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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Voids in a neutrino-dominated universe

In a neutrino-dominated universe, galaxies are expected to form only in large-scale sheets and filaments. Most of space should be filled by low-density regions devoid of galaxies. In this paper, N-body simulations are used to estimate the size distribution for these regions for quantitative comparison with the observed voids in recent red-shift surveys. The theoretical distribution depends very weakly on the mode or epoch of galaxy formation. With very conservative assumptions, at best marginal consistency is found even for cosmological parameters as extreme as Omega(v) = 1 and H(0) = 100 km/s per/Mpc. Any significant reduction in either H(0) or Omega(v) leads to predicted void sizes much larger than those observed. This difficulty arises because the observed voids are rarely completely empty.

Zeng, Ning↗
NASA NTRS1993

Modeling void growth and movement with phase change in thermal energy storage canisters

A scheme was developed to model the thermal hydrodynamic behavior of thermal energy storage salts. The model included buoyancy, surface tension, viscosity, phases change with density difference, and void growth and movement. The energy, momentum, and continuity equations were solved using a finite volume formulation. The momentum equation was divided into two pieces. The void growth and void movement are modeled between the two pieces of the momentum equations. Results showed this scheme was able to predict the behavior of thermal energy storage salts.

Darling, Douglas↗
NASA NTRS1993

Modeling void growth and movement with phase change in thermal energy storage canisters

A scheme was developed to model the thermal hydrodynamic behavior of thermal energy storage salts. The model included buoyancy, surface tension, viscosity, phases change with density difference, and void growth and movement. The energy, momentum, and continuity equations were solved using a finite volume formulation. The momentum equation was divided into two pieces. The void growth and void movement are modeled between the two pieces of the momentum equations. Results showed this scheme was able to predict the behavior of thermal energy storage salts.

Darling, Douglas↗
NASA NTRS1997

Comparison of the Effects of Debonds and Voids in Adhesive Joints

An analytical model is developed to compare the effects of voids an debonds on the interfacial shear stresses between the adherends and the adhesive in simple lap joints. Since the adhesive material above the debond may undergo some extension (either due to applied load or thermal expansion or both), a modified shear lag model, where the adhesive can take an extensional as well as shear deformation, is used in the analysis. The adherends take on only axial loads and act as membranes. Two coupled nondimensional differential equations are derived, and in general, five parameters govern the stress distribution in the overlap region. As expected, the major differences between the debond and the void occur for the stresses near the edge of the defect itself. Whether the defect is a debond or a void, is hardly discernible by the stresses at the overlap region. If the defect occurs precisely at or very close to either end of the overlap, however, differences of the order of 20 percent in the peak stresses can be obtained.

Rossettos, J. N.↗
NASA NTRS2001

Detection of X-Ray Emission from Galaxies Inside the Bootes Void

We report the X-ray properties of Bootes void galaxies detected by the ROSAT All-Sky Survey (RASS). By searching the fields of 26 radio and 27 emission-line-selected void galaxies, we have detected nine X-ray-emitting void galaxies at greater than 2.7 sigma confidence level. Five of these nine galaxies are in the IRAS subsample. Two of the IRAS galaxies were previously identified in positional cross-correlation studies of the RASS and IRAS Point Source Catalogue sources. Three of the X-ray-emitting galaxies are active galactic nuclei (AGNs; IRAS 14288+5255, Mrk 845, and IRAS 15195+5050), three are emission-line galaxies (PC 1357+4641, CG 547, and CG 922), and the remaining three are of unknown spectral type (IRAS 14500+4804, CG 637, and MAS 15092+3940). The far-infrared flux levels of the AGN sources imply that most of the observed X-ray emissivity is from starburst activity. We have carried out timing and spectral analysis for the Seyfert 1 galaxy Mrk 845. Poor statistics prevents detailed analysis of the remaining sources. Only two galaxies in our sample, BHI 1514+3819 and FSS 1515+3823, were observed during ROSAT pointed observations resulting in a nondetection at the 1 sigma level.

Kim, Chulhee↗
NASA NTRS1999

Quantifying Void Ratio Variation in Sand using Computed Tomography

A series of displacement-controlled, conventional, drained axisymmetric (triaxial) experiments were conducted on dry Ottawa sand specimens at very low effective confining stresses in a microgravity environment aboard the Space Shuttle during the NASA STS-89 mission. Post-flight analysis included studying the internal fabric and failure patterns of these specimens using Computed Tomography (CT). The CT scans of three specimens subjected to different compression levels (uncompressed specimen, a specimen compressed to 3.3% nominal axial strain (epsilon(sub a)), and a specimen compressed to 25% epsilon(sub a)) are presented to investigate the evolution of instability patterns and to quantify void ratio variation. The progress of failure is described and discussed. Also, specimens' densities were calibrated using standard ASTM procedures and void ratio spatial variation was calculated and represented by contour maps and histograms. The CT technique demonstrated good ability to detect specimen inhomogeneities, localization patterns, and quantifying void ratio variation within sand specimens.

