17,778 research outputs found
Dynamics of a 1-D model for the emergence of the plasma edge shear flow layer with momentum conserving Reynolds stress
A one-dimensional version of the second-order transition model based on the
sheared flow amplification by Reynolds stress and turbulence supression by
shearing is presented. The model discussed in this paper includes a form of the
Reynolds stress which explicitly conserves momentum. A linear stability
analysis of the critical point is performed. Then, it is shown that the
dynamics of weakly unstable states is determined by a reduced equation for the
shear flow. In the case in which the flow damping term is diffusive, the
stationary solutions are those of the real Ginzburg-Landau equation.Comment: 21 pages, 8 figure
A Statistical Study of Photospheric Magnetic Field Changes During 75 Solar Flares
Abrupt and permanent changes of photospheric magnetic fields have been
observed during solar flares. The changes seem to be linked to the
reconfiguration of magnetic fields, but their origin is still unclear. We
carried out a statistical analysis of permanent line-of-sight magnetic field
() changes during 18 X-, 37 M-, 19 C- and 1 B-class flares using
data from Solar Dynamics Observatory/Helioseismic and Magnetic Imager. We
investigated the properties of permanent changes, such as frequency, areas, and
locations. We detected changes of in 59/75 flares. We find that
strong flares are more likely to show changes, with all flares M1.6
exhibiting them. For weaker flares, permanent changes are observed in 6/17
C-flares. 34.3\% of the permanent changes occurred in the penumbra and 18.9\%
in the umbra. Parts of the penumbra appeared or disappeared in 23/75 flares.
The area where permanent changes occur is larger for stronger flares. Strong
flares also show a larger change of flux, but there is no dependence of the
magnetic flux change on the heliocentric angle. The mean rate of change of
flare-related magnetic field changes is 20.7 Mx cm min. The
number of permanent changes decays exponentially with distance from the
polarity inversion line. The frequency of the strength of permanent changes
decreases exponentially, and permanent changes up to 750 Mx cm were
observed. We conclude that permanent magnetic field changes are a common
phenomenon during flares, and future studies will clarify their relation to
accelerated electrons, white light emission, and sunquakes to further
investigate their origin.Comment: Piblished in Ap
Flow Blurring-Enabled Production of Polymer Filaments from Poly(ethylene oxide) Solutions
Flow blurring (FB) atomizers are relatively
simple yet robust devices used for the generation of sprays
from solutions of a wide range of viscosities. In this work, we
have demonstrated that FB devices may also be applied for
massive production of liquid filaments from polymeric
solutions. They can later be transformed into solid filaments
and fibers, leading to the production of so-called fiber mats.
The liquid precursors consisted of poly(ethylene oxide)
(PEO) solutions of varying molecular weights (105 [100k]
to 4 × 106 g/mol [4M]) and concentrations. The FB device
was operated in the gas pressure range of 3−6 bar. Except for
solutions of PEO 100k, all solutions exhibited a shear thinning
behavior. For massive filament production, a threshold
polymer concentration (ct) was identified for each molecular
weight. Below such concentration, the atomization resulted in droplets (the classical FB functioning mode). Such a threshold
value decreased as the PEO molecular weight increased, and it coincides with the polymer coil overlap concentration, c*. The
viscoelastic nature of the solutions was also observed to increase with the molecular weight. A 3.2 dependency of the zero-shear
rate viscosity on a so-called Bueche parameter was found for filament production, whereas a nearly linear dependency was found
for droplet production. In general, the mean diameter of the filaments decreased as they traveled downstream from the
atomization point. Furthermore, at a given distance from the atomizer outlet and gas pressure, the mean filament diameter
slightly shifted toward larger sizