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At least 217 records · Page 12

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NASA NTRS1976

Interpretation of broad-band polarimetry of solar coronal transients - Importance of H-alpha emission

An eruptive prominence and coronal transient was observed in H-alpha, He II 304-A, and coronal white light (3700-7000 A). This event was generically similar to many other coronal transients associated with eruptive prominences in that loops of material propagated outward through the corona. It differed in that some of the prominence material was observed above 1.75 solar radii in both coronal white light and He II. Polarization analysis of the white-light data shows that the observed radiance from the loop-like transient was entirely due to free electron or Thomson scattering, whereas the white-light radiance from the prominence material was dominated by H-alpha emission. By comparing the white-light observations with model calculations, the densities along the loop and limits on the temperature and density of the prominence are obtained. Material observed in both white light and He II is shown to be cool prominence material, whereas analysis indicates that the transient-loop material is hot and coronal in origin. The time sequence of observations leads to the hypothesis that the prominence material is heated as it is ejected from the sun.

Poland, A. I.↗

NASA NTRS1981

Some observations in high pressure rheology of lubricants

Experimental data are presented on viscosity, elastic shear modulus, and limiting shear stress of 12 liquid lubricants. It is shown that transition histories do affect the limiting shear stress of the materials in the form of isothermal compression resulting in a lower density and lower limiting stress than isobaric cooling. The measured limiting shear stress agrees with EHD traction data at slide-to-roll ratios of 0.1 or more. In pressure viscosity measurements of the polymer solutions, it is found that for some temperatures, the pressure viscosity coefficient of the blend is slightly less than that of the base, which results in the crossing of the viscosity-pressure isotherms at high pressures.

Bair, S.↗

DOE OSTI2025

Modeling of the high column density systems in the Lyman-Alpha forest

The Lyman-α forests observed in the spectra of high-redshift quasars can be used as a tracer of the cosmological matter density to study baryon acoustic oscillations (BAO) and the Alcock-Paczynski effect. Extraction of cosmological information from these studies requires modeling of the forest correlations. While the models depend most importantly on the bias parameters of the intergalactic medium (IGM), they also depend on the numbers and characteristics of high-column-density systems (HCDs) ranging from Lyman-limit systems with column densities log N HI /1cm -2 > 17 to damped Lyman-α systems (DLAs) with log N HI /1cm -2 > 20.2. These HCDs introduce broad damped absorption characteristic of a Voigt profile. Consequently they imprint a component on the power spectrum whose modes in the radial direction are suppressed, leading to a scale-dependent bias. Using mock data sets of known HCD content, we test a model that describes this effect in terms of the distribution of column densities of HCDs, the Fourier transforms of their Voigt profiles and the bias of the halos containing the HCDs. Our results show that this physically well-motivated model describes the effects of HCDs with an accuracy comparable to that of the ad-hoc models used in published forest analyses. We also discuss the problems of applying the model to real data, where the HCD content and their bias is uncertain.

Lyman alpha forest↗

NASA NTRS1983

Spectral consequences of photoreceptor sampling in the rhesus retina

Optical transforms were used to compute the power spectra of rhesus cones treated as arrays of image sampling points. Spectra were obtained for the central fovea, parafovea, periphery, and far periphery. All were consistent with a novel spatial sampling principle that introduces minimal noise for spatial frequencies below the Nyquist limits implied by local receptor densities, while frequencies above the nominal Nyquist limits are not converted into conspicuous moiree patterns, but instead are scattered into broadband noise. This sampling scheme allows the visual system to escape aliasing distortion despite a large mismatch between retinal image bandwidth and the Nyquist limits implied by extrafoveal cone densities.

Yellott, J. I., Jr.↗

NASA NTRS2016

Improving Free-Piston Stirling Engine Power Density

Analyses and experiments demonstrate the potential benefits of optimizing piston and displacer motion in a free piston Stirling Engine. Isothermal analysis shows the theoretical limits of power density improvement due to ideal motion in ideal Stirling engines. More realistic models based on nodal analysis show that ideal piston and displacer waveforms are not optimal, often producing less power than engines that use sinusoidal piston and displacer motion. Constrained optimization using nodal analysis predicts that Stirling engine power density can be increased by as much as 58% using optimized higher harmonic piston and displacer motion. An experiment is conducted in which an engine designed for sinusoidal motion is forced to operate with both second and third harmonics, resulting in a maximum piston power increase of 14%. Analytical predictions are compared to experimental data showing close agreement with indirect thermodynamic power calculations, but poor agreement with direct electrical power measurements.

