2,501 research outputs found
Cobalt base superalloy has outstanding properties up to 1478 K (2200 F)
Alloy VM-103 is especially promising for use in applications requiring short time exposure to very high temperatures. Its properties over broad range of temperatures are superior to those of comparable commercial wrought cobalt-base superalloys, L-605 and HS-188
The Post-Common Envelope and Pre-Cataclysmic Binary PG 1224+309
We have made extensive spectroscopic and photometric observations of PG
1224+309, a close binary containing a DA white dwarf primary and an M4+
secondary. The H alpha line is in emission due to irradiation of the M-star by
the hot white dwarf and is seen to vary around the orbit. From the radial
velocities of the H alpha line we derive a period of P = 0.258689 +/- 0.000004
days and a semi-amplitude of K_Halpha = 160 +/- 8 km/s. We estimate a
correction Delta_K = 21 +/- 2 km/s, where K_M = K_Halpha + Delta_K. Radial
velocity variations of the white dwarf reveal a semi-amplitude of K_WD = 112
+/- 14 km/s. The blue spectrum of the white dwarf is well fit by a synthetic
spectrum having T_eff = 29,300 K and log(g) = 7.38. The white dwarf contributes
97% of the light at 4500 Angstroms and virtually all of the light blueward of
3800 Angstroms. No eclipses are observed. The mass inferred for the white dwarf
depends on the assumed mass of the thin residual hydrogen envelope: 0.40 < M_WD
< 0.45 solar masses for hydrogen envelope masses of 0 < M_H < 4.0E-4 solar
masses. We argue that the mass of the white dwarf is closer to 0.45 solar
masses, hence it appears that the white dwarf has a relatively large residual
hydrogen envelope. The mass of the M-star is then M_M = 0.28 +/- 0.05 solar
masses, and the inclination is i = 77 +/- 7 degrees. We discuss briefly how PG
1224+309 may be used to constrain theories of close binary star evolution, and
the past and future histories of PG 1224+309 itself. The star is both a
``post-common envelope'' star and a ``pre-cataclysmic binary'' star. Mass
transfer by Roche-lobe overflow should commence in about 10 Gyr.Comment: 17 pages, 8 figures, AAS LaTeX, to appear in AJ, March 199
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Diffusion Kinetics Studies in the Al-Ni-U System
A diffusion couple technique was developed and standardized for the purpose of obtaining meaningful data regarding the diffusion kinetics of the Al-- Ni--U system. The technique entailed hot pressing under a dynamic vacuum and subsequent isothermal soaking treatments within evacuated quartz tubes for periods of 10 to 7000 hr at temperatures of 850, 750, 850, andd 950 deg F. In addition, the nickel barrier bond between Type 1100 aluminum cladding and cast U- 3.5% Mo fuel alloy was metallographically examined on four fuel cylinders fabricated according to Piqua procedure. One cylinder was inspected in the as fabricated'' conditions, and the three others after 1000, 2000, and 4000 hr of isothemnal soaking in Santowax OMP at 750 deg F in an autoclave under 100 psig of nitrogen. The following conclusions were obtained: 1) The penetration of aluminum into nickel followed a parabolic relationship with time. The penetration coefficient was graphically determined as a function of temperature. 2) The nickel penetration data generated by the diffusion couple technique were slightly higher thand those derived from the cylinder bond study. The maximum discrepancy was 20%. This disagreement was attributed to the geometry of the tubular specimens and to the interface displacements which occurred during the annealing process due to differential thermal expandsion between fuel and cladding, both at the OD and ID. 3) A low-aluminum phase AlN/sub 3/ or AlNi/sub 3/ nucleated at the Al/sub 3/Ni/Al/sub 3/Ni/sub 2/ interf ace when the supply of additional free Al into the system was restricted due to void formation or separation between intermetallics and Al. The growth of the low-aluminum phase progressed at the expense of the Al/sub 3/Ni until all this phase was consumed, then consumption of the Al/sub 3/Ni/sub 2/ phase began. 4) The interdiffusion rate of uranium and nickel also followed the parabolic relationship with time. The diffusion coefficient as a function of temperature was plotted. (auth
Lattice-Boltzmann and finite-difference simulations for the permeability for three-dimensional porous media
Numerical micropermeametry is performed on three dimensional porous samples
having a linear size of approximately 3 mm and a resolution of 7.5 m. One
of the samples is a microtomographic image of Fontainebleau sandstone. Two of
the samples are stochastic reconstructions with the same porosity, specific
surface area, and two-point correlation function as the Fontainebleau sample.
