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At least 883 records · Page 49

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NASA NTRS1986

Spinning disk calibration method and apparatus for laser Doppler velocimeter

A method and apparatus for calibrating laser Doppler velocimeters having one or more intersecting beam pairs are described. These velocimeters measure fluid velocity by observing the light scattered by particles in the fluid stream. Moving fluid particulates are simulated by fine taut wires that are radially mounted on a disk that is rotated at a known velocity. The laser beam intersection locus is first aimed at the very center of the disk and then the disk is translated so that the locus is swept by the rotating wires. The radial distance traversed is precisely measured so that the velocity of the wires (pseudo particles) may be calculated.

Snyder, P. K.↗

NASA NTRS1986

Dwarf nova burst asymmetry and the physics of accretion disks

Far-ultraviolet observations of a burst of SS Cyg obtained with the Voyager spacecraft are presented along with optical observations. Representative V band and far-UV light curves are computed based on a thermal instability model, and the dependence of the far-UV delay and the shape of the outburst on the location in the disk where the outburst starts is examined. The results show that burst initiated at large radii produce asymmetric light curves for which the far-UV flux lags the optical flux by about half a day, as observed. For bursts starting at smaller radii, the delay is smaller and the outburst is more rounded and symmetric. For both burst types, the delay with respect to the optical is longer for larger accretion disks and at shorter wavelengths. The physical processes that will promote burst initiation in the outer disk are discussed, emphasizing that this can occur in principle with no modulation of the mass transfer rate from the companion.

Cannizzo, J. K.↗

NASA NTRS1986

Accretion disks and periodic outbursts of active galactic nuclei

The local thermal stability of accretion disks around supermassive black holes in active galactic nuclei is examined. Such disks are unstable at radii where the surface temperature is several thousand degrees. Supermassive disks therefore should undergo limit-cycle outbursts similar to those believed to occur in dwarf novae. Operating on a time scale of about 10,000 to 10 million yr and at radii of about 10 to the 15th to 10 to the 16th cm, this mechanism will result in alternating periods of higher and lower accretion rate onto the black hole and, consequently, higher and lower luminosity. Quasi-periodic outbursts on this time scale may be recorded in the structure of extended radio sources, a possible example being 4C 29.47. For accretion rates greater than 0.1 solar masses/yr, the situation is complicated by instabilities caused by self-gravitation and by the dominance of radiation pressure and electron scattering opacity.

Lin, D. N. C.↗

NASA NTRS1986

Compton-heated winds and coronae above accretion disks. II - Instability and oscillations

The stability and evolution of windy accretion disks is investigated in detail. The basic disk evolution equations are briefly recapitulated, and an idealized analytic treatment of the wind and viscosity is used to show that steady disk flow is indeed unstable for sufficiently large ratio of the mass loss rate in the wind to the central accretion rate. Numerical solutions for a more realistic and appropriate expression for the mass loss rate and the standard ad hoc alpha model prescription for the viscosity are presented. The application of these results to real systems with Compton-heated winds is discussed, and a general formula for the oscillation period is given. The prediction is compared with observed periodic behavior of Galactic X-ray sources and AGNs.

Shields, G. A.↗

NASA NTRS1986

Effect of design variables, temperature gradients and speed of life and reliability of a rotating disk

A generalized methodology to predict the fatigue life and reliability of a rotating disk such as used for aircraft engine turbines and compressors is advanced. The approach incorporates the computed life of elemental stress volumes to predict system life and reliability. Disk speed and thermal gradients as well as design variables such as disk diameter and thickness and bolt hole size, number and location are considered.

Zaretsky, E. V.↗

NASA NTRS1987

Collisional dynamics of perturbed particle disks in the solar system

Investigations of the collisional evolution of particulate disks subject to the gravitational perturbation of a more massive particle orbiting within the disk are underway. Both numerical N-body simulations using a novel collision algorithm and analytical kinetic theory are being employed to extend our understanding of perturbed disks in planetary rings and during the formation of the solar system. Particular problems proposed for investigation are: (1) The development and testing of general criteria for a small moonlet to clear a gap and produce observable morphological features in planetary rings; (2) The development of detailed models of collisional damping of the wavy edges observed on the Encke division of Saturn's A ring; and (3) The determination of the extent of runaway growth of the few largest planetesimals during the early stages of planetary accretion.

