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At least 1,513 records · Page 84

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NASA NTRS1983

Stagnation-point ablation of carbonaceous flat disks. I Theory

The process of ablation is calculated for the stagnation region of a flat disk in a radiation-dominated, massive-blowing environment produced in a ballistic range filled with argon. Flow environments are determined by solving the boundary-layer equations while radiative transfer is calculated through a line-by-line spectral computation. The resulting wall heat-transfer rates are coupled with an existing material's response code to determine surface recession and char thickness. The calculation is performed for six 5-cm-diam models made of carbon-phenolic and carbon-carbon composite launched in the Track-G facility at the Arnold Engineering Development Center. Significant surface recessions are predicted to occur for these models due mostly to radiative heating.

Park, C.↗

NASA NTRS1983

Stagnation-point ablation of carbonaceous flat disks. II Experiment

Six flat-disk models made of carbon-carbon and carbon-phenolic materials were launched in an argon-filled track-range facility to test ablation characteristics in a radiation-dominated, massive-blowing environment. The shock standoff distances deduced from the shadowgraphs agree with theoretical predictions during the earlier portion of the flight, while the wall temperatures determined by the image-converter photographs agree with predictions during the later portion. The measured surface recessions exceed the calculated values by about 60 percent for carbon-phenolic and 30 percent for carbon-carbon. The discrepancies are attributed to spallation. The measured char thicknesses agree with theoretical predictions.

Park, C.↗

NASA NTRS1983

Stellar evolution in N-body simulations of disk galaxies. I

The Kalnajs (1972, 1976) Omega models of global mass and velocity distributions are employed in the present two-dimensional N-body simulation, which allows for a spectrum of particle masses, stellar explosions, explosion remnant interactions with an interstellar medium, and the creation of new stars from the gas. Two sequences of runs using the Omega values of 0.8 and 0.9 examine the separate and combined effects of particle mass distribution, the gravitational influence of an interstellar gas distribution on the N-body particles, and stellar evolution, allowing for stellar explosions and star formation from the gas. It is found that both Omega values' nonequilibrium results dramatically change when evolution is allowed to occur. These results call for more realistic coupled N-body and evolution simulations in order to improve the understanding of disk galaxy evolution.

Comins, N. F.↗

NASA NTRS1984

Evaluation of the solar disk sextant concept

In this paper the viability of the solar disk sextant concept is evaluated, the optimum parameters required to carry out solar variability studies, which are the mission objectives, are derived. The experimental environment is first discussed, followed by the application of the finite Fourier transform definition (FFTD) to the detector array data. The requirements on the optical system are studied next. A computer program was carried out simulating solar edge data and FFTD. From this study, it is concluded that the required accuracy of measurement may be reached using currently available detector array technology, a focal ratio of the optical system in excess of 90, and an entrance aperture of 22 cm. The guidance error must be small enough to require no more than a correction rate of 0.1 arcsec/sec. All these conditions are well within current technology.

Chiu, H.-Y.↗

NASA NTRS1984

Solar disk sextant optical configuration

In this paper the performance of a plausible configuration for the solar disk sextant, an instrument to be used to monitor the solar diameter, is evaluated. Overall system requirements are evaluated, and tolerable uncertainties are obtained. It is concluded that by using a beam splitting wedge, a folded optics design can be used to measure the solar diameter to an accuracy of 10 to the -6th, despite the greater aberrations present in such optical systems.

Chiu, H.-Y.↗

NASA NTRS1984

Solar disk sextant

This paper presents the conceptual design of an instrument, called the solar disk sextant, to be used in space to measure the shape and the size of the sun and their variations. The instrumental parameters required to produce sufficient sensitivity to address the problems of solar oblateness, solar pulsations, and global size changes of climatic importance are given.

Sofia, S.↗

NASA NTRS1984

Evidence for an infrared disk in the core of the extraordinary planetary nebula Abell 30

High spatial resolution infrared maps of the core of the evolved planetary nebula Abell 30, which has been shown to be composed of heavily processed material, are presented. These maps reveal an elongated, disklike structure that is not evident on optical pictures. Multicolor photometry at several positions in this disk shows that this emission is due to dust, some of which is near the evaporation temperature of graphite. Lyman-alpha trapping is shown to be inadequate as a heating source for these grains, but direct absorption of stellar photons is sufficient. In view of the unusual chemical composition, A30 represents a late stage in the evolution of a planetary nebula progenitor. The observed infrared structure may provide clues to the ejection mechanism at this stage of development.

Lester, D. F.↗

NASA NTRS1984

High-resolution maps of the 1.5 GHz emission from Jupiter's disk and radiation belt

VLA maps of four different faces of Jupiter made with a resolution of about 0.3 Jovian radius show new features of the radiation belt emission. A synchrotron model which reproduces these features serves to define the major characteristics of the relativistic electrons in the radiation belt. The observations provide the best determination to date of the atmospheric emission at 1.5 GHz and yield a disk brightness temperature of 425 + or - 100 K.

Roberts, J. A.↗

NASA NTRS1984

Transient thermal stresses in a reinforced hollow disk or cylinder containing a radial crack

The transient thermal stress problem in a hollow cylinder or a disk containing a radial crack is considered. It is assumed that the cylinder is reinforced on its inner boundary by a membrane which has thermoelastic constants different than those of the base material. The transient temperature, thermal stresses and the crack tip stress intensity factors are calculated in a cylinder which is subjected to a sudden change of temperature on the inside surface. The results are obtained for various dimensionless parameters and material constants. The special cases of the crack terminating at the cylinder-membrane interface and of the broken membrane are separately considered and some examples are given.

Tang, R.↗

NASA NTRS1984

High spatial resolution IR observations of young stellar objects - A possible disk surrounding HL Tauri

High spatial resolution images of the T Tauri star HL Tau were obtained at 1.6 microns and 2.2 microns. The original images as well as maximum entropy image reconstructions reveal a circumstellar envelope structure, similar at both wavelenghts, and extended along P.A. = 112 deg; the 10 percent width of the structure is 1.9 sec (300 AU at 160 pc). The extended structure is interpreted as light scattered toward earth by dust in a disk surrounding this young stellar object. Polarization measurements made at 2.2 microns support this hypothesis. The total solid particle mass is, at minimum, 5 x 10 to the -7th solar mass.

Grasdalen, G. L.↗

NASA NTRS1984

Voyager disk-integrated photometry of Io

The results of a comprehensive analysis of disk-integrated photometry of Io derived from Voyager images are presented. Phase curves, rotation curves, and geometric albedos consistent with earth-based observations of Io are obtained, except for a previously noted tendency for the Voyager data to yield somewhat redder colors. It is demonstrated that the phase coefficient decreases with increasing wavelength. The phase integral ranges from about 0.7 in the ultraviolet to about 0.8 in the orange. The Bond albedo is estimated at 0.50 + or -0.10, a value consistent with, though slightly below, previous earth-based estimates.

Simonelli, D. P.↗

NASA NTRS1984

Circumstellar Dust Disks Around Young Stars

Kuiper Airborne Observatory observations of several resolved far-infrared structures that are likely to represent disks formed during the process of the collapse of a cloud to form a star are discussed. An infrared radiation nebula, star HH57, and SVS13 were studied. Forty seven and 95 micron maps of far infrared radiation are given.

Cohen, M.↗

NASA NTRS1985

The structure of the shower disk observed at Mt. Norikura

The structure of the EAS shower disk, the arrival time distribution of charged particles at the core of the small or middle size shower, is measured at Mt. Norikura in Japan. Four fast scintillation counters with an area of 0.25 sq m and a fast trigger system are added to the Mt. Norikura EAS array for the study.

Sasaki, H.↗

NASA NTRS1984

Eddy current jet engine disk-crack monitor

A disk-crack monitor is described that is suitable for use on the ground or in flight. The system consists of an engine-mounted eddy current sensor, a series capacitance in each leg of the sensor circuit, and a capacitance-conductance bridge followed by an oscilloscope capable of advanced signal processing. It was applied to the detection of service-induced cracks in the first-stage turbine wheel of a helicopter engine. A 3.5 mm long radial fatigue crack plus smaller cracks, all located in the blade root region of the wheel, were detected during engine test stand operation at ground idle speed and temperature. The calculation of the value of series capacitance is also presented.

Barranger, J. P.↗

NASA NTRS1984

Gravitational spurs and resonances - Effects of small mass disturbers in spiral galaxy disks

In the present simulations of a disturber in a complete stellar disk without the restrictive assumption, the disturber parameters of the NGC 206 cloud in M 31 were assumed as a realistic example. The resulting spur around the disturber was comparable in shape, size, and strength to Julian and Toomre's (1966) results. In addition, a complicated evolving pattern of strong density peaks appeared well inside and outside the disturber's orbit. Simulation with a ten-times-more-massive disturber showed a more clearly defined version of the same initial pattern, two spiral arms of density peaks rotating with the disturber in the stronger arm. The orbital radii of the density peaks correspond to those of epicyclic resonances with the orbiting disturber potential.

Byrd, G. G.↗

NASA NTRS1984

Hot subdwarfs in detached binary systems and thick-disk cataclysmic variables from the Palomar-Green survey

A total of 19 stars from the Palomar Green (PG) survey which were found to show composite spectra, generally with a hot subdwarf primary and a cool main-sequence secondary star, are discussed. The primary components have Teff values between 27,000 and 38,000 K (Mv = +5 to +7) and log g = about 6.5, while the secondaries generally show spectral types G8 to K4.5; however, these ranges are undoubtedly limited by selection effects, and extremely helium-rich subdwarfs were excluded as well. Aproximately half of all the PG subdwarfs could be in undetected binaries. Only two thick-disk cataclysmic variables with predominantly absorption-line spectra were found in the PG survey, which provides strong evidence that this kind of cataclysmic variable is luminous and confined to the Galactic plane (Population I). On the other hand, a few of the detached binaries show evidence for spectroscopic variations. Those which are close enough for a significant reflection effect on the secondary star may be progenitors to cataclysmic variable systems.

Ferguson, D. H.↗

NASA NTRS1985

The natural angular momentum distribution in the study of thick disks around black holes

The thick-disk model is reviewed with an angular momentum distribution chosen to have a power-law relation with the von Zeipel parameter lambda = sq rt(l/Omega), where l denotes the specific angular momentum and Omega denotes the angular velocity. This choice permits the integration of the relativistic Euler equation independent of the background geometry. The consequences with regard to the Schwarzschild and Kerr black-hole geometries are briefly investigated.

Chakrabarti, S. K.↗

NASA NTRS1984

The solar nebula and the planetesimal disk

Two popular theories of solar system formation are briefly reviewed, then used as background in an examination of several new developments related to planetary ring dynamics that promise to have great impact on future research. Most important are the incorporation of accretion disk and density wave theories into cosmogonic theory. A successful integration of these mechanisms may significantly constrain evolutionary models of the early solar system and also provide new insight into the mechanisms themselves.

Ward, W. R.↗