Alshibli, Khalid A.↗
NASA NTRS2004

Gas-liquid Phase Distribution and Void Fraction Measurements Using the MRI

We used a permanent-magnet MRI system to estimate the integral and spatially- and/or temporally-resolved void-fraction distributions and flow patterns in gas-liquid two-phase flows. Air was introduced at the bottom of the stagnant liquid column using an accurate and programmable syringe pump. Air flow rates were varied between 1 and 200 ml/min. The cylindrical non-conducting test tube in which two-phase flow was measured was placed in a 2.67 kGauss MRI with MRT spectrometer/imager. Roughly linear relationship has been obtained for the integral void-fraction, obtained by volume-averaging of the spatially-resolved signals, and the air flow rate in upward direction. The time-averaged spatially-resolved void fraction has also been obtained for the quasi-steady flow of air in a stagnant liquid column. No great accuracy is claimed as this was an exploratory proof-of-concept type of experiment. Preliminary results show that MRI a non-invasive and non-intrusive experimental technique can indeed provide a wealth of different qualitative and quantitative data and is especially well suited for averaged transport processes in adiabatic and diabatic multi-phase and/or multi-component flows.

Daidzic, N. E.↗
NASA NTRS2007

Using Digital Radiography To Image Liquid Nitrogen in Voids

Digital radiography by use of (1) a field-portable x-ray tube that emits low-energy x rays and (2) an electronic imaging x-ray detector has been found to be an effective technique for detecting liquid nitrogen inside voids in thermal-insulation panels. The technique was conceived as a means of investigating cryopumping (including cryoingestion) as a potential cause of loss of thermal insulation foam from space-shuttle external fuel tanks. The technique could just as well be used to investigate cryopumping and cryoingestion in other settings. In images formed by use of low-energy x-rays, one can clearly distinguish between voids filled with liquid nitrogen and those filled with gaseous nitrogen or other gases. Conventional film radiography is of some value, but yields only non-real-time still images that do not show time dependences of levels of liquids in voids. In contrast, the present digital radiographic technique yields a succession of images in real time at a rate of about 10 frames per second. The digitized images can be saved for subsequent analysis to extract data on time dependencies of levels of liquids and, hence, of flow paths and rates of filling and draining. The succession of images also amounts to a real-time motion picture that can be used as a guide to adjustment of test conditions.

Cox, Dwight↗
NASA NTRS2022

Effects of Debulking on the Fiber Microstructure and Void Distribution in Carbon Fiber Reinforced Plastics

Carbon Fiber Reinforced Plastics (CFRPs) are widely used due to their high stiffness to weight ratios. A common process manufacturers use to increase the strength to weight ratio is debulking. Debulking is the process of compacting a dry fibrous reinforcement prior to resin infusion. This process is meant to decrease the average inter-fiber distance, effectively increasing the fiber volume fraction of the sample. While this process is widely understood macroscopically its effects on fibrous microstructures have not yet been well characterized. The aim of this work is to compare the microstructures of three CFRP laminates, varying only the debulking step in the manufacturing process. High resolution serial sections of all three laminates were taken for analysis. Using these scans, the fiber positions were reconstructed. Statistical descriptors such as local fiber and void volume fractions, fiber orientation, and void distribution and morphology were then generated for each sample. Fiber clusters present within the material were identified and analyzed for each level of debulking applied. Using these descriptors, the effects of debulking on the morphology and organization of the composite microstructure was evaluated.

carbon fiber↗
NASA NTRS1976

Phase relationship in three-phase composites which include a void phase

The paper shows the relationship among polymer, particles, and voids in a three-phase composite and how some of the properties of a composite may be changed by changing the proportions of the phases. The three-phase composite is an aggregate of microspheres bonded together with a small amount of polymer which may not form a continuous matrix. The void space (third phase) is obtained by limiting the amount of polymer which is mixed with the microspheres. A ternary phase diagram is used to show the proportional relationship among the three phases, with each apex representing a volume fraction of unity for a constituent while the side opposite the apex represents a volume fraction of zero for that constituent. The vertical dimension represents some composite property such as density or strength. The effect of composition on composite properties is shown by plotting them on a binary phase diagram which represents a perpendicular plane coincident with the 0.60 volume fraction microsphere line.

