3,613 research outputs found
Viscous Withdrawal of Miscible Liquid Layers
In viscous withdrawal, a converging flow imposed in an upper layer of viscous
liquid entrains liquid from a lower, stably stratified layer. Using the idea
that a thin tendril is entrained by a local straining flow, we propose a
scaling law for the volume flux of liquid entrained from miscible liquid
layers. A long-wavelength model including only local information about the
withdrawal flow is degenerate, with multiple tendril solutions for one
withdrawal condition. Including information about the global geometry of the
withdrawal flow removes the degeneracy while introducing only a logarithmic
dependence on the global flow parameters into the scaling law.Comment: 4 pages, 4 figure
Axially-homogeneous Rayleigh-Benard convection in a cylindrical cell
Previous numerical studies have shown that the "ultimate regime of thermal
convection" can be attained in a Rayleigh-Benard cell when the kinetic and
thermal boundary layers are eliminated by replacing the walls with periodic
boundary conditions (homogeneous Rayleigh-Benard convection). Then, the heat
transfer scales like Nu ~ Ra^{1/2} and turbulence intensity as Re ~ Ra^{1/2},
where the Rayleigh number Ra indicates the strength of the driving force.
However, experiments never operate in unbounded domains and it is important to
understand how confinement might alter the approach to this ultimate regime.
Here we consider homogeneous Rayleigh-Benard convection in a laterally confined
geometry - a small aspect-ratio vertical cylindrical cell - and show evidence
of the ultimate regime as Ra is increased: In spite of the confinement and the
resulting kinetic boundary layers, we still find Nu ~ Re ~ Ra^{1/2}. The system
supports exact solutions composed of modes of exponentially growing vertical
velocity and temperature fields, with Ra as the critical parameter determining
the properties of these modes. Counterintuitively, in the low Ra regime, or for
very narrow cylinders, the numerical simulations are susceptible to these
solutions which can dominate the dynamics and lead to very high and unsteady
heat transfer. As Ra is increased, interaction between modes stabilizes the
system, evidenced by the increasing homogeneity and reduced fluctuations in the
r.m.s. velocity and temperature fields. We also test that physical results
become independent of the periodicity length of the cylinder, a purely
numerical parameter, as the aspect ratio is increased
Riparian vegetation, Colorado River, and climate: Five decades of spatiotemporal dynamics in the Grand Canyon with river regulation
Documentation of the interacting effects of river regulation and climate on riparian vegetation has typically been limited to small segments of rivers or focused on individual plant species. We examine spatiotemporal variability in riparian vegetation for the Colorado River in Grand Canyon relative to river regulation and climate, over the five decades since completion of the upstream Glen Canyon Dam in 1963. Long-term changes along this highly modified, large segment of the river provide insights for management of similar riparian ecosystems around the world. We analyze vegetation extent based on maps and imagery from eight dates between 1965 and 2009, coupled with the instantaneous hydrograph for the entire period. Analysis confirms a net increase in vegetated area since completion of the dam. Magnitude and timing of such vegetation changes are river stage-dependent. Vegetation expansion is coincident with inundation frequency changes and is unlikely to occur for time periods when inundation frequency exceeds approximately 5%. Vegetation expansion at lower zones of the riparian area is greater during the periods with lower peak and higher base flows, while vegetation at higher zones couples with precipitation patterns and decreases during drought. Short pulses of high flow, such as the controlled floods of the Colorado River in 1996, 2004, and 2008, do not keep vegetation from expanding onto bare sand habitat. Management intended to promote resilience of riparian vegetation must contend with communities that are sensitive to the interacting effects of altered flood regimes and water availability from river and precipitation. å©2015. American Geophysical Union. All Rights Reserved
A Composite Seyfert 2 X-ray Spectrum: Implications for the Origin of the Cosmic X-ray Background
We present a composite 1-10 keV Seyfert 2 X-ray spectrum, derived from ASCA
observations of a distance-limited sample of nearby galaxies. All 29 observed
objects were detected. Above ~3 keV, the composite spectrum is inverted,
confirming that Seyfert 2 galaxies as a class have the spectral properties
necessary to explain the flat shape of the cosmic X-ray background spectrum.
