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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Fast pulsars with disks

The observed properties of the pulsar PSR1937+214 are compared with predictions of the disk model. It is assumed that an isolated magnetized rotating neutron star is ringed by a fluid disk with a 0.00001 solar mass, and relative rotations of the star and the disk produce potential differences across the disk. A Faraday disk dynamo is also formed between the disk and the star, and allows the polar cap current to return from the disk to the star through auroral arcing. Preferential regions of the star are recipients of a return current controlled by the surface magnetic field structure, which configures the pulsing emissions. The disk model predicts the average luminosity to be 10 to the 31st erg/sec, and an emission of 3 x 10 to the 30th erg/sec was detected. Only one-millionth of the output of the emissions is in the radio region, and the X and gamma ray emissions are in the normal range for pulsars. It is concluded that PSR1937+214 behaves within the predictions of the disk model and is not a new kind of object.

Michel, F. C.↗
NASA NTRS1984

Vibration and flutter of mistuned bladed-disk assemblies

An analytical model for investigating vibration and flutter of mistuned bladed disk assemblies is presented. This model accounts for elastic, inertial and aerodynamic coupling between bending and torsional motions of each individual blade, elastic and inertial couplings between the blades and the disk, and aerodynamic coupling among the blades. The disk was modeled as a circular plate with constant thickness and each blade was represented by a twisted, slender, straight, nonuniform, elastic beam with a symmetric cross section. The elastic axis, inertia axis, and the tension axis were taken to be noncoincident and the structural warping of the section was explicitly considered. The blade aerodynamic loading in the subsonic and supersonic flow regimes was obtained from two-dimensional unsteady, cascade theories. All the possible standing wave modes of the disk and traveling wave modes of the blades were included. The equations of motion were derived by using the energy method in conjunction with the assumed mode shapes for the disk and the blades. Continuities of displacement and slope at the blade-disk junction were maintained. The equations were solved to investigate the effects of blade-disk coupling and blade frequency mistuning on vibration and flutter. Results showed that the flexibility of practical disks such as those used for current generation turbofans did not have a significant influence on either the tuned or mistuned flutter characteristics. However, the disk flexibility may have a strong influence on some of the system frequencies and on forced response.

Kaza, K. R. V.↗
NASA NTRS1984

Stagger angle dependence of inertial and elastic coupling in bladed disks

Conditions which necessitate the inclusion of disk and shaft flexibility in the analysis of blade response in rotating blade-disk-shaft systems are derived in terms of nondimensional parameters. A simple semianalytical Rayleigh-Ritz model is derived in which the disk possesses all six rigid body degrees of freedom, which are elastically constrained by the shaft. Inertial coupling by the rigid body motion of the disk on a flexible shaft and out-of-plane elastic coupling due to disk flexure are included. Frequency ratios and mass ratios, which depend on the stagger angle, are determined for three typical rotors: a first stage high-pressure core compressor, a high bypass ratio fan, and an advanced turboprop. The stagger angle controls the degree of coupling in the blade-disk system. In the blade-disk-shaft system, the stagger angle determines whether blade-disk motion couples principally to the out-of-plane or in-plane motion of the disk on the shaft. The Ritz analysis shows excellent agreement with experimental results.

Crawley, E. F.↗
NASA NTRS1984

Vibration and flutter of mistuned bladed-disk assemblies

An analytical model for investigating vibration and flutter of mistuned bladed disk assemblies is presented. This model accounts for elastic, inertial and aerodynamic coupling between bending and torsional motions of each individual blade, elastic and inertial couplings between the blades and the disk, and aerodynamic coupling among the blades. The disk was modeled as a circular plate with constant thickness and each blade was represented by a twisted, slender, straight, nonuniform, elastic beam with a symmetric cross section. The elastic axis, inertia axis, and the tension axis were taken to be noncoincident and the structural warping of the section was explicitly considered. The blade aerodynamic loading in the subsonic and supersonic flow regimes was obtained from two-dimensional unsteady, cascade theories. All the possible standing wave modes of the disk and traveling wave modes of the blades were included. The equations of motion were derived by using the energy method in conjunction with the assumed mode shapes for the disk and the blades. Continuities of displacement and slope at the blade-disk junction were maintained. The equations were solved to investigate the effects of blade-disk coupling and blade frequency mistuning on vibration and flutter. Results showed that the flexibility of practical disks such as those used for current generation turbufans did not have a significant influence on either the tuned or mistuned flutter characteristics. However, the disk flexibility may have a strong influence on some of the system frequencies and on forced response.