The fourth sample is a physical model which mimics the processes of
sedimentation, compaction and diagenesis of Fontainebleau sandstone. The
permeabilities of these samples are determined by numerically solving at low
Reynolds numbers the appropriate Stokes equations in the pore spaces of the
samples. The physical diagenesis model appears to reproduce the permeability of
the real sandstone sample quite accurately, while the permeabilities of the
stochastic reconstructions deviate from the latter by at least an order of
magnitude. This finding confirms earlier qualitative predictions based on local
porosity theory. Two numerical algorithms were used in these simulations. One
is based on the lattice-Boltzmann method, and the other on conventional
finite-difference techniques. The accuracy of these two methods is discussed
and compared, also with experiment.Comment: to appear in: Phys.Rev.E (2002), 32 pages, Latex, 1 Figur
Bubble collisions and measures of the multiverse
To compute the spectrum of bubble collisions seen by an observer in an
eternally-inflating multiverse, one must choose a measure over the diverging
spacetime volume, including choosing an "initial" hypersurface below which
there are no bubble nucleations. Previous calculations focused on the case
where the initial hypersurface is pushed arbitrarily deep into the past.
Interestingly, the observed spectrum depends on the orientation of the initial
hypersurface, however one's ability observe the effect rapidly decreases with
the ratio of inflationary Hubble rates inside and outside one's bubble. We
investigate whether this conclusion might be avoided under more general
circumstances, in particular placing the observer's bubble near the initial
hypersurface. We find that it is not. As a point of reference, a substantial
appendix reviews relevant aspects of the measure problem of eternal inflation.Comment: 24 pages, two figures, plus 16-page appendix with one figure; v2:
minor improvements and clarifications, conclusions unchanged (version to
appear in JCAP
High altitude diving in river otters: coping with combined hypoxic stresses
River otters (Lontra canadensis) are highly active, semi-aquatic mammals indigenous to a range of elevations and represent an appropriate model for assessing the physiological responses to diving at altitude. In this study, we performed blood gas analyses and compared blood chemistry of river otters from a high-elevation (2357 m) population at Yellowstone Lake with a sea-level population along the Pacific coast. Comparisons of oxygen dissociation curves (ODC) revealed no significant difference in hemoglobin-oxygen (Hb-O2) binding affinity between the two populations - potentially because of demands for tissue oxygenation. Instead, high-elevation otters had greater Hb concentrations (18.7 g dl-1) than sea-level otters (15.6 g dl-1). Yellowstone otters displayed higher levels of the vasodilator nitric oxide (NO), and half the concentration of the serum protein albumin, possibly to compensate for increased blood viscosity. Despite compensation in several hematological and serological parameters, theoretical aerobic dive limits (ADL) were similar between high-elevation and sea-level otters because of the lower availability of O2 at altitude. Our results suggest that recent disruptions to the Yellowstone Lake food web could be detrimental to otters because at this high elevation, constraints on diving may limit their ability to switch to prey in a deep-water environment
Variance as a predictor of health outcomes: Subject-level trajectories and variability of sex hormones to predict body fat changes in peri- and post-menopausal women
Longitudinal biomarker data and cross-sectional outcomes are routinely
collected in modern epidemiology studies, often with the goal of informing
tailored early intervention decisions. For example, hormones such as estradiol
and follicle-stimulating hormone may predict changes in womens' health during
the midlife. Most existing methods focus on constructing predictors from mean
marker trajectories. However, subject-level biomarker variability may also
provide critical information about disease risks and health outcomes. In this
paper, we develop a joint model that estimates subject-level means and
variances of longitudinal biomarkers to predict a cross-sectional health
outcome. Simulations demonstrate excellent recovery of true model parameters.
The proposed method provides less biased and more efficient estimates, relative
to alternative approaches that either ignore subject-level differences in
variances or perform two-stage estimation where estimated marker variances are
treated as observed. Analyses of women's health data reveal larger variability
of E2 or larger variability of FSH were associated with higher levels of fat
mass change and higher levels of lean mass change across the menopausal
transition
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