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At least 1,585 records · Page 88

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NASA NTRS1990

Numerical simulation of transition in a rotating disk flow

An investigation of the laminar-turbulence transition process in a three-dimensional boundary layer on a rotating disk is initiated. Preliminary results from a direct numerical simulation using spectral collocation method are presented. The simulation results in the linear regime is tested against linear stability theory. Saturation of the primary disturbance energy suggests the importance of modal interaction in initiating transition.

Balachandar, S.↗

NASA NTRS1991

Constraints on r-process nucleosynthesis in accretion disks

Systems in which accretion drives an outflow from a region near a compact object may enrich the interstellar medium in r-process elements. A detailed assessment of the efficacy of this mechanism for the r-process is presented here, taking into account the constraints imposed by typical accretion-disk conditions. It is concluded that r-process elements are unlikely to have been made in this way, largely because the total production is too low, by a factor of about 100,000, to explain the observed abundances.

Jin, Liping↗

NASA NTRS1991

The Solar Disk Sextant - Monitoring the size and shape of the sun

The Solar Disk Sextant (SDS) is a space instrument whose objective is to measure the solar diameter, at different orientations, as a function of time. Results would include the solar oblateness, the oscillation spectrum for use in helioseismology, and the rate of the secular variation of the solar diameter. The required instrumentation precision (a few milliarcsec) is attained by means of an objective beam splitting wedge which produces multiples solar images through consecutive reflections. In order to test the SDS concept a balloonborne version of the instrument has been fabricated and flown on three occasions. Preliminary results of the May 1990 flight are presented.

Sofia, Sabatino↗

NASA NTRS1991

Detection of silicates in the Beta Pictoris disk

A broad 10-micron emission feature has been detected within 3 arcsec of the star Beta Pic which is known to have a circumstellar, possibly protoplanetary, disk. The observations were made using four narrow-band filters in the MSFC bolometer array at the IRTF. The emission feature closely resembles silicate features observed in Comets Halley and Kohoutek and in a variety of Galactic sources. The feature is strong, with the total flux density emitted by dust at 10.3 microns being at least twice the estimated continuum at that wavelength, and must be emitted by particles with characteristic radii less than 10 microns.

Telesco, C. M.↗

NASA NTRS1991

Performance consequences of parity placement in disk arrays

The performance of a variety of parity placement schemes are defined and investigated to demonstrate that, at relatively large request sizes of hundreds of kilobytes, the choice of parity placement significantly affects performance (20 to 30 percent for the disk array configurations that are common today). It is shown that the left-symmetric, extended-left-symmetric and flat-left-symmetric are the best RAID level 5 parity placements. The placement with the highest read performance, flat-left-symmetric, has the lowest write performance, while the placement with the lowest read performance, left-symmetric, has the highest write performance. Suggestions for optimizing parity placements are included.

Lee, Edward K.↗

NASA NTRS1991

A powerful local shear instability in weakly magnetized disks. I - Linear analysis. II - Nonlinear evolution

A broad class of astronomical accretion disks is presently shown to be dynamically unstable to axisymmetric disturbances in the presence of a weak magnetic field, an insight with consequently broad applicability to gaseous, differentially-rotating systems. In the first part of this work, a linear analysis is presented of the instability, which is local and extremely powerful; the maximum growth rate, which is of the order of the angular rotation velocity, is independent of the strength of the magnetic field. Fluid motions associated with the instability directly generate both poloidal and toroidal field components. In the second part of this investigation, the scaling relation between the instability's wavenumber and the Alfven velocity is demonstrated, and the independence of the maximum growth rate from magnetic field strength is confirmed.

Balbus, Steven A.↗

NASA NTRS1991

Luminosity limit for alpha-viscosity accretion disks

The existence of a luminosity limit for alpha-viscosity physically thin accretion disks around black holes is established, using a new formulation of the radiation equation bridging optically thick and thin regimes. For alpha close to unity, this limit can be lower than the Eddington limit. Physically, this limit is due to the combined effects of gas and radiation pressure which become too large to satisfy vertical hydrostatic balance at intermediate optical depths for sufficiently high luminosities. This effect was overlooked in previous treatments using only the optically thin or thick limits of the radiative equation.