Rao, K.↗
NASA NTRS1988

Gravitational instability in two-phase disks and the origin of the moon

Two-phase disks may be gravitationally unstable at temperatures or surface densities at which a disk composed of either single phase would be highly stable. It is argued that two-phase disks can achieve a marginally unstable state (in addition to a highly unstable state that leads to fragmentation), limited by the ability of the photosphere to radiate the energy dissipated in the disk. A self-consistent prescription for the viscosity induced by the slow instabilities is provided. Two-phase disks are more centrally condensed than single-phase disks, and their secular cooling time may be comparable to their spreading time. A circumterrestrial disk of sufficient mass to form the moon provides a detailed example of all the preceding points. Its stability, structure, and dynamical evolution are investigated, and it is concluded that its spreading time is short (about 100 yr); the moon is formed molten, or partially molten; the moon's initial orbit lies in the earth's equatorial plane; and only a small fraction of the disk mass is lost in a wind, although this may represent a substantial fraction of volatiles. Most of these conclusions are independent of how the disk was formed, e.g., from a giant impact.

Thompson, Christopher↗
NASA NTRS1991

Nonlinear evolution of accretion disks induced by radiative feedback processes

The existence of accretion disks around young stellar objects has recently become widely accepted. The luminosity of some young stellar objects is highly variable and is generally attributed to the release of gravitational energy from matter funneled onto them by accretion disks. The inward transport of matter through these disks is coupled to the outward transfer of angular momentum. This transfer is most likely to be regulated by the mixing of adjacent annuli through the process of large-scale turbulence. Most of the accretion energy generated by this process emerges near the inner edge of the disk. This radiation may be intercepted by the disk and may modify the vertical and viscous evolution of the disk itself. If surface heating can stabilize the disk against the dominant viscous process, then in systems with large accretion rates, angular momentum transport and mass flow through the disk will be quenched. Using this result, it is shown that such a mechanism can induce feedback through the disk which produces oscillations in the luminosity of the central object. This oscillation can become chaotic in certain regimes and might thus explain the highly variable nature of many T Tauri systems and their outbursting counterparts: the FU Orionis objects.

Bell, K. R.↗
NASA NTRS1990

Redundant disk arrays: Reliable, parallel secondary storage

During the past decade, advances in processor and memory technology have given rise to increases in computational performance that far outstrip increases in the performance of secondary storage technology. Coupled with emerging small-disk technology, disk arrays provide the cost, volume, and capacity of current disk subsystems, by leveraging parallelism, many times their performance. Unfortunately, arrays of small disks may have much higher failure rates than the single large disks they replace. Redundant arrays of inexpensive disks (RAID) use simple redundancy schemes to provide high data reliability. The data encoding, performance, and reliability of redundant disk arrays are investigated. Organizing redundant data into a disk array is treated as a coding problem. Among alternatives examined, codes as simple as parity are shown to effectively correct single, self-identifying disk failures.

Gibson, Garth Alan↗
NASA NTRS1992

Model scattering envelopes of young stellar objects. I - Method and application to circumstellar disks

We describe a Monte Carlo code that accurately treats multiple scattering, absorption, and polarization by dust, and use this code to calculate images of dusty disks around young stellar objects. We present some approximate analytic results that describe the behavior of the Monte Carlo calculations. A geometrically thin disk illuminated by a central T Tauri star scatters very little light at distances of many AU from the star. Viewed at any inclination, the flux scattered by such a disk at the distance to the nearest star-forming region will be overwhelmed by the stellar image. An optically thick disk that has a flaring surface may be observable, especially if viewed nearly edge-on so that the stellar source becomes occulted. An optically thin disk with a finite opening angle, similar to the one surrounding beta Pictoris, is about as observable as the typical flared optically thick disk at a similar distance from the earth. The polarization position angle is perpendicular to the disk plane in all of the models, in contrast to observations of many young stellar objects which have the position angle oriented parallel to the presumed disk plane. We suggest that the scattered light structures observed around many premain-sequence objects are dusty envelopes rather than disks.

Whitney, Barbara A.↗
NASA NTRS1994

Astronomical observations of phenomena in disks

Astronomical constraints on disks around young stellar objects are reviewed in this abstract. Since disks around the youngest stars cannot generally be resolved spatially, most results are based on analyses of the infrared and radio emission from circumstellar dust and gas. At least half of all young stars seem to have substantial circumstellar disks at an age of approximately 10(exp 6) yr. The young objects without disks tend to be binary stellar systems, though not exclusively, and some close binaries may have outer disks that are not tidally disrupted. Probably most, if not all, stars formed originally with circumstellar disks, but evolutionary timescales may differ dramatically. All the results, disk disappearance, disk mass around young stars, and long-wavelength dust opacities, are subject to the additional uncertainty of the effects of infalling dusty envelopes, which produce the disks in the first place.

Hartmann, L.↗
NASA NTRS1994

The stability of thick, self-gravitating disks in protostellar systems

Using numerical hydrodynamics techniques, we perform a nonlinear stability analysis of accretion disk systems that contain thick, self-gravitating disks. The systems are initially represented by a point mass M(sub c) at the center and a geometrically thick, axisymmetric disk of mass M(sub d) that supports uniform specific angular momentum and obeys an n = 3/2, polytropic equation of state. The equilibrium disk structure is uniquely defined upon the specification of two key dimensionless system parameters: M(sub d)/M(sub c) and T/absolute value of W (the ratio of rotational kinetic energy if the disk to the gravitational potential energy of the system). The focus of this work is on the identification of systems within this two-dimensional parameter space that are marginally unstable toward the development of nonaxisymmetric distortions. The geometric form and relative pattern speed of the disk's distortion as well as the likelihood of disk fragmentation as a result of such instabilities is examined, particularly in the context of protostellar systems. The value of T/absolute value of W at which thick disks first become dynamically unstable to nonaxisymmetric distortions is found to vary significantly with the mass ratio of the accertion disk system. Nonaxisymmetric eigenmodes with four distinctly different characters are identified in systems with mass ratios in the range 0.2 less than or equal to (M(sub d)/M(sub c)) less than or equal to 5.

Woodward, John W.↗
NASA NTRS2001

Relativistic Iron Emission and Disk Reflection in Galactic Microquasar XTE J1748-288

We report evidence for an Fe K-alpha fluorescence line feature and disk reflection in the very high, high-, and low-state X-ray spectra of the Galactic microquasar XTE J1748 - 288 during its 1998 June outburst. Spectral analyses are made on data gathered throughout the outburst by the Rossi X-Ray Timing Explorer Proportional Counter Array. Gaussian line, relativistic disk emission line, and ionized disk reflection models are fitted to the data. In the very high state the line profile appears strongly redshifted, consistent with disk emission from the innermost stable orbits around a maximally rotating Kerr black hole. In the high state the line profile is less redshifted and increasingly prominent. The low-state line profile is very strong (approx. 0.5 keV equivalent width) and centered at 6.7 +/- 0.10 keV; disk line emission model fits indicate that the inner edge of the disk fluctuates between approx. 20R(sub g) and - approx. 100R(sub g) in this state. The disk reflection fraction is traced through the outburst; reflection from an ionized disk is preferred in the very high and high states, and reflection from a relatively neutral disk is preferred in the low state. We discuss the implications of our findings for the binary system dynamics and accretion flow geometry in XTE J1748 - 288.