Stirling engines↗

NASA NTRS1978

Global circulation and distribution of hydrogen in thermosphere of Venus

The global density distribution of atomic H in the upper atmosphere of Venus is determined in terms of a global circulation model. It is shown that H produced on the dayside is efficiently advected to the nightside by the major gas CO2 where it is then convected to lower altitudes and recombines. A maximum night/day H density ratio of about 5 is derived, in which case exospheric return flow from night to day strongly limits the nightside density enhancement. A previously postulated constraint connecting the eddy diffusion coefficient with the escape flux of H in one dimensional models is no longer required when global circulation of H is considered.

Hartle, R. E.↗

NASA NTRS1982

Radio emission from Io

Radio observations of Io taken with the VLA at 2, 6, and 21 cm show flux densities of 11.6 + or - 0.2 mJy, and less than 0.5 mJy, respectively, where the latter is a 2 sigma upper limit. These flux densities correspond to Io disk brightness temperatures of T(b) (2 cm) = 98 + or - 17 K, T(b) (6 cm) = 85 + or - 16 K, and T(b) (21 cm) less than 400 K, respectively. These radio brightness temperatures are consistent with thermal emission expected from Io's surface on the basis of its infrared brightness temperature and its radio emissivity derived from radar studies. No evidence for nonthermal radio emission from Io is found, such as has been reported by Mingaliev et al (1979). By using a model for the generation of synchrotron emission at 21 cm by energetic electrons in a hypothetical Io magnetosphere, an upper limit of Io's dipole magnetic moment less than 2 x 10 to the 25th gauss/cu cm is found.

De Pater, I.↗

NASA NTRS1981

A compressed cloud in the Vela supernova remnant

Strong interstellar absorption lines of C I, arising from the two excited fine-structure levels, are found in IUE observations of HD 72350 (type B4 III). An analysis of the excited-level populations of C I gives local temperature and pressure limits, and auxiliary data on the limit of column density for excited O I and the carbon ionization help to establish that (1) the local temperature is within the limits of 25-100 K, and (2) the pressure/Boltzmann's constant ratio is at least 10 to the 4.3/cu cm K, despite its small size. This high-pressure cloud is discussed in terms of shock compression by the Vela supernova blast wave, along with the relationship of this kind of cloud compression to star formation and to the origin of the characteristic filamentary emission arcs seen in Vela and other supernova remnants

Jenkins, E. B.↗

NASA NTRS1992

The role of electron-positron pairs in parsec-scale radio jets

We investigate the hypothesis that the parsec-scale jets observed in extragalactic objects consist of an electron-positron pair plasma outflowing from the compact, X-ray emitting region of the source. Due to annihilation in the innermost regions, there is an upper limit on the number of electron-positron pairs that can reach the parsec-scale jet. On the other hand, there is a lower limit on the density of particles in the jet: their density must be at least equal to that needed to emit the synchrotron radiation we observe. We derive the latter limit by assuming a power-law electron distribution, with a low-energy cut-off. By comparing these two limits, we find that the constituents of parsec-scale jets may be electron-positron pairs produced in the innermost regions only if their minimum energy is greater than approximately 50 MeV. Remarkably, the same limit on the minimum particle energy is found in the case of jets consisting of electrons and protons in order that Faraday depolarization is avoided.

Ghisellini, G.↗

NASA NTRS1994

Far-ultraviolet emission from the Taurus molecular cloud

Far-ultraviolet spectra of diffuse emission from a region in the Taurus molecular cloud are presented. These data were obtained with the Berkeley spectrometer on NASA's Ultraviolet Experiment (UVX) payload. The intensity of the continuum emission drops as the optical depth increases toward the denser part of the cloud, an effect which is the opposite of that observed for the diffuse flux at high galactic latitudes. At the center of the aperture, the optical depth is very large, and the continuum emission must be produced by backscattering. At the edges of the cloud, the continuum intensity rises by a factor of 2. The data are interpreted with a new radiative transfer model, which is significantly more sophisticated than its predecessors. If the grain scattering properties are constant across the cloud, a unique best-fit combination can be determined from the data. The best-fit albedo is 0.6 +/- 0.1 at 1600 A. The best-fit value for the phase factor g is 0.5 +/- 0.15. The intensity of the flourescence of H2, combined with an upper limit to the column density of atomic hydrogen associated with the cloud, sets a lower limit of about 130/cm to the density in the flourescence region. Our estimate of the flux incident on the cloud at 1000 A is a factor of 2 to 3 lower than the value required to produce the measured flourescence intensity. Most mechanisms which might reconcile the discrepancy reduce the derived albedo and phase factor.