Roberts, W. W.↗

NASA NTRS1987

Influence of disk leakage path on labyrinth seal inlet swirl ratio

The results of numerous investigators have shown the importance of labyrinth seal inlet swirl on the calculated dynamic stiffness of labyrinth seals. These results have not included any calculation of inlet leakage swirl as a function of geometry and sealing conditions of the given seal. This paper outlines a method of calculating the inlet swirl at a given seal by introducing a radial chamber to predict the gas swirl as it goes from the stage tip down to the seal location. For a centrifugal compressor, this amounts to including the flow path from the impeller discharge, down the back of the disk or front of the cover, then into the shaft seal or eye packing, respectively. The solution includes the friction factors of both the disk and stationary wall with account for mass flow rate and calculation of radial pressure gradients by a free vortex solution. The results of various configurations are discussed and comparisons made to other published results of disk swirl.

Kirk, R. Gordon↗

NASA NTRS1987

Survey of the galactic disk from 1 = -150 deg to 1 = 82 deg in the submillimeter range

The first almost complete survey of the galactic disk from 1 = -150 deg to 1 = 82 deg in the submillimeter range (effective wavelength = 380 microns), performed with the AGLAE balloon-borne instrument modified to include a submillimeter channel, is reported. The instrumentation and observational procedures are described, as are the signal processing and calibration. The results are presented as a profile of the submillimeter brightness of the galactic disk displayed as a function of the galactic longitude. This profile exhibits diffuse emission all along the disk with bright peaks associated with resolved sources. The averaged galactic spectrum is in agreement with a temperature distribution of the interstellar cold dust.

Caux, Emmanuel↗

NASA NTRS1987

On the origin of the 40-120 micron emission of galaxy disks: A comparison with H-alpha fluxes

A comparison of 40 to 120 micron Infrared Astronomy Satellite (IRAS) fluxes with published H alpha and UBV photometry shows that the far infrared emission of galaxy disks consists of at least two components: a warm one associated with OB stars in HII-regions and young star-forming complexes, and a cooler one from dust in the diffuse, neutral interstellar medium, heated by the more general interstellar radiation field of the old disk population (a cirrus-like component). Most spiral galaxies are dominated by emission from the cooler component in this model. A significant fraction of the power for the cool component must originate with non-ionizing stars. For a normal spiral disk there is a substantial uncertainty in a star formation rate derived using either the H alpha or the far infrared luminosity.

Persson, Carol J. Lonsdale↗

NASA NTRS1986

Formation and evolution of a circumterrestrial disk Constraints on the origin of the moon in geocentric orbit

A data base of about 25,000 numerically integrated trajectories of earth-encountering planetesimals is used to study the angular momentum problem of forming the moon out of material captured into a circumterrestrial disk from heliocentric orbits. Mass-orbital element distributions of incoming planetesimals are combined with this data base to calculate, as a function of distance from earth, the net geocentric specific angular momentum of disk-encountering material on heliocentric orbits. Results suggest that a permanent disk population is not possible.

Herbert, Floyd↗

NASA NTRS1987

Timescales for planetary accretion and the structure of the protoplanetary disk

No self-consistent scenario for all stages of planetary accretion which satisfies observational constraints currently exists. An attempt is accordingly made here to resolve the timescale problems and to outline a planet formation scenario consistent with current theories of star formation as well as related models of the protoplanetary disk. For accretion to have proceeded in the manner presently hypothesized, the surface mass density of planetessimals would have had to to be relatively uniform in the Venus-Jupiter region of the protoplanetary disk, consistent with viscous accretion disk models of the solar nebula. The outer regions of the nebula would still have contained enough solid matter to account for the growth of Uranus and Neptune in 5 to 500 million years.

Lissauer, Jack J.↗

NASA NTRS1987

A linear thermal stability analysis for the vertical structure of alpha model accretion disks

The linear equations that describe the thermal stability of the convective and radiative vertical structure of an alpha model accretion disk are derived. For a specific model chosen to be representative of a dwarf nova disk, it is found that the eigenfunction for the temperature shows large excursions in the zone of partial ionization, but that the eigenfunctions for the unstable fundamental mode perturbations of the vertical distance and flux are nearly constant over the vertical extent of the disk. The growth of any instability is thus nearly homologous and the stability criteria derived for vertically averaged structures represent an adequate approximation.