Liang, Edison P.↗

NASA NTRS1991

Radiative-hydrodynamical simulations of accretion disk coronae

An algorithm has been developed that will carry out detailed, two-dimensional, fully self-consistent radiative-hydrodynamical simulations of accretion disk coronae in X-ray-luminous compact sources. The calculation reported here, for an accreting neutron star radiating at 0.5 times the Eddington luminosity, reveals several striking features. (1) The corona is comprised of two main regions - an inner highly dynamic portion whose vertical structure varies cyclically on a dynamical time scale, and an outer, more stable zone in which the evaporated plasma rises to form a 'sheath' that gradually merges into a wind at large radii. (2) The coronal structure shows a density inversion that contrasts sharply with the Gaussian profiles of earlier hydrostatic models. (3) Interestingly, flow velocities as high as a few billion cm/s are not uncommon in portions of the corona. The relevance of this point to the large measured width of the emission features seen in low-mass X-ray binaries and the Galactic black hole candidates is discussed.

Melia, Fulvio↗

NASA NTRS1992

Study Of Dc Modulation Noise In Magnetic Recording Disks

Report discusses dc modulation noise in thin-film magnetic medium on magnetic recording disk. Presents study of statistical and spectral characteristics of noise and describes study of dependence of noise upon applied magnetic field, thickness of magnetic layer, and roughness of surface of layer. DC modulation noise attributed to nucleation of isolated regions of reversal of magnetization in recording medium, and to growth and coalescence of regions with increasing reverse applied magnetic field. Inhomogeneities in magnetic recording media, not roughnesses of surfaces, are dominant sources of noise.

Katti, Romney R.↗

NASA NTRS1991

Manifestations of dynamo driven large-scale magnetic field in accretion disks of compact objects

A turbulent dynamo nonlinear theory of turbulence was developed that shows that in the compact objects of accretion disks, the generated large-scale magnetic field (when the generation takes place) has a practically toroidal configuration. Its energy density can be much higher than turbulent pulsations energy density, and it becomes comparable with the thermal energy density of the medium. On this basis, the manifestations to which the large-scale magnetic field can lead at the accretion onto black holes and gravimagnetic rotators, respectively, are presented.

Chagelishvili, G. D.↗

NASA NTRS1992

Results of examination of the A276 white and Z306 black thermal control paint disks flown on LDEF

Specimens of A276 white and Z306 black thermal control paints were analyzed for the effects of space environmental exposure as part of the Long Duration Exposure Facility (LDEF) Materials Special Investigation Group activity. The specimens, actually disks or spots of paint on tray clamps, were located at regular intervals on all LDEF longerons and intercostals. The principle conclusions from the analysis are: UV exposure degraded the surface resin of A276 paint, with coating solar absorptance increasing with UV exposure; contamination, though detected, was not enough to have adversely affected optical properties; atomic oxygen eroded resin on specimens with incidence angles of up to 100 deg; the erosion of Z306 paint on leading edge specimens removed a minimum of 10 microns of that coating; and the erosion of A276 paint at up to 80 deg incidence angle resulted in near original condition solar absorptance readings.

Johnny L Golden↗

NASA NTRS1991

Analysis of a slot loaded microstrip disk antenna for circular polarisation

A method is presented to analyze a circular microstrip disk antenna loaded with a narrow, inclined slot at its center and fed by a coaxial probe. Making use of the exact dyadic Green's functions for a grounded dielectric slab due to electric and magnetic currents, Richmond's reaction integral equation in conjunction with the method of moments (Galerkin), a generalized h-parameter equivalent network for the loaded patch is obtained. An expression for the input impedance of a slot loaded microstrip patch is obtained from the equivalent parameters. The theoretical results are compared with the experiment.