Hurwitz, M.↗

NASA NTRS1993

Engine materials characterization and damage monitoring by using x ray technologies

X ray attenuation measurement systems that are capable of characterizing density variations in monolithic ceramics and damage due to processing and/or mechanical testing in ceramic and intermetallic matrix composites are developed and applied. Noninvasive monitoring of damage accumulation and failure sequences in ceramic matrix composites is used during room-temperature tensile testing. This work resulted in the development of a point-scan digital radiography system and an in situ x ray material testing system. The former is used to characterize silicon carbide and silicon nitride specimens, and the latter is used to image the failure behavior of silicon-carbide-fiber-reinforced, reaction-bonded silicon nitride matrix composites. State-of-the-art x ray computed tomography is investigated to determine its capabilities and limitations in characterizing density variations of subscale engine components (e.g., a silicon carbide rotor, a silicon nitride blade, and a silicon-carbide-fiber-reinforced beta titanium matrix rod, rotor, and ring). Microfocus radiography, conventional radiography, scanning acoustic microscopy, and metallography are used to substantiate the x ray computed tomography findings. Point-scan digital radiography is a viable technique for characterizing density variations in monolithic ceramic specimens. But it is very limited and time consuming in characterizing ceramic matrix composites. Precise x ray attenuation measurements, reflecting minute density variations, are achieved by photon counting and by using microcollimators at the source and the detector. X ray computed tomography is found to be a unique x ray attenuation measurement technique capable of providing cross-sectional spatial density information in monolithic ceramics and metal matrix composites. X ray computed tomography is proven to accelerate generic composite component development. Radiographic evaluation before, during, and after loading shows the effect of preexisting volume flaws on the fracture behavior of composites. Results from one-, three-, five-, and eight-ply ceramic composite specimens show that x ray film radiography can monitor damage accumulation during tensile loading. Matrix cracking, fiber-matrix debonding, fiber bridging, and fiber pullout are imaged throughout the tensile loading of the specimens. In situ film radiography is found to be a practical technique for estimating interfacial shear strength between the silicon carbide fibers and the reaction-bonded silicon nitride matrix. It is concluded that pretest, in situ, and post-test x ray imaging can provide greater understanding of ceramic matrix composite mechanical behavior.

Baaklini, George Y.↗

NASA NTRS1981

The influence of cosmological gravitational waves on a Newtonian binary system

The interaction of a continuous gravitational wave with a Newtonian binary system is discussed, and the possibility of using the orbital perturbations to detect cosmological gravitational waves is investigated. The response of the binary system is dominated at late times by secular terms that appear in the orbital perturbations. The dominant secular terms are calculated, and it is shown that they can be used to put interesting upper limits on the energy density of cosmological gravitational waves. In particular, the recent studies of the Earth-Moon and Earth-Mars distances tentatively limit the energy density of the waves, in units of the closure density, to be less than 10 and 0.05 for incoherent waves with periods of 1 month and 1 year, respectively. The possibility of existence of cosmological waves with these periods is discussed.

Mashhoon, B.↗

NASA NTRS1989

Big Bang nucleosynthesis and the Quark-Hadron transition

An examination and brief review is made of the effects of quark-hadron transistion induced fluctuations on Big Bang nucleosynthesis. It is shown that cosmologically critical densities in baryons are difficult to reconcile with observation, but the traditional baryon density constraints from homogeneous calculations might be loosened by as much as 50 percent, to 0.3 of critical density, and the limit on the number of neutrino flavors remains about N(sub nu) is less than or approximately 4. To achieve baryon densities of greater than or approximately 0.3 of critical density would require initial density contrasts R is much greater the 10(exp 3), whereas the simplest models for the transition seem to restrict R to less than of approximately 10(exp 2).