Saio, Hideyuki↗

NASA NTRS1988

Predicting Life And Reliability Of A Rotating Disk

Report of generalized method for prediction of fatigue life and reliability of rotating disk. Method incorporates computation of life, at given probability of survival, of elemental stressed volumes within body of disk. Effects of design variables, temperature gradients, and speeds examined. Useful in analysis, design, and development of disks for aircraft-engine turbines and compressors and flywheels for industrial and automotive applications.

Zaretsky, Erwin V.↗

NASA NTRS1988

Life and reliability of rotating disks

In aerospace applications, an engineer must be especially cognizant of size and weight constraints which affect design decisions. Although designing at or below the material fatigue limit may be desirable in most industrial applications, in aerospace application it is almost mandatory to design certain components for a finite life at an acceptable probability of survival. Zaretsky outlined such a methodology based in part on the work of W. Weibull (1939, 1951) and G. Lundberg and A. Palmgren (1947a, 1947b, 1952). It is the objective of this work to apply the method of Zaretsky (1987) to statistically predict the life of a generic solid disk with and without bolt holes; determine the effect of disk design variables, thermal loads, and speed on relative life; and develop a generalized equation for determining disk life by incorporating only these variables.

Zaretsky, Erwin V.↗

NASA NTRS1987

Reconnection-driven oscillations in dwarf nova disks

A class of oscillations observed during eruption of dwarf novae has been interpreted as oscillations of the accretion disks in these systems.These oscillations are quasi-periodic with coherence times typically between three and 15 cycles. It is shown that magnetic field reconnection at high magnetic Reynolds number can drive disk oscillations. The expected stochastic geometry of disk magnetic fields could naturally produce the observed phase incoherency.

Tajima, T.↗

NASA NTRS1986

Spaceborne optical disk controller development

The current status and potential applications of an optical-disk buffer (ODB) memory system being developed by an interagency consortium including NASA and the USAF are reviewed. The design goals for the ODB include usable capacity 1 Tb, maximum data rate 1.6 Gb/s, read error rate less than 10 to the -12th, time to initial access less than 100 ms, and unlimited read/write cycles. Present efforts focus on a brassboard ODB which employs 12 14-inch magnetooptic disks and 24 nine-diode read/write heads. A typical space application of an optical disk mass memory system (ODMMS) is discussed: as communications buffer, temporary storage, and/or multiuser I/O buffer for data management on the Space Station Earth Observing System. Environmental, operational, system-architecture, and functional-separation factors; critical design issues; and standardization questions for spaceborne ODMMSs are examined in detail.

Shull, Thomas A.↗

NASA NTRS1988

On disk-planet interactions and orbital eccentricities

While Lindblad resonances both within and without a perturber's orbit excite its eccentricity, the present study of the eccentricity evolution due to the density wave interaction between a planetesimal and a Keplerian disk notes that coronation resonances in these regions lose their eccentricity damping effectiveness if the object is embedded in a continuous disk without a gap. Attention is given to another class of Lindblad resonances which, under these conditions, operates on disk material coorbiting with the perturber; these resonances thereby become the most important source of eccentricity damping. A model problem indicates that eccentricity ultimately undergoes decay.

Ward, William R.↗

NASA NTRS1988

Infrared radiative transfer in dense disks around young stars

A two-dimensional radiative transfer program has been used to determine the temperature distribution within cylindrically symmetric, centrally heated dust clouds. In particular, the disk-shaped structures observed around young luminous stars have been modeled. Changing the dust distribution in these disks primarily affected the observed morphology in the near-infrared and far-infrared, and at millimeter wavelengths. The overall cloud spectrum, however, was mainly determined by the characteristics of the grains themselves. Comparison with published far-infrared and molecular line data has indicated that the dust density can generally be modeled by a power-law distribution in r with index of -2 and an exponential in z with disk thickness proportional to 1/r. When observed nearly edge-on, scattered direct stellar radiation is observed in the polar regions in the form of comet-shaped lobes of emission.

Dent, William R. F.↗