Miller, J. M.↗
NASA NTRS2001

Relativistic Iron Emission and Disk Reflection in Galactic Microquasar XTE J1748-288

We report evidence for an Fe K(alpha) fluorescence line feature and disk reflection in the very high, high-, and low-state X-ray spectra of the Galactic microquasar XTE J1748-288 during its 1998 June outburst. Spectral analyses are made on data gathered throughout the outburst by the Rossi X-Ray Timing Explorer Proportional Counter Array. Gaussian line, relativistic disk emission line, and ionized disk reflection models are fitted to the data. In the very high state the line profile appears strongly redshifted, consistent with disk emission from the innermost stable orbits around a maximally rotating Kerr black hole. In the high state the line profile is less redshifted and increasingly prominent. The low-state line profile is very strong (approx. 0.5 keV equivalent width) and centered at 6.7 +/- 0.10 keV; disk line emission model fits indicate that the inner edge of the disk fluctuates between approx. 20Rg and approx. 100Rg in this state. The disk reflection fraction is traced through the outburst; reflection from an ionized disk is preferred in the very high and high states, and reflection from a relatively neutral disk is preferred in the low state. We discuss the implications of our findings for the binary system dynamics and accretion flow geometry in XTE J1748-288.

Miller, J. M.↗
NASA NTRS2001

A Realistic Accretion Disk Model for AGNs

We present a new relativistic accretion disk model of AGNs, based on alpha modified accretion disk theory, where alpha is a parameter that characterizes the efficiency of the mechanism of angular momentum transport. In this model we have considered a geometrically-thin, high-luminosity (alpha-accretion disk, around a supermassive black hole. The geometrically thin limit assumes a local energy balance within the accretion disk, which has been used to derive the plasma temperature of the disk. The energy balance equation has three solutions: low, medium and high temperature, depending on the heating rate of the accretion disk. We predict that the inner part of a high luminosity accretion disk is in the high- temperature (10(exp 7) to 10(exp 9) K) state and for this we obtain the high temperature solution of the energy balance equation using a Comptonization process. We find the local spectrum of each ring of the disk (we have divided the high temperature region of the disk into 50 rings) to be a diluted Wien spectrum. However, the emergent integral spectrum of the high temperature region is a power-law with a high-energy cutoff that depends on the basic parameters of AGNs (the accretion rate, the incidence angle, and the mass and the angular momentum of the central black hole). We have fitted the observed spectra of 28 AGNs using the present model and have derived the values of the basic parameters of these AGNs. Results of these spectral fittings have been discussed in the framework of the unification model of AGNs.

Suleimanov, V.↗
NASA NTRS2002

High Temperature Spin Testing of a Superalloy Disk With a Dual Grain Structure

Comparative spin tests were run on superalloy disks at an elevated temperature to determine the benefits of a DMHT disk, with a fine grain bore and coarse grain rim, versus a traditional subsolvus disk with a fine grain structure in the bore and rim. The results of these tests showed that the DMHT disk exhibited significantly lower growth at 1500 F. Further, the results of these tests could be accurately predicted using a 2D viscoelastic finite element analysis. These results indicate DMHT technology can be used to extend disk operating temperatures when compared to traditional subsolvus heat treatment options for superalloy disks. However, additional research is required to ensure the safe operation of a DMHT disk under more realistic engine operating conditions. This includes testing to determine the burst margin and cyclic capability of DMHT disks in a spin pit, at a minimum, and ultimately running an engine test with a DMHT disk.

Gayda, John↗
NASA NTRS2003

Coronagraphic Observations of Optically Thin Debris Disks

We present the first observations of the debris disks around HD141569A and HD100546 made with the Advanced Camera for Surveys (ACS) Coronagraph. Both HD141569A and HD100546 are known to possess circumstellar disks with complex structures. HD141569A's disk was shown to exhibit a a gap from NICMOS observations (Weinberger et ai. 1999), while HD100546 has been shown to exhibit a disk with spiral strucure (Grady et ai. 2001). The ACS observations were made as part of the GTO program to study the formation of planetary systems. The performance of the ACS Coronagraph meets pre-launch expectations, and allows us t o achieve images with both unparalleled contrast and, a fully-sampled point spread function. We will present new, multi-color images of each object obtained with the F435W, F606W and F814W filters. These images present a significant improvement over the previous observations with NICMOS and STIS. We fully resolve the complex spiral structures observed within these disks and determine the true angular extent of the disks. Disk properties including color variations, disk asymmetries and morphology are determined. We discuss the formation history of these disks in the light of,our new observations and review the evidence for the possibility that the systems harbor planets.

Clampin, Mark↗
NASA NTRS2003

Coronagraphic Observations of Optically Thin Debris Disks

We present the first observations of the debris disks around HD141569A and HD100546 made with the Advanced Camera for Surveys (ACS) Coronagraph. Both HD141569A and HD100546 are known to possess circumstellar disks with complex structures. HD141569A's disk was shown to exhibit a a gap from NICMOS observations, while HD100546 has been shown to exhibit a disk with spiral structure (Grady et al. 2001). The ACS observations were made as part of the GTO program to study the formation of planetary systems. The performance of the ACS Coronagraph meets pre-launch expectations, and allows us to achieve images with both unparalleled contrast and, a fully-sampled point spread function. We will present new, multi-color images of each object obtained with the F435W, F606W and F814W filters. These images present a significant improvement over the previous observations with NICMOS and STIS. We fully resolve the complex spiral structures observed within these disks and determine the true angular extent of the disks. Disk properties including color variations, disk asymmetries and morphology are determined. We discuss the formation history of these disks in the light of our new observations and review the evidence for the possibility that the systems harbor planets.

Clampin, Mark↗
NASA NTRS2006

Burst Testing and Analysis of Superalloy Disks With a Dual Grain Microstructure

Elastic-plastic finite element analyses of room temperature burst tests on four superalloy disks were conducted and reported in this paper. Two alloys, Rene 104 (General Electric Aircraft Engines) and Alloy 10 (Honeywell Engines & Systems), were studied. For both alloys an advanced dual microstructure disk, fine grain bore and coarse grain rim, were analyzed and compared with conventional disks with uniform microstructures, coarse grain for Rene 104 and fine grain for Alloy 10. The analysis and experimental data were in good agreement up to burst. At burst, the analysis underestimated the speed and growth of the Rene 104 disks, but overestimated the speed and growth of the Alloy 10 disks. Fractography revealed that the Alloy 10 disks displayed significant surface microcracking and coalescence in comparison to Rene 104 disks. This phenomenon may help explain the differences between the Alloy 10 disks and the Rene 104 disks, as well as the observed deviations between analytical and experimental data at burst.

Gayda, John↗
NASA NTRS2006

Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope

We presented the results of an infrared imaging survey of Tr 37 and NGC 7160 using the IRAC and MIPS instruments on board the Spitzer Space Telescope. Our observations cover the wavelength range from 3.6 to 24 microns, allowing us to detect disk emission over a typical range of radii 0.1 to 20 AU from the central star. In Tr 37, with an age of about 4 Myr, about 48% of the low-mass stars exhibit detectable disk emission in the IRAC bands. Roughly 10% of the stars with disks may be "transition" objects, with essentially photospheric fluxes at wavelengths i 4.5 microns but with excesses at longer wavelengths, indicating an optically thin inner disk. The median optically thick disk emission in Tr 37 is lower than the corresponding median for stars in the younger Taurus region; the decrease in infrared excess is larger at 6-8 microns than at 24 microns, suggesting that grain growth and/or dust settling has proceeded faster at smaller disk radii, as expected on general theoretical grounds. Only about 4% of the low-mass stars in the 10 Myr old cluster NGC 7160 show detectable infrared disk emission. We also find evidence for 24 micron excesses around a few intermediate-mass stars, which may represent so-called "debris disk" systems. Our observations provided new constraints on disk evolution through an important age range.

Sicilia-Aguilar Aurora↗