56,897 research outputs found
Dispersion of biased swimming microorganisms in a fluid flowing through a tube
Classical Taylor-Aris dispersion theory is extended to describe the transport
of suspensions of self-propelled dipolar cells in a tubular flow. General
expressions for the mean drift and effective diffusivity are determined exactly
in terms of axial moments, and compared with an approximation a la Taylor. As
in the Taylor-Aris case, the skewness of a finite distribution of biased
swimming cells vanishes at long times. The general expressions can be applied
to particular models of swimming microorganisms, and thus be used to predict
swimming drift and diffusion in tubular bioreactors, and to elucidate competing
unbounded swimming drift and diffusion descriptions. Here, specific examples
are presented for gyrotactic swimming algae.Comment: 20 pages, 4 figures. Published version available at
http://rspa.royalsocietypublishing.org/content/early/2010/02/09/rspa.2009.0606.short?rss=
Self-similar solutions for the interaction of relativistic ejecta with an ambient medium
We find self-similar solutions to describe the interaction of spherically
symmetric ejecta expanding at relativistic speeds with an ambient medium having
a power law density distribution. Using this solution, the time evolution of
the Lorentz factor of the outer shock is derived as a function of the explosion
energy, the mass of the ejecta, and parameters for the ambient medium. These
solutions are an ultra-relativistic version of the solutions for the
circumstellar interaction of supernova ejecta obtained by Chevalier and
extensions of the relativistic blast wave solutions of Blandford & Mckee.Comment: 12 pages, 1 figure, accepted for publication in Ap
Fossil Imprints of the First Generation Supernova Ejecta in Extremely Metal-Deficient Stars
Using results of nucleosynthesis calculations for theoretical core-collapse
supernova models with various progenitor's masses, it is shown that abundance
patterns of C, Mg, Si, Ca, and H seen in extremely metal-deficient stars with
[Fe/H] < -2.5 follow those seen in the individual first generation supernova
remnants (SNRs). This suggests that most of the stars with [Fe/H] < -2.5 were
made from individual supernova (SN) events. To obtain the ratio of heavy
elements to hydrogen, a formula is derived to estimate the mass of hydrogen
swept up by a SNR when it occurs in the interstellar matter with the primordial
abundances. We use [Mg/H] to indicate the metallicities instead of [Fe/H]. The
metallicities [Mg/H] predicted from these SNRs range from ~-4 to ~-1.5 and the
mass of Mg in a SN is well correlated with its progenitor's mass. Thus the
observed [Mg/H] in an extremely metal deficient star has a correspondence to
the progenitor's mass. A larger [Mg/H] corresponds to a larger progenitor's
mass. Therefore, so called `age-metallicity relation' does not hold for stars
with [Fe/H] < -2.5. In contrast, the [Mg/Fe] ratios in the theoretical SNRs
have a different trend from those in extremely metal-deficient stars. It is
also shown that the observed trend of [Mg/Fe] can predict the Fe yield of each
SN given the correspondence of [Mg/H] to the progenitor's mass. The Fe yields
thus obtained are consistent with those derived from SN light curve analyses.
This indicates that there is still a problem in modelling a core-collapse
supernova at its beginning of explosion or mass cut.Comment: 6 pages, 4 figures, 1 table; Accepted for publication in the
Astrophysical Journal Letter
Stability of an Ultra-Relativistic Blast Wave in an External Medium with a Steep Power-Law Density Profile
We examine the stability of self-similar solutions for an accelerating
relativistic blast wave which is generated by a point explosion in an external
medium with a steep radial density profile of a power-law index > 4.134. These
accelerating solutions apply, for example, to the breakout of a gamma-ray burst
outflow from the boundary of a massive star, as assumed in the popular
collapsar model. We show that short wavelength perturbations may grow but only
by a modest factor <~ 10.Comment: 12 pages, 3 figures, submitted to Physical Review
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
Credit Availability for Potential Irrigators in North Dakota
Agricultural Finance, Resource /Energy Economics and Policy,
POLICY HARMONIZATION, CONVERGENCE, AND COMPATIBILITY ISSUES IN NORTH AMERICAN HORTICULTURE
Agricultural and Food Policy,
Gravitational oscillations of a liquid column
We report gravity oscillations of a liquid column partially immersed in a
bath of liquid. We stress in particular some peculiarities of this system,
namely (i) the fact that the mass of this oscillator constantly changes with
time; (ii) the singular character of the beginning of the rise, for which the
mass of the oscillator is zero; (iii) the sources of dissipation in this
system, which is found to be dominated at low viscosity by the entrance (or
exit) effects, leading to a long-range damping of the oscillations. We conclude
with some qualitative description of a second-order phenomenon, namely the
eruption of a jet at the beginning of the rise.Comment: 22 pages, pdf. Submitted to Physics of Fluid
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Refinement and preliminary evaluation of two tablet-based tests of real-world visual function
PURPOSE: To describe, refine, evaluate, and provide normative control data for two freely available tablet-based tests of real-world visual function, using a cohort of young, normally-sighted adults.
METHODS: Fifty young (18-40Â years), normally-sighted adults completed tablet-based assessments of (1) face discrimination and (2) visual search. Each test was performed twice, to assess test-retest repeatability. Post-hoc analyses were performed to determine the number of trials required to obtain stable estimates of performance. Distributions were fitted to the normative data to determine the 99% population-boundary for normally sighted observers. Participants were also asked to rate their comprehension of each test.
RESULTS: Both tests provided stable estimates in around 20 trials (~1-4Â min), with only a further reduction of 14%-17% in the 95% Coefficient of Repeatability (CoR95 ) when an additional 40 trials were included. When using only ~20 trials: median durations for the first run of each test were 191Â s (Faces) and 51Â s (Search); test-retest CoR95 were 0.27Â d (Faces) and 0.84Â s (Search); and normative 99% population-limits were 3.50Â d (Faces) and 3.1Â s (Search). No participants exhibited any difficulties completing either test (100% completion rate), and ratings of task-understanding were high (Faces: 9.6 out of 10; Search: 9.7 out of 10).
CONCLUSIONS: This preliminary assessment indicated that both tablet-based tests are able to provide simple, quick, and easy-to-administer measures of real-world visual function in normally-sighted young adults. Further work is required to assess their accuracy and utility in older people and individuals with visual impairment. Potential applications are discussed, including their use in clinic waiting rooms, and as an objective complement to Patient Reported Outcome Measures (PROMs)
Two-dimensional solitons on the surface of magnetic fluids
We report an observation of a stable soliton-like structure on the surface of
a ferrofluid, generated by a local perturbation in the hysteretic regime of the
Rosensweig instability. Unlike other pattern-forming systems with localized 2D
structures, magnetic fluids are characterized by energy conservation; hence
their mechanism of soliton stabilization is different from the previously
discussed gain/loss balance mechanism. The radioscopic measurements of the
soliton's surface profile suggest that locking on the underlying periodic
structure is instrumental in its stabilization.Comment: accepted for publication by Physical Review Letter
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