Price, H. L.↗
NASA NTRS1982

Large-scale superclusters surrounding the giant galaxy void in Bootes

The overdensity of galaxies observed by Kirshner et al. (1981) on both redshift sides of the galaxy void in Bootes is found to coincide in redshift space with similar overdensities of clusters and superclusters of galaxies in the northern hemisphere. The main contributors to these overdensities are the rich Hercules and Corona Borealis superclusters. It is suggested that large-scale density enhancements of galaxies associated with the Hercules and Corona Borealis superclusters may extend to more than about 100-150/h Mpc in projection and surround the giant volume of galaxy depletion in Bootes. Previous observational evidence, together with the results presented here, suggests that galaxy voids may generally be associated with surrounding galaxy excesses.

Bahcall, N. A.↗
NASA NTRS1983

Dielectric Scanning Locates Voids in Glass Foam

Capacitive probes succeed where X-ray and ultrasonic equipment do not. Crossbar switching of pairs of conductors enables detection of dielectric inhomogeneities in selected small volume elements of slab of material. Large void in sample detected as low capacitance reading for volume element containing void. Applicable in nondestructive testing of other dielectric objects for internal defects that exhibit dielectric inhomogeneities.

Gammell, P. M.↗
NASA NTRS1983

The properties of coronal voids

Skylab coronagraph observations of the outer solar corona have revealed dark, ray-like structures whose identification as a true coronal phenomenon rather than a subtle photographic effect is presently undertaken. Photometric analysis indicates that these features represent a reduction of 5% in coronal radiance, and the use of a previously developed model of the coronal electron component allows the specification of void electron densities over a 2.5-4.5 solar radius range. Projection of the phenomena onto synoptic surface maps indicates a close relationship with filaments and neutral lines, and a potentially significant temporal correlation between void formation and the formation of the underlying prominence is noted.

Macqueen, R. M.↗
NASA NTRS1986

Statistics of voids in hierarchical universes

As one alternative to the N-point galaxy correlation function statistics, the distribution of holes or the probability that a volume of given size and shape be empty of galaxies can be considered. The probability of voids resulting from a variety of hierarchical patterns of clustering is considered, and these are compared with the results of numerical simulations and with observations. A scaling relation required by the hierarchical pattern of higher order correlation functions is seen to be obeyed in the simulations, and the numerical results show a clear difference between neutrino models and cold-particle models; voids are more likely in neutrino universes. Observational data do not yet distinguish but are close to being able to distinguish between models.

Fry, J. N.↗
NASA NTRS1988

Artificial Voids In Ceramic Materials

Method for creating voids in ceramic specimens developed. Silicon carbide and silicon nitride are high-temperature structural ceramic materials considered for applications in advanced gas-turbine engines. Ability to detect and characterize voids (by sizes, shapes, and locations) in structural ceramics vital for increasing strengths and reliabilities of materials. Small holes made deliberately to help quantify techniques of nondestructive evaluation.

Roth, Don J.↗
NASA NTRS1988

Clustering and voids in the Lyman-alpha forest

In a statistical study of quasar absorption lines, no evidence is found for large voids in the Ly-alpha forest, contrary to some recent suggestions. The 28 Mpc gap found by Crotts (1987) in the spectrum of Q0420-388 is not statistically significant, and it is possible, from a sample of 18 quasar spectra, to reject at the 2 sigma level the hypothesis that comoving voids of 30 Mpc fill as much as 15 percent of the universe. However, a significant (at the 99 percent level) excess of small line intervals of velocity width 200-600 km/s is found. This provides evidence either for the gravitationally induced correlations proposed in some models or for the fragmented shells proposed in others.

Ostriker, Jeremiah P.↗
NASA NTRS1989

The generalized drift flux approach: Identification of the void-drift closure law

The main characteristics and the potential advantages of generalized drift flux models are presented. In particular it is stressed that the issue on the propagation properties and on the mathematical nature (hyperbolic or not) of the model and the problem of closure are easier to tackle than in two fluid models. The problem of identifying the differential void-drift closure law inherent to generalized drift flux models is then addressed. Such a void-drift closure, based on wave properties, is proposed for bubbly flows. It involves a drift relaxation time which is of the order of 0.25 s. It is observed that, although wave properties provide essential closure validity tests, they do not represent an easily usable source of quantitative information on the closure laws.

Boure, J. A.↗