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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

An unsupervised feature extraction method for high dimensional image data compaction

A new on-line unsupervised feature extraction method for high-dimensional remotely sensed image data compaction is presented. This method can be utilized to solve the problem of data redundancy in scene representation by satellite-borne high resolution multispectral sensors. The algorithm first partitions the observation space into an exhaustive set of disjoint objects. Then, pixels that belong to an object are characterized by an object feature. Finally, the set of object features is used for data transmission and classification. The example results show that the performance with the compacted features provides a slight improvement in classification accuracy instead of any degradation. Also, the information extraction method does not need to be preceded by a data decompaction.

Ghassemian, Hassan↗
NASA NTRS1991

Gravitational microlensing of high-redshift supernovae by compact objects

An analysis of the effect of microlensing by a cosmologically dominant density of compact objects is performed, using high-redshift Type Ia supernovae (SN Ia's) as probes. The compact objects are modeled as a three-dimensional distribution of point masses, and Monte Carlo simulations are done to calculate the resulting amplification probability distributions for several column densities and cosmologies. By combining these distributions with the intrinsic SN Ia luminosity function and comparing with the results for a perfectly smooth universe, estimates are made of the number of supernovae that would need to be observed to confirm or rule out this lensing scenario. It is found that about 1000 SN Ia's with redshifts of z = 1 would be needed to perform this test, which is beyond what current searches can hope to accomplish. Observations of many fewer high-redshift supernovae, used merely as standard candles, appears a promising way of distinguishing between different cosmological models.

Rauch, Kevin P.↗
NASA NTRS1991

Collisions of giant stars with compact objects - Hydrodynamical calculations

Smooth particle hydrodynamics are used here to model encounters between an 0.8 solar mass giant star and a 1.4 solar mass compact object on a parabolic orbit. It is found that all encounters with periastron distance r(p) less than about 2.5 stellar radii R(G) lead to significant disruption of the stellar envelope, including some mass loss. When r(p)/R(G) is less than about 0.5, the envelope is completely destroyed during the collision. About 40 percent of the gas escapes, while the rest forms a massive disk around the compact object, leaving behind the bare stellar core on a highly eccentric orbit. Subsequent close passages lead to continuous extraction of gas from the disk at a large rate, and the binary orbit may eventually stabilize while still retaining a large eccentricity. It is suggested that many globular cluster pulsars may have been spun up by accretion from a massive disk rather than from a binary companion, thereby providing a possible solution to the birthrate problem.

Rasio, Frederic A.↗
NASA NTRS1991

Offset dual-shaped reflectors for dual chamber compact ranges

The application of the theory of the synthesis of offset dual-shaped reflectors to the design of compact ranges is examined. The object of the compact range is to provide a uniform plane wave with minimum amplitude and phase ripple over as large a volume as possible for a given size reflector. Ripple can be lowered by reducing the edge diffraction from the reflector producing the plane wave. This has been done either by serrating or rolling the edge. An alternative approach is to use dual offset-shaped reflector synthesis techniques to produce a reflector aperture distribution that is uniform over most of the aperture, but with a Gaussian taper near the edge. This approach can be used together with rolling and/or serration if desirable. The amount of phase and amplitude ripple obtained with two different dual-shaped reflector designs is studied as a function of position in the plane wave zone and reflector size in wavelengths. The amount of both transverse and longitudinal (z-component) cross polarization is studied.

Galindo-Israel, Victor↗
NASA NTRS1991

Compact starbursts in ultraluminous infrared galaxies

The 40 ultraluminous galaxies in the IRAS Bright Galaxy Sample of sources stronger than S = 5.24 Jy at lambda = 60 microns were mapped with approximately 0.25 arcsec resolution at 8.44 GHz. Twenty-five contain diffuse radio sources obeying the FIR-radio correlation; these are almost certainly starburst galaxies. Fourteen other galaxies have nearly blackbody FIR spectra with color temperatures between 60 and 80 K so their (unmeasured) FIR angular sizes must exceed approximately 0.25 arcsec, yet they contain compact (but usually resolved) radio sources smaller than this limit. The unique radio and FIR properties of these galaxies can be modeled by ultraluminous nuclear starbursts so dense that they 67 are optically thick to free-free absorption at about 1.49 GHz and dust absorption at about 25 microns. Only one galaxy (UGC 08058 = Mrk 231) is a dominated by a variable radio source too compact to be an ultraluminous starburst; it must be powered by a 'monster'.

Condon, J. J.↗
NASA NTRS1991

The stability of numerical boundary treatments for compact high-order finite-difference schemes

The stability characteristics of various compact fourth and sixth order spatial operators are assessed using the theory of Gustafsson, Kreiss and Sundstrom (G-K-S) for the semi-discrete Initial Boundary Value Problem (IBVP). These results are then generalized to the fully discrete case using a recently developed theory of Kreiss. In all cases, favorable comparisons are obtained between the G-K-S theory, eigenvalue determination, and numerical simulation. The conventional definition of stability is then sharpened to include only those spatial discretizations that are asymptotically stable. It is shown that many of the higher order schemes which are G-K-S stable are not asymptotically stable. A series of compact fourth and sixth order schemes, which are both asymptotically and G-K-S stable for the scalar case, are then developed.

Carpenter, Mark H.↗
NASA NTRS1991

Planar near-field scanning for compact range bistatic radar cross-section measurement

The design, construction, and testing of a low cost, planar scanning system to be used in a compact range environment for bistatic radar cross-section (bistatic RCS) measurement data are discussed. This scanning system is similar to structures used for measuring near-field antenna patterns. A synthetic aperture technique is used for plane wave reception. System testing entailed comparison of measured and theoretical bistatic RCS of a sphere and a right circular cylinder. Bistatic scattering analysis of the ogival target support, target and pedestal interactions, and compact range room was necessary to determine measurement validity.

Tuhela-Reuning, S. R.↗
NASA NTRS1992

Bistatic image processing for a 32 x 19 inch model aircraft using scattered fields obtained in the OSU-ESL compact range

Inverse Synthetic Aperture Radar (ISAR) images for a 32 in long and 19 in wide model aircraft are documented. Both backscattered and bistatic scattered fields of this model aircraft were measured in the OSU-ESL compact range to obtain these images. The scattered fields of the target were measured for frequencies from 2 to 18 GHz with a 10 MHz increment and for full 360 deg azimuth rotation angles with a 0.2 deg step. For the bistatic scattering measurement, the compact range was used as the transmitting antenna; while, a broad band AEL double ridge horn was used as the receiving antenna. Bistatic angles of 90 deg and 135 deg were measured. Due to the size of the chamber and target, the receiving antenna was in the near field of the target; nevertheless, the image processing algorithm was valid for this case.

Lee, T-H.↗
NASA NTRS1992

Radiation dynamics in X-ray binaries. I - Type 1 bursts. II - Type 2 bursts. III - Extremely compact objects

Equations describing the evolution of a thin, axisymmetric, viscous, relativistic, irradiated accretion disk are presented, as well as numerical solutions of these equations in the case where irradiation results from a thermonuclear flash on the surface of the accreting neutron star. These calculations verify the notion that the radiation torque induces a substantial increase in the accretion rate, during a type 1 X-ray burst, and provide insight into the factors which influence the dynamical response of the disk. A new model for the source XBT 1730-335, the rapid burster, is presented. Temporal and spectral properties are calculated. The rapid burster is found to be a nonmagnetic, 'critically compact', slowly rotating neutron star in a highly eccentric binary system with a period of 6 mo. The spectral modifications which arise from the scattering of photons by accretion disks around nonmagnetic neutron stars are calculated. The 'black hole candidates' are interpreted as extremely compact stars.

Walker, Mark A.↗
NASA NTRS1993

Compact Raman Spectrometers Would Detect Hydrogen

Compact Raman spectrometers developed to measure concentrations of hydrogen as low as hundreds of parts per million in air, nitrogen, or other carrier gases. Advantages include speed, dynamic range, and ease of calibration. Design concept incorporates Raman-scattering apparatus into compact instrument of hydrogen leaking into stream of gas or into gas enclosed in small space. Should hydrogen-fueled cars and trucks come into widespread use, instruments used to detect leaks from vehicles and supply equipment, to help prevent explosions. Similar spectrometers developed to detect other gases emitting characteristic Raman spectra.

Helms, William R.↗
NASA NTRS1993

Time-stable boundary conditions for finite-difference schemes solving hyperbolic systems: Methodology and application to high-order compact schemes

We present a systematic method for constructing boundary conditions (numerical and physical) of the required accuracy, for compact (Pade-like) high-order finite-difference schemes for hyperbolic systems. First, a roper summation-by-parts formula is found for the approximate derivative. A 'simultaneous approximation term' (SAT) is then introduced to treat the boundary conditions. This procedure leads to time-stable schemes even in the system case. An explicit construction of the fourth-order compact case is given. Numerical studies are presented to verify the efficacy of the approach.

Carpenter, Mark H.↗
NASA NTRS1992

Verification of effective thicknesses for side-grooved compact specimens

The definition of effective thicknesses of the ASTM standard 25 percent side-grooved compact specimens to calculate the elastic compliance, elastic SIF, and elastic-plastic J integral is reevaluated. 3D elastic-plastic analyses of polane-strain, smooth, and 25 percent side-grooved compact specimen models are conducted using the ZIP3D code. Calculated compliance, SIFs, and J-integrals are compared with E-813 solutions.

Shivakumar, Kunigal N.↗
NASA NTRS1992

Observations on infiltration of silicon carbide compacts with an aluminium alloy

The melt infiltration of ceramic particulates permits an opportunity to observe such fundamental materials phenomena as nucleation, dynamic wetting and growth in constrained environments. Experimental observations are presented on the infiltration behavior and matrix microstructures that form when porous compacts of platelet-shaped single crystals of alpha- (hexagonal) silicon carbide are infiltrated with a liquid 2014 Al alloy. The infiltration process involved counter gravity infiltration of suitably tamped and preheated compacts of silicon carbide platelets under an external pressure in a special pressure chamber for a set period, then by solidification of the infiltrant metal in the interstices of the bed at atmospheric pressure.

Asthana, R.↗
NASA NTRS1992

Deep Hubble Space Telescope imaging of a compact radio galaxy at z = 2.390

The radio galaxy with the highest redshift in the Leiden-Berkeley Deep Survey, 53W002, is described and examined in terms of UV profile in relation to an early-type galaxy. The HST WFC images have a resolution of 0.2 arcsec FWHM, and the I- and V-band structures are assessed. The source is elongated in a manner similar to the Ly alpha cloud in V, and the structure is highly compact in I. The present object with a young starburst has very high central UV surface brightnesses relative to nearby luminous early-type galaxies, while the light profiles are similar. The data are concluded to suggest that 53W002 is a young galaxy that has a regular light profile at z = 2.390 even though it has been forming stars since not more than about 0.5 Gyr before z = 2.390. Such a scenario is consistent with concurrent dynamical collapse and star formation in the compact radio galaxy.

Windhorst, Rogier↗
NASA NTRS1993

Melt infiltration of silicon carbide compacts. II - Evaluation of solidification microstructures

Microstructural aspects of alloy solidification within the interstices of porous compacts of platelet-shaped single crystals of alpha-SiC, when the latter are infiltrated with a hot metal under pressure, have been described. Microstructural evidence is presented of selective reorientation of platelets and nonhomogeneous solute distribution under shear of pressurized melt, of constrained growth of primary solid within finite width zones, and of the modulation of coring due to microsegregation as a result of variations in the pore size of compacts.

Asthana, Rajiv↗
NASA NTRS1992

Compact jets and the AGN paradigm

The nature of compact, nonthermal jets in active galactic nuclei is reviewed from both observational and theoretical standpoints. Despite rapid variations in brightness, the smallest emission regions in a jet may be relatively far removed from the central engine. Certain models for the formation of a relativistic particle beam that evolves downstream into the observed radio-infrared jet do, however, predict substantial high energy radiation from regions in the beam close to the accretion disk. Observations of such emission, and the general requirement that the jets be well collimated and accelerated to relativistic bulk velocities by the point where they are observed in the radio, can provide indirect constraints on the massive black hole accretion paradigm. Observations of well collimated, semi-relativistic jets in the accreting stellar systems SS 433 and Cyg X-3, as well as slower jets emanating from star forming regions, encourage one to associate jet formation with accretion of gas onto a compact object.

Marscher, Alan P.↗
NASA NTRS1993

Searching gamma-ray bursts for gravitational lensing echoes - Implications for compact dark matter

The first available 44 gamma-ray bursts (GRBs) detected by the Burst and Transient Source Experiment on board the Compton Gamma-Ray Observatory have been inspected for echo signals following shortly after the main signal. No significant echoes have been found. Echoes would have been expected were the GRBs distant enough and the universe populated with a sufficient density of compact objects composing the dark matter. Constraints on dark matter abundance and GRB redshifts from the present data are presented and discussed. Based on these preliminary results, a universe filled to critical density of compact objects between 10 exp 6.5 and 10 exp 8.1 solar masses are now marginally excluded, or the most likely cosmological distance paradigm for GRBs is not correct. We expect future constraints to be able either to test currently popular cosmological dark matter paradigms or to indicate that GRBs do not lie at cosmological distances.

Nemiroff, R. J.↗
NASA NTRS1993

LREE variability in CM matrices: Another look at meteorite compaction ages

Recent Neon-21 analyses indicate that individual grains in some CM meteorites show the effects of enhanced exposures to energetic particles beyond that which can be attributed to conventional cosmic ray exposure. The exposures clearly occurred prior to the final compaction of the CM meteorites. These exposures were of such a magnitude that they require that we either have to revise our understanding of regolith dynamics and the time scales involved in the regolith activity of meteorite parent bodies, or must invoke an exposure of the irradiated grains to an enhanced energetic particle environment, an environment which would most likely be produced by an early active sun. Constraints on the compaction ages of the meteorites which exhibit the large precompaction exposures could play a critical role in resolving this issue.

Woolum, Dorothy S.↗