Integrating the composite spectrum over redshift, we find that the total
emission from Seyfert 2 galaxies, combined with the expected contribution from
unabsorbed type 1 objects, provides an excellent match to the spectrum and
intensity of the hard X-ray background. The principal uncertainty in this
procedure is the cosmic evolution of the Seyfert 2 X-ray luminosity function.
Separate composite spectra for objects in our sample with and without polarized
broad optical emission lines are also presented.Comment: 11 pages (AASTeX), including 3 figures. Accepted for publication in
ApJ Letter
Spectroscopic confirmation of an ultra-faint galaxy at the epoch of reionization
Within one billion years of the Big Bang, intergalactic hydrogen was ionized
by sources emitting ultraviolet and higher energy photons. This was the final
phenomenon to globally affect all the baryons (visible matter) in the Universe.
It is referred to as cosmic reionization and is an integral component of
cosmology. It is broadly expected that intrinsically faint galaxies were the
primary ionizing sources due to their abundance in this epoch. However, at the
highest redshifts (; lookback time 13.1 Gyr), all galaxies with
spectroscopic confirmations to date are intrinsically bright and, therefore,
not necessarily representative of the general population. Here, we report the
unequivocal spectroscopic detection of a low luminosity galaxy at . We
detected the Lyman- emission line at {\AA} in two separate
observations with MOSFIRE on the Keck I Telescope and independently with the
Hubble Space Telescope's slit-less grism spectrograph, implying a source
redshift of . The galaxy is gravitationally magnified by
the massive galaxy cluster MACS J1423.8+2404 (), with an estimated
intrinsic luminosity of mag and a stellar mass of
solar masses. Both are an order of
magnitude lower than the four other Lyman- emitters currently known at
, making it probably the most distant representative source of
reionization found to date
Physical Activity and Recurrent Pain in Children and Adolescents in Germany—Results from the MoMo Study
Recurrent pain can be a significant disruption in the activities of daily life, and is not only a health problem in adults but also in children and adolescents. This study analyzed the prevalence of recurrent pain in the current sample (n = 1516; 11–17 years (mean = 14.4 ± 2.0 years); 50.8% female) of a nationwide study in Germany, evaluated the association of participants’ device-based physical activity (PA) with the prevalence of recurrent pain, and assessed whether children and adolescents who reported pain for the last three months accumulated less PA than those who did not. A higher prevalence was found in girls for recurrent headaches (42.2% vs. 28.7%), abdominal pain (28.2% vs. 20.1%), and back pain (26.9% vs. 19.5%). We found higher odds for recurrent headaches in girls (OR = 1.54) and in participants that did not reach at least 60 min of moderate to vigorous PA (MVPA) per day (OR = 2.06). Girls who reported recurrent headaches accumulated 4.7 min less MVPA per day than those without. The prevalence of pain remains at a high level in the German youth and underscores the need for interventions to improve the health situations of children and adolescents
Collapse of a non-axisymmetric, impact-created air cavity in water
The axisymmetric collapse of a cylindrical air cavity in water follows a
universal power law with logarithmic corrections. Nonetheless, it has been
suggested that the introduction of a small azimuthal disturbance induces a long
term memory effect, reflecting in oscillations which are no longer universal
but remember the initial condition. In this work, we create non-axisymmetric
air cavities by driving a metal disc through an initially-quiescent water
surface and observe their subsequent gravity-induced collapse. The cavities are
characterized by azimuthal harmonic disturbances with a single mode number
and amplitude . For small initial distortion amplitude (1 or 2% of the
mean disc radius), the cavity walls oscillate linearly during collapse, with
nearly constant amplitude and increasing frequency. As the amplitude is
increased, higher harmonics are triggered in the oscillations and we observe
more complex pinch-off modes. For small amplitude disturbances we compare our
experimental results with the model for the amplitude of the oscillations by
Schmidt et al. (2009) and the model for the collapse of an axisymmetric
impact-created cavity previously proposed by Bergmann et al. (2009b). By
combining these two models we can reconstruct the three-dimensional shape of
the cavity at any time before pinch-off.Comment: 18 pages, 13 figure
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