence of wind on the evolution of mechanically generated regular (m.g.r.) waves is reported. Surface elevation measurements were made at four fetches for steep (0.1 less than ak(bar) less than 0.2, 2 Hz) m.g.r. waves, moderate (15 less than u(asterisk) less than 25 cm/s, 3-6 Hz) wind waves, and combinations of the m.g.r. and wind waves. The m.g.r. wave spectra exhibit Benjamin-Feir sidebands that grow exponentially with fetch and whose growth rate increases as the initial wave steepness increases. As fetch increases for the wind cases, total energy increases and the frequency of the spectral maximum downshifts, but no spectral lines representing Benjamin-Feir sidebands were detected even though the wave steepness and fetch were similar to the m.g.r. waves whose spectra displayed sidebands. As wind speed increased over the m.g.r. waves, sideband magnitude, sideband growth rate, and low-frequency perturbation components associated with the instability mechanism were reduced.

Bliven, L. F.↗

NASA NTRS1986

A numerical study of shock wave diffraction by a circular cylinder

The nonstationary shock wave diffraction patterns generated by a blast wave impinging on a circular cylinder are numerically simulated using a second-order hybrid upwind method for solving the two-dimensional inviscid compressible Euler equations of gasdynamics. The complete diffraction patterns, including the transition from regular to Mach reflection, trajectory of the Mach triple point and the complex shock-on-shock interaction at the wake region resulting from the Mach shocks collision behind the cylinder are reported in detail. Pressure-time history and various contour plots are also included. Comparison between the work of Bryson and Gross (1961) which included both experimental schlieren pictures and theoretical calculations using Whitham's ray-shock theory and results of the present finite difference computation indicate good agreement in every aspect except for some nonideal gas and viscous effects which are not accounted for by the Euler equations.

Yang, J.-Y.↗

NASA NTRS1985

Coherent structures in a turbulent mixing layer - A comparison between direct numerical simulations and experiments

An eduction scheme has been developed in an attempt to determine the characteristics of large-scale vortical structures in a turbulent mixing layer. This analysis scheme has been applied to a set of experimental data taken in a new, larger mixing layer facility designed to minimize boundary and resonance effects. A similar scheme has been developed to apply to the results of a direct numerical simulation of a temporally growing mixing layer. A comparison of the two approaches shows important similarities in the coherent structures. The numerical simulations indicate that low levels of coherent forcing can dramatically change the evolution of the mixing layer. In the absence of such forcing, the numerical simulations and experiments show a lack of regularity in the transverse position, spacing, amplitude, shape and spanwise coherence of the large-scale vortical structures.

Metcalfe, R. W.↗

NASA NTRS1986

Wave-front correctors by diamond turning

The production of wave-front correctors by single-point diamond turning is reported. Interferograms are shown which demonstrate excellent agreement between the diamond-turned surface and the desired surface. It is concluded from this experiment that it is now feasible by means of single-point diamond turning to make generalized wave-front control surfaces or to produce any unusual surface desired by the optical designer. The maximum departure from the nearest regular surface is set by the dynamic range and maximum diamond point acceleration permitted by the machine.

Meinel, A. B.↗

NASA NTRS1986

Modeling of dust halo formation following comet outbursts Preliminary results

Evolution of gas and dust distributions following a spatially and temporally localized comet outburst was calculated using a hybrid kinetic - hydrodynamic method. It was found that a comet outburst resulted in significantly increased dust terminal velocities. As these higher terminal velocity values result in larger apex distances, therefore the outburst generates distinct dust envelopes in front of the regular dust coma. Such envelopes were observed at several comets (cf. comet Donati).

Gombosi, T. I.↗

NASA NTRS1986

A search for the prewetting line

This paper describes efforts to locate the prewetting line in a binary liquid system (isopropanol-perfluoromethylcyclohexane) at the vapor-liquid interface. Tight upper bounds were placed on the temperature separation (0.2 K) between the prewetting line and the line of bulk liquid phase separation. The prewetting line in systems at equilibrium was not detected. Experimental signatures indicative of the prewetting line occurred only in nonequilibrium situations. Several theories predict that the adsorption of one of the components (the fluorocarbon, in this case) at the liquid-vapor interface should increase abruptly, at a temperature sightly above the temperature at which the mixture separates into two liquid phases. A regular solution calculation indicates that this prewetting line should have been easily detectable with the instruments used in this experiment. Significant features of the experiment are: (1) low-gradient thermostatting, (2) in situ stirring, (3) precision ellipsometry from the vapor-liquid interface, (4) high resolution differential index of refraction measurements using a novel cell design, and (5) computer control.

Schmidt, J. W.↗

NASA NTRS1986

Multiband far-infrared observations of the NGC 6334 complex

Multiband far-infrared (20-155 microns) observations of the large H II region/molecular cloud complex NGC 6334 are presented. The observations were performed simultaneously in four bands with effective wavelengths 21, 42, 71, and 134 microns. Eight far-infrared sources were detected which cover the range from sources with little of no continuum radio emission to well-developed H II regions. Maps of dust temperature and far-infrared optical depth of the complex are presented which reveal large-scale variations in the color temperature and optical depth. Star formation in the complex is discussed and it is concluded that a Rayleigh-Taylor instability generated by the gravitational attraction of the galactic plane can explain the regular separation of the far-IR sources.