Kurki-Suonio, Hannu↗

NASA NTRS1990

Big bang nucleosynthesis and the quark-hadron transition

An examination and brief review is made of the effects of quark-hadron transition induced fluctuations on Big Bang nucleosynthesis. It is shown that cosmologically critical densities in baryons are difficult to reconcile with observation, but the traditional baryon density constraints from homogeneous calculations might be loosened by as much as 50 percent, to 0.3 of critical density, and the limit on the number of neutrino flavors remains about N(sub nu) is less than or approximately 4. To achieve baryon densities of greater than or approximately 0.3 of critical density would require initial density contrasts R is much greater the 10(exp e), whereas the simplest models for the transition seem to restrict R to less than of approximately 10(exp 2).

Kurki-Suonio, Hannu↗

NASA NTRS1991

Limits to the lunar atmosphere

Apollo UV spectrometer experiment set limits on the density of oxygen of less than 500/cu cm, and the Apollo Lunar Atmospheric Composition Experiment data imply a value less than 50/cu cm above the subsolar point. These limits are surprisingly small relative to the measured value for sodium. A simple consideration of sources and sinks predicts significantly greater densities of oxygen. It is possible but doubtful that the Apollo measurements occurred during an epoch in which source rates were small. A preferential loss process for oxygen on the darkside of the moon is considered in which ionization by electron capture in surface collisions leads to escape through acceleration in the local electric field. Cold trapping in permanently shadowed regions as a net sink is considered and discounted, but the episodic nature of cometary insertion may allow formation of ice layers which act as a stabilized source of OH. On the basis of an assumed meteoroid impact source, a possible emission brightness of 50 R in the OH(A - X)(0,0) band above the lunar bright limb is predicted.

Morgan, T. H.↗

NASA NTRS1995

Effect Of Transition Density On Performance Of A DTTL

One estimates minimum transition density needed to limit symbol-error rate. Theoretical calculations and accompanying computer simulations provide numerical basis for specification of minimum data-transition density needed for adequate performance of radio receiver processing noisy signal containing non-return-to-zero (NRZ) stream of data symbols (bits). By use of digital-data-transition-tracking loop (DTTL), receiver strives to synchronize operation with data symbols. Performance of receiver in terms of ability to keep symbol-error rate (SER) in output of receiver below specified level depends on symbol signal-to-noise ratio (SSNR) of incoming signal and on degree of synchronization. Ability of DTTL to maintain synchronization depends partly on SSNR and partly on data-transition density.

Nguyen, Tien M.↗

NASA NTRS2016

Improving Power Density of Free-Piston Stirling Engines

Analyses and experiments demonstrate the potential benefits of optimizing piston and displacer motion in a free piston Stirling Engine. Isothermal analysis shows the theoretical limits of power density improvement due to ideal motion in ideal Stirling engines. More realistic models based on nodal analysis show that ideal piston and displacer waveforms are not optimal, often producing less power than engines that use sinusoidal piston and displacer motion. Constrained optimization using nodal analysis predicts that Stirling engine power density can be increased by as much as 58 using optimized higher harmonic piston and displacer motion. An experiment is conducted in which an engine designed for sinusoidal motion is forced to operate with both second and third harmonics, resulting in a maximum piston power increase of 14. Analytical predictions are compared to experimental data showing close agreement with indirect thermodynamic power calculations, but poor agreement with direct electrical power measurements.

space technology↗

NASA NTRS2016

Improving Power Density of Free-Piston Stirling Engines

Analyses and experiments demonstrate the potential benefits of optimizing piston and displacer motion in a free-piston Stirling Engine. Isothermal analysis shows the theoretical limits of power density improvement due to ideal motion in ideal Stirling engines. More realistic models based on nodal analysis show that ideal piston and displacer waveforms are not optimal, often producing less power than engines that use sinusoidal piston and displacer motion. Constrained optimization using nodal analysis predicts that Stirling engine power density can be increased by as much as 58 percent using optimized higher harmonic piston and displacer motion. An experiment is conducted in which an engine designed for sinusoidal motion is forced to operate with both second and third harmonics, resulting in a piston power increase of as much as 14 percent. Analytical predictions are compared to experimental data and show close agreement with indirect thermodynamic power calculations, but poor agreement with direct electrical power measurements.

aerospace engineering↗