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At least 1,135 records · Page 63

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NASA NTRS1982

The gravitational field of a disk

The gravitational potential of a disk is given, as is the gravitational field at a point in space. It is pointed out that formulas for a ring can be obtained as the difference between the results presented here for two different values of the disk radius. Results are obtained in terms of elliptic integrals and it is shown how these functions can be computed efficiently. Formulas necessary for the computation of partial derivatives are also presented.

Krough, F. T.↗

NASA NTRS1982

Ultraviolet spectrophotometry of 2A 1822-371 - A bulge on the accretion disk

It is suggested that the 5.57-hour modulation of the X-ray source 2A 1822-371 is caused by the combined effects of (1) an occultation of the emitting region by a companion star, and (2) a bulge on the accretion disk surrounding the X-ray source. It is speculated that the changing aspect of the X-ray-heated inner face of the bulge with orbital phase may also contribute to the modulation at UV and optical wavelengths. The bulge's position angle suggests it to have been the result of turbulence caused by the impact of a gas stream transferring matter from the companion, and 2A 1822-371 is held to provide the most direct indication extant of such a structure in an accretion disk. It is speculated that comparison of high-time resolution UV observations with optical and X-ray light curves will allow further deductions as to the size, structure and location of the far-UV emitting region.

Mason, K. O.↗

NASA NTRS1982

RW Sextantis, a disk with a hot, high-velocity wind

The continuum spectrum of the flickering blue variable RW Sex was observed from 10,000 to 1150 A. The star is a cataclysmic variable currently stabilized at maximum, and the spectrum is dominated by an accretion disk, with flat spectrum in the ultraviolet, except at more than 5000 A, where a blackbody near 7000 K is seen. A distance of 400 pc is derived, if the latter arises from an F type main sequence star. The accretion rate required is near 10 to the -8th solar masses per year. Only weak emission is seen, except for Lyman alpha; strong, broad UV absorption lines are seen with centers displaced up to -3000 km/s, with terminal velocities up to -4500 km/s, the velocity of escape from a white dwarf. The low X-ray flux may arise from absorption within an unusually dense, hot wind from the innermost portions of the disk. The estimated mass loss rate is nearly 10 to the -12th solar masses per year.

Greenstein, J. L.↗

NASA NTRS1982

Physical conditions in the accretion disk of V603 Aquilae

Ultraviolet and optical spectra of the old nova V603 Aql are discussed. The UV-optical continuum is dominated by emission from the accretion disk. Emission lines from ions of H, He, C, N, and O are identified. These lines are probably formed in a circumstellar shell with radius comparable to the binary separation, density ten billion per cu cm, and a roughly solar chemical composition. This corona is probably heated by radiation emitted by the underlying accretion disk. Photoionization calculations of the structure and emission-line spectrum of the corona are presented, and the effects of this gas on the X-ray continuum are discussed.

Ferland, G. J.↗

NASA NTRS1982

Dynamics and mechanism of cavitation erosion on perspex and epoxy resins tested in a rotating disk device

The cavitation erosion behavior including the initiation, dynamics and mechanism of damage process on perspex and epoxy resin specimens tested in a rotating disk device were discussed with respect to exposure time. The inception of erosion always took place at the location nearest to the center of rotation of the disk. Subsequently, as exposure time increased, erosion initiated at other locations as well. Light optical photographs and scanning electron micrographs clearly indicate that most of the material loss appears to occur form the networks of cracks due to their interaction and pits indicate particle debris. The optical degradation (loss of transmittance) on perspex was observed to be more on the rear side than on the front side.

Rao, P. V.↗

NASA NTRS1982

The investigation of flow instabilities on a rotating disk with curvature in the radial direction

The major objective is to explore any visible differences of the flow field with wall curvature of the test body, including possible interaction between Taylor-Gortler instabilities present along concave walls and the inflexional instabilities investigated here. An experimental study was conducted with emphasis placed on making visual observations and recording photographically the flow instabilities present under three different rotating bodies: a flat disk, a concave paraboloid, and a convex paraboloid. The data collected for the three test bodies lead to the conclusion that the wall curvature of the concave and convex paraboloids did not alter the observed flow field significantly from that observed on the flat disk.