Rao, Y. R.↗

NASA NTRS1992

Preliminary results of a balloon flight of the solar disk sextant

Preliminary results of a balloon flight on October 11, 1991, of the solar disk sextant (SDS) experiment are reported. The SDS is an instrument which measures the solar diameter at different orientations with respect to the solar polar axis. Fitting straight lines through two fixed-angle data sets with time as the independent variable yields slopes of (7.1 +/ - 1.5) x 10 exp -3 and (6.7 +/- 1.6) x 10 exp -3/mas s, consistent with the value of 6.47 x 10 exp -3/mas s expected from the earth's approach to the sun due to the orbital motion toward perihelion. Upon the instrument's rotation on its axis a sinusoidal component of the diameter measurement was observed in each rotation cycle, with a variable amplitude of about 150 mas. The present result is epsilon of (5.6 +/- 6.3) x 10 exp -6, about 30 deg offset from the polar-equator position. The absolute diameter obtained by means of the FFT definition is found to be 1919.269 +/- 0.240 arcsec or 1919.131 +/- 0.240 arcsec, depending on the orientation mode of the measurement.

Maier, E.↗

NASA NTRS1992

On the Galactic disk and halo models of gamma-ray bursts

Statistical properties of the distribution of gamma-ray bursts are calculated for a large range of different luminosity functions assumed for the sources. Surprisingly, the observable parameters are almost the same as in the 'standard-candle' scenario. In particular, no way is found to reconcile the observed distribution of gamma-ray bursts with any Galactic disk model. A Galactic halo distribution must have a core radius greater than 18 kpc to have no measurable dipole anisotropy. A cosmological distribution of the bursters is a more reasonable alternative.

Mao, Shude↗

NASA NTRS1992

Propagation of cosmic-ray nuclei in a diffusing galaxy with convective halo and thin matter disk

A diffusion model for cosmic-ray propagation in the galaxy that includes the effects of convection in the halo is presented. Calculations are made for 13 primary and secondary nuclei with rigidities between 1 and 1000 GV using interaction loss rates, secondary production rates, and radioactive decay on the basis of recent new cross-section measurements. It is found that, in order to fit the rather weak radial dependence of cosmic-ray protons derived from gamma-ray data, the radial profile of the cosmic-ray sources must also have a weak radial dependence. It is suggested that convection perpendicular to the disk of the Milky Way Galaxy may not be important even at rigidities less than a few GV. The obtained limits on halo thicknesses are consistent with what can be determined for the distribution of cosmic-ray electrons in the halo based on the distribution of radio synchrotron emission in this and other galaxies.

Webber, W. R.↗

NASA NTRS1992

The radial distribution of oxygen in disk galaxies

The shape of the oxygen abundance profile is examined for seven nearby spiral galaxies in which at least 15 H II regions have been observed. The radial abundance profile for three of these galaxies has a distinct change in slope. The others show no deviation from a single scale-length exponential. This behavior is explained within the framework of the star-forming viscous disk model of galaxy evolution. A prediction is made that radial abundance profiles of noninteracting unbarred spiral galaxies change slope at the radius where the rotation curve changes from linearly rising to flat. For barred and interacting galaxies the situation is complicated by additional mixing mechanisms of the interstellar medium.

Zaritsky, Dennis↗

NASA NTRS1992

Disk instability and the time-dependent X-ray emission from the intermediate polar GK Persei

The correlation between the disk instability model for the 1981-1989 optical outbursts of the intermediate polar GK Per and the accompanying X-ray emission is examined, and the self-consistency of the combined optical-X-ray model is investigated. Special attention is given to the nature of the transition in the X-ray emission due to the time-dependent accretion rates in the simple column accretion model. The large variation in the efficiency of hard X-ray production is explained.

Yi, Insu↗

NASA NTRS1992

Radiative-hydrodynamical simulations of X-ray burst-induced accretion disk coronae

A fully self-consistent radiative hydrodynamical algorithm developed earlier is used to study the 'steady' state coronae in order to simulate the response of the evaporated outflow to a time-dependent irradiation of the disk's surface. It is found that the burst definition is altered at both low and high inclination angles i. At i greater than or approximately equal to 45 deg, the detectable flux is enhanced (as much as 50 percent) by X-rays scattered into the line of sight from the radiation propagation through the corona, and (greater than or approximately equal to 30 percent) by X-rays reflected off the corona when i is less than or approximately equal to 45 deg.

Mella, Fulvio↗