Loughran, L.↗

NASA NTRS1986

Fast transients - A search in X-rays for short flares, bursts, and related phenomena

The HEAO 1 A-2 database on fast high energy X-ray transients was examined for any discernible regularities. The data were taken over the interval 1977-79 with six collimated multiwire, multilayer, proportional counters that had a sensitivity sufficient for detecting events lasting 1-5 sec at energies as low as 8-120 keV. The entire sky was surveyed completely three times in the observational period. Best-fit position, error box corners, mean transient flux and quiescent flux data are provided for all six of the type 3 events that were found. All the sources were within the Galaxy. The duration of the events ranged from 60-2000 sec. The limited number of events observed leads to estimates of 10,000-200,000 events per year. It is suggested that all the events originate from hard flares occurring at a rate of 20,000/yr on dMe/dKe stars.

Connors, A.↗

NASA NTRS1985

The Serendip II design

Serendip II is an automated system designed to perform a real time search for narrow band radio signals in the spectra of sources in a regularly scheduled, non-Seti, astronomical observing program. Because Serendip II is expected to run continuously without requiring dedicated observing time, it is hoped that a large portion of the sky will be surveyed at high sensitivity and low cost. Serendip II will compute the power spectrum using a 65,536 channel fast Fourier transform processor with a real time bandwidth of 128 KHz and 2 Hz per channel resolution. After searching for peaks in a 100 KHz portion of the radio telescope's IF band, Serendip II will move to the next 100 KHz portion using a programmable frequency synthesizer; when the whole IF band has been scanned, the process will start again. Unidentified peaks in the power spectra are candidates for further study and their celestial coordinates will be recorded along with the time and power, IF and RF frequency, and bandwidth of the peak.

Werthimer, D.↗

NASA NTRS1986

Tidal reorientation and the fracturing of Jupiter's moon Europa

The lineaments on Europa are discussed in terms of the orientation of the lineaments relative to the tensile stress trajectories due to tidal distortions and to nonsynchronous rotation. The cracks are noticeable by their darker albedo compared to the presumed water ice surrounding them. The stress trajectories for tidal distortion of a thin elastic shell are superimposed on Mercator projection maps of the lineaments. It is shown that the lineaments are mainly oriented at high angles to the tensile stress trajectories that would be expected for regularly occurring nonsynchronous rotation, i.e., extensional fractures would appear. The reorientation motions which would cause the fractures are estimated. It is suggested that the fractures occur episodically to release stresses built up on the tensile surface of the crust during the continuous nonsynchronous rotation of Europa.

Mcewen, A. S.↗

NASA NTRS1986

The largest molecular cloud complexes in the first galactic quadrant

The Columbia CO survey of the first Galactic quadrant was used to determine the locations and physical properties of the largest molecular complexes in the inner Galaxy. Within the range of the survey (l = 12-60 deg), 26 complexes were detected with masses greater than 5 x 10 to the 5th solar masses, and roughly several hundred such complexes are deduced to exist throughout the Galaxy within the solar circle. These complexes are the parent objects of much of the Population I in the Galaxy. Distances to most of the complexes were determined kinematically, the distance ambiguity being resolved with the aid of associated H II regions, OB associations, masers, and other early Population I objects. The largest complexes are good tracers of spiral structure, the Sagittarius arm in particular being delineated with unprecedented clarity. A total of 17 large complexes are distributed rather uniformly along a 15 kpc stretch of the arm with a spacing comparable to that of the strings of regularly spaced H Ii regions observed in external galaxies. Power-law relations exist between the line widths and sizes of the complexes and between their densities and sizes. The forms of these relations are in good agreement with those found previously and are extended by roughly an order of magnitude in cloud mass.

Dame, T. M.↗

NASA NTRS1986

Variation of the drag coefficient and its dependence on sea state

Using a Gill propeller vane anemometer and resistance wave wires over a water column depth of 15 m, simultaneous measurements of the momentum flux and sea surface wave spectra were acquired from the Pisa mast, 28 km offshore in the German Bight during autumn and winter 1979. These data were analyzed to identify the relationship between wind stress and surface waves. It was found that wind stresses for wind speeds above 15 m/s were regularly higher than open ocean wind stresses as reported by Smith (1980) and by Large and Pond (1981) for the same mean wind speed. These results, when described in terms of the drag coefficient, compared closely with the results of Sheppard et al. (1972), who collected surface layer statistics over Lough Neagh, Northern Ireland. After modeling the surface waves of the North Sea as a function of wave saturation (or wave age), it became evident that variations in the magnitude of the drag coefficient could be explained by coincident variations in the surface wave energy spectrum. By applying the wave dependent roughness length model described by Kitaigorodskii (1973), the North Sea drag coefficient was predicted to be larger than drag coefficients reported from the open sea.

Geernaert, G. L.↗

NASA NTRS1986

Theoretical electric dipole moments of SiH, GeH and SnH

Accurate theoretical dipole moments have been computed for the X2Pi ground states of Si(-)H(+) (0.118 D), Ge(+)H(-) (0.085 D), and Sn(+)H(-) (0.357 D). The trend down the periodic table is regular and follows that expected from the electronegativities of the group IV atoms. The dipole moment of 1.24 + or - 0.1 D for GeH recently derived by Brown, Evenson and Sears (1985) from the relative intensities of electric and magnetic dipole transitions in the 10-micron spectrum of the X2Pi state is seriously questioned.

Pettersson, L. G. M.↗