Intemann, P. A.↗

NASA NTRS1982

Numerical simulation of a conducting disk partially covered by an insulator interacting with a plasma

A numerical simulation has been made of the 'pinhole effect' which produces the characteristic 'S-shaped' current-voltage curve. A disk-shaped conducting probe immersed in a plasma is modeled using a particle-in-cell (PIC) code. A probe partially covered by a very thin insulating layer is considered, as well as a probe mounted on an insulating disk. The simulation uses a cylindrical particle mover and allows for a variable number of particles in the system. The simulation space grid uses three different mesh sizes, the coarsest being away from the probe and the finest near the probe, in order to accurately calculate the trajectories of the simulation particles contributing the current to the probe and to the surface charge density on the dielectric. The calculation of the electrostatic potential is done self-consistently using successive over-relaxation (SOR). Backscattering and secondary electron emission are included for the case of positive probe voltage.

Chaky, R. C.↗

NASA NTRS1982

Performance characteristics of a microwave plasma disk ion source

The development, design, and preliminary performance characteristics of a microwave plasma disk ion source are presented. Several important design concepts indigenous to microwave plasmas have been utilized in the development of this ion source: generation of a resonantly sustained microwave discharge inside a microwave/plasma coupler, probe and length tuning of the microwave/plasma coupler, and minimization of plasma volume. The experimental results for a 2.45 GHz, eight cm ion source excited in the TE(211) and TM(011) modes are described. Results are presented for electromagnetic mode excitation, electron density, coupling efficiency and loaded cavity Q, and I-V characteristics and efficiencies. It is preliminarily shown that a microwave disk-like plasma can be sustained adjacent to the grids with either TE or TM cavity modes at pressures below 2 x 10 to the -4th torr. Over 80% of the power absorbed in the cavity is coupled into the plasma.

Asmussen, J.↗

NASA NTRS1983

The ultraviolet excess of luminous quasars. II - Evidence for massive accretion disks

This paper presents new infrared, optical, and ultraviolet spectra of three high-redshift quasars. They are analyzed along with three published quasar spectra. After accounting for other sources of radiation, the continua are fitted with spectra predicted for optically thick steady-state accretion disks which include the effects of general relativity. The two fitting parameters, the mass of the accreting black hole and the accretion rate, are determined by the data with a formal accuracy of 20 percent. The masses range from 0.2-0.5 billion solar mass for 3C 273 and PKS 0405-123 to 1-3 billion solar masses for the high-redshift quasars. The values depend on the assumed disk inclination and the angular momentum of the hole. The high luminosities of these six quasars are all within a factor of 2 of their Eddington limits, and this conclusion is not extremely sensitive to the modeling assumptions.

Malkan, M. A.↗

NASA NTRS1983

Perturbed particle disks

The velocity ellipsoid in a particle disk near an isolated satellite resonance is determined by solving the Boltzmann moment equations, and solutions are obtained that are stationary functions of the azimuthal angle in a coordinate frame which rotates with the pattern speed of the perturbation potential. The magnitude of the deformation rate tensor in a perturbed particle disk is bounded from above by an expression which includes the orbital angular velocity, the optical depth, and a dimensionless constant of order unity. It is also found that, in sufficiently perturbed regions, there are ranges of azimuthal angle over which the radial component of the angular momentum flux is negative. It is also possible for the angular momentum luminosity to be negative. These results are pertinent to the understanding of sharp edges and density wave decay in planetary rings.

Borderies, N.↗

NASA NTRS1983

Cavitation erosion characteristics of poly(methyl methacrylate) in a rotating disk device

Veerabhadra Rao, P.↗

NASA NTRS1984

MPP disk subsystem

A disk subsystem for the Massively Parallel processor (MPP) is designed to the block diagram level. The subsystem is capable of storing 4,992 megabytes of data, expandable to 39,936 megabytes. The subsystem is capable of transferring data to the MPP Staging Memory at a rate of 25 megabytes/second, expandable to 100 megabytes/second. A lower cost disk subsystem is also presented. This alternate subsystem is capable of storing 3,744 megabytes with a transfer rate of 10.6 megabyte/second.

Hudgins, W. A.↗

NASA NTRS1984

An infrared disk at the center of the bipolar planetary nebula NGC 6302

Broad-band infrared photometry and high-resolution infrared maps are presented for NGC 6302, a bipolar planetary nebula in which material is being driven outward by a powerful stellar wind. The infrared morphology is very unlike that seen optically. Maps at 1.25, 2.2, 3.4, and 10 microns show evidence for a disk of material with its short axis oriented parallel to the supersonic flow and located between the optical lobes. This structure is not optically evident partly because of the large extinction along the line of sight to the nebular core. It is suggested that the observed disk is either an expanded fossil toroid that once confined the flow, or else is stellar envelope material that was not in the path of the bipolar flow and that remains in the vicinity of the stellar remnant.

Lester, D. F.↗

NASA NTRS1984

Voyager disk resolved photometry of the Saturnian satellites

The scattering properties of the icy satellites of Saturn are studied. Scans of intensity along the photometric equator near opposition are analyzed, and disk-resolved measurements of intensity are fitted to simple photometric functions. Histograms of the albedo distribution across the satellite disks are constructed, and the geometric albedos are determined as a function of wavelength. It is found that, at low phase angles, Rhea and Dione closely follow lunar behavior with almost no limb darkening. Mimas, Tethys, and especially Enceladus show significant limb darkening at low phase angles, which suggest multiple scattering is important for their surfaces. No satellite's surface can be described by Lambert's law. The darker satellites are redder, and the darker areas of each satellite are redder. A longitudinal dark stripe which represents a 15 percent decrease in albedo is situated near the center of the trailing side of Tethys. Mimas and Enceladus both have remarkably flat spectral reflectances into the ultraviolet.

Buratti, B. J.↗

NASA NTRS1984

A possible relationship between metal abundance and luminosity for disk galaxies

Near-infrared colors have been measured for a sample of 31 late-type galaxies in the Pegasus I and Pisces clusters; system luminosities in the sample cover the range of M(H) between -19 and -23.5. The color index (J - K) correlates strongly with the absolute H magnitude; lower-luminosity systems have bluer colors. These observations are consistent with the assumption that the mean metal abundance of the old disk population decreases systematically with luminosity. The systematic variation (B - H) with absolute H magnitude reported recently by Tully et al. (1982) derives in part from this proposed systematic change of metallicity with luminosity. However, a relative increase in the number of newly formed stars and/or a systematic smaller age for lower-luminosity disks must still be posited in order to fully explain the observed (B - H), H relation.

Bothun, G. D.↗

NASA NTRS1982

Hot isostatically pressed manufacture of high strength MERL 76 disk and seal shapes

The performance of a HIP MERL 76 disk installed in an experimental engine and exposed to realistic operating conditions in a 150 hour, 1500 cycle endurance test is examined. Post test analysis, based on visual, fluorescence penetrant and dimensional inspection, indicates that the disk performs satisfactorily.

Evans, D. J.↗

NASA NTRS1985

Solar activity beyond the disk and variations of the cosmic ray gradient

Part of galactic cosmic rays (CR) observed near the Earth and on the Earth come from beyond-disk regions of circumsolar space. But CR of those energies which undergo substantial modulation cover too large a path across the lines of force of the interplanetary magnetic field (IMF) in order that they could provide an effective transfer of information about beyond-disk solar activity. And if it is still possible, the most probable channel for transferring such information must be a neutral layer of heliomagnetosphere in which the transverse CR transport is facilitated by their drift in an inhomogeneous magnetic field. A simple diffusion model for an expected CR variation in a neutral layer near the Earth is discussed. It is of importance that variations of the CR gradient are not at all always accompanied by considerable variations of IMF and solar wind velocity at the point of observation.

Belov, A. V.↗

NASA NTRS1985

The wave pattern produced by a point source on a rotating disk

It is pointed out that the boundary layer on a rotating disk is important in stability theory because it provides a particularly simple way to study the important phenomenon of crossflow instability. This type of instability is responsible for early transition on sweptback wings. Mack and Kendall (1983) have studied the wave patterns formed by harmonic point sources in a Blasius boundary layer on the basis that the source uniformly excites all oblique normal modes of the source frequency. The calculation procedure for planar boundary layers was modified to fit the different geometry of the rotating disk and the lack of an axis of symmetry. Calculations were performed of the wave pattern produced by a zero-frequency point source located at the Reynolds number of the artificial roughness element in an experiment conducted by Wilkinson and Malik (1983). The results provided in the present investigation confirm that the experimental wave pattern is a superposition of the complete azimuthal wavenumber spectrum of zero-frequency normal modes with uniform initial amplitude and phase.

Mack, L. M.↗