379 research outputs found
Transition from film boiling to nucleate boiling in forced convection vertical flow
The mechanism of collapse of forced cnnvection annular vertical flow film boiling, with liquid core, is investigated using liquid nitrogen at low pressures. The report includes the effect of heat flux from the buss bar. Tests include runs with mass fluxes varying from 44,000 lbm/hr-ft2 2 2 to 186,000 lbm/hr-ft , and buss bar heat fluxes from 0 to 107,000 BTU/hr-ft The channel was a 0.4 inch I. D. by 0.5 inch O.D. by 8 feet long Inconel 600 tube. Two modes of collapse were isolated, in the absence of rewet by dispersed cooling within the mist flow region. These were axial conduction controlled collapse originating at the entrance to the test section for zero or negative buss bar heat flux; and impulse cooling collapse originating downstream of the entrance for heat into the test section from the buss bar. Collapse heat flux was found to be a function of mass flux, but the collapse wall temperature difference (T - T ) was independent w S of mass flux and could be successfully predicted within 6% by pool boiling minimum transition correlation (e.g. Berenson [5]).Sponsored by the National Science Foundation DSR Projec
Long-Term Evolution of Massive Black Hole Binaries. II. Binary Evolution in Low-Density Galaxies
We use direct-summation N-body integrations to follow the evolution of binary
black holes at the centers of galaxy models with large, constant-density cores.
Particle numbers as large as 400K are considered. The results are compared with
the predictions of loss-cone theory, under the assumption that the supply of
stars to the binary is limited by the rate at which they can be scattered into
the binary's influence sphere by gravitational encounters. The agreement
between theory and simulation is quite good; in particular, we are able to
quantitatively explain the observed dependence of binary hardening rate on N.
We do not verify the recent claim of Chatterjee, Hernquist & Loeb (2003) that
the hardening rate of the binary stabilizes when N exceeds a particular value,
or that Brownian wandering of the binary has a significant effect on its
evolution. When scaled to real galaxies, our results suggest that massive black
hole binaries in gas-poor nuclei would be unlikely to reach gravitational-wave
coalescence in a Hubble time.Comment: 13 pages, 8 figure
Doping and dimensionality effects on the core-level spectra of layered ruthenates
Core-level spectra of the Mn-doped Sr3Ru2O7 and Srn+1RunO3n+1 (n = 1, 2 and
3) crystals are investigated with X-ray photoelectron spectroscopy. Doping of
Mn to Sr3Ru2O7 considerably affects the distribution of core-level spectral
weight. The satellite of Ru 3d core levels exhibits a substantial change with
doping, indicating an enhanced electron localization across the doping- induced
metal-insulator transition. However, the Ru 3p core levels remain identical
with Mn-doping, thus showing no sign of doping-induced multiple Ru valences. In
the Srn+1RunO3n+1 (n = 1, 2 and 3), the Ru 3d core-level spectra are similar,
indicating that the chemical bonding environment around Ru ions remains the
same for different layered compounds. Meanwhile the Sr 3d shallow core levels
shift to higher binding energy with increasing n, suggesting their
participation in Sr-O bonding with structural evolution.Comment: 6 pages with 6 figures, to be published in PR
Psychosocial outcomes of an inclusive adapted sport and adventurous training course for military personnel.
PURPOSE: To explore the psychosocial outcomes of an inclusive adapted sport and adventurous training course that aims to support the rehabilitation and personal development of military personnel who have sustained physical and/or psychological disability. METHOD: Narrative life story interviews were conducted with 11 men aged 20-43 taking part in one of the 5-day courses. A thematic narrative analysis was conducted, focusing on accounts that provided insights into personally meaningful psychosocial outcomes of the course. FINDINGS: We identified six themes, falling into two distinct clusters. "Bringing me back to myself" was achieved through the themes of (1) returning to activity, (2) rediscovering a sense of purpose, and (3) reconnecting to others. "New rooms to explore" was realised through (4) experiencing new activities, (5) being valued/respected/cared for and (6) being inspired by other people. CONCLUSION: Involvement in the course stimulated a balance of present- and future-oriented psychosocial outcomes through which participants both recreated aspects of themselves that had been lost through injury/trauma and moved forward with their lives as a result of new horizons of possibility. IMPLICATIONS FOR REHABILITATION: This 5-day inclusive adapted sport and adventurous training course offered meaningful psychosocial outcomes among military personnel who had experienced physical and/or psychological disability. The course helped participants recover aspects of their previous life and self through becoming physically active again, rediscovering a sense of purpose and reconnecting to others. Participants describe a broadening of life horizons as a result of the course, through new activities, being valued/respected/cared for, and being inspired by other people
A Note on Gravitational Brownian Motion
Chandrasekhar's theory of stellar encounters predicts a dependence of the
Brownian motion of a massive particle on the velocity distribution of the
perturbing stars. One consequence is that the expectation value of the massive
object's kinetic energy can be different from that of the perturbers. This
effect is shown to be modest however, and substantially smaller than claimed in
a recent study based on a more approximate treatment of the encounter
equations.Comment: 7 pages, 3 figure
A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: IV. Modeling the Sagittarius Tidal Tails
M giants recovered from the Two Micron All-Sky Survey (2MASS) have recently
been used to map the position and velocity distributions of tidal debris from
the Sagittarius (Sgr) dwarf spheroidal galaxy entirely around the Galaxy. We
compare this data set to both test particle orbits and N-body simulations of
satellite destruction run within a variety of rigid Milky Way potentials and
find that the mass of the Milky Way within 50 kpc of its center should be
3.8-5.6 x 10^11 Msun in order for any Sgr orbit to simultaneously fit the
velocity gradient in the Sgr trailing debris and the apocenter of the Sgr
leading debris. Orbital pole precession of young debris and leading debris
velocities in regions corresponding to older debris provide contradictory
evidence in favor of oblate/prolate Galactic halo potentials respectively,
leading us to conclude that the orbit of Sgr has evolved over the past few Gyr.
Based upon the velocity dispersion and width along the trailing tidal stream
we estimate the current bound mass of Sgr to be M_Sgr = 2 - 5 x 10^8 Msun
independant of the form of the Galactic potential; this corresponds to a range
of mass to light ratios (M/L)_Sgr = 14 - 36 (M/L)_Sun for the Sgr core. Models
with masses in this range best fit the apocenter of leading Sgr tidal debris
when they orbit with a radial period of roughly 0.85 Gyr and have periGalactica
and apoGalactica of about 15 kpc and 60 kpc respectively. These distances will
scale with the assumed distance to the Sgr dwarf and the assumed depth of the
Galactic potential. The density distribution of debris along the orbit in these
models is consistent with the M giant observations, and debris at all orbital
phases where M giants are obviously present is younger (i.e. was lost more
recently from the satellite) than the typical age of a Sgr M giant star.Comment: 42 pages, 13 figures; Accepted for publication by ApJ (October 08,
2004; originally submitted May 10, 2004). Fixed typos and added references.
PDF file with high resolution figures may be downloaded from
http://www.astro.caltech.edu/~drlaw/Papers/Sgr_paper4.pd
Envelope Structure of Starless Core L694-2 Derived from a Near-Infrared Extinction Map
We present a near-infrared extinction study of the dark globule L694-2, a
starless core that shows strong evidence for inward motions in molecular line
profiles. The J,H, and K band data were taken using the European Southern
Observatory New Technology Telescope. The best fit simple spherical power law
model has index p=2.6 +/- 0.2, over the 0.036--0.1 pc range in radius sampled
in extinction. This power law slope is steeper than the value of p=2 for a
singular isothermal sphere, the initial condition of the inside-out model for
protostellar collapse. Including an additional extinction component along the
line of sight further steepens the inferred profile. Fitting a Bonnor-Ebert
sphere results in a super-critical value of the dimensionless radius xi_max=25
+/- 3. The unstable configuration of material may be related to the observed
inward motions. The Bonnor-Ebert model matches the shape of the observed
profile, but significantly underestimates the amount of extinction (by a factor
of ~4). This discrepancy in normalization has also been found for the nearby
protostellar core B335 (Harvey et al. 2001). A cylindrical density model with
scale height H=0.0164+/- 0.002 pc viewed at a small inclination to the cylinder
axis provides an equally good radial profile as a power law model, and
reproduces the asymmetry of the core remarkably well. In addition, this model
provides a basis for understanding the discrepancy in the normalization of the
Bonnor-Ebert model, namely that L694-2 has prolate structure, with the full
extent (mass) of the core being missed by assuming symmetry between the
profiles in the plane of the sky and along the line-of-sight. If the core is
sufficiently magnetized then fragmentation may be avoided, and later evolution
might produce a protostar similar to B335.Comment: 38 pages, 7 figures, accepted to Astrophysical Journa
Core Collapse via Coarse Dynamic Renormalization
In the context of the recently developed "equation-free" approach to
computer-assisted analysis of complex systems, we extract the self-similar
solution describing core collapse of a stellar system from numerical
experiments. The technique allows us to side-step the core "bounce" that occurs
in direct N-body simulations due to the small-N correlations that develop in
the late stages of collapse, and hence to follow the evolution well into the
self-similar regime.Comment: 5 pages, 3 figure
The need to promote behaviour change at the cultural level: one factor explaining the limited impact of the MEMA kwa Vijana adolescent sexual health intervention in rural Tanzania. A process evaluation
Background - Few of the many behavioral sexual health interventions in Africa have been rigorously evaluated. Where biological outcomes have been measured, improvements have rarely been found. One of the most rigorous trials was of the multi-component MEMA kwa Vijana adolescent sexual health programme, which showed improvements in knowledge and reported attitudes and behaviour, but none in biological outcomes. This paper attempts to explain these outcomes by reviewing the process evaluation findings, particularly in terms of contextual factors.
Methods - A large-scale, primarily qualitative process evaluation based mainly on participant observation identified the principal contextual barriers and facilitators of behavioural change.
Results - The contextual barriers involved four interrelated socio-structural factors: culture (i.e. shared practices and systems of belief), economic circumstances, social status, and gender. At an individual level they appeared to operate through the constructs of the theories underlying MEMA kwa Vijana - Social Cognitive Theory and the Theory of Reasoned Action – but the intervention was unable to substantially modify these individual-level constructs, apart from knowledge.
Conclusion - The process evaluation suggests that one important reason for this failure is that the intervention did not operate sufficiently at a structural level, particularly in regard to culture. Recently most structural interventions have focused on gender or/and economics. Complementing these with a cultural approach could address the belief systems that justify and perpetuate gender and economic inequalities, as well as other barriers to behaviour change
Solution Geochemistry of the Water of Limestone Terrains
Limestone groundwater flows mainly in openings it has solutionally enlarged, thus an understanding of the water\u27s state of saturation relative to calcite (the principal mineral component of limestone) is fundamental to an understanding of the nature and evolution of the limestone aquifer. This study investigated the Mammoth Cave-Sinkhole Plain (MCSP) and Cave Hollow (CH) aquifers in Kentucky, both in Missippian limestones.
Both aquifers were always undersaturated with calcite. Except for completely ventilated vadose flows (usually) and some vadose seepage (occasionally), all recharges sampled (sinking streams, vadose flows, and vadose seepage) were also undersaturated. The lack of saturation in the MCSP aquifer was due to the introduction of carbon dioxide into the water in amounts difficult to explain by the carbon dioxide content of the above recharges. In both vadose flows and seepage, undersaturatlon tended to correlate directly with flow volume, and there was an inverse correlation between the amount of carbon dioxide and calcite saturation in most of the waters sampled. In vadose seepage this relationship was so strong as to suggest seasonal invariance of carbon dioxide content of the water prior to out gassing.
Results suggest solutional enlargement is greatest near recharge points in ventilated aquifers (CH) but the carbon dioxide introduction phenomenon (MCSP) allows solution over wide areas in unventilated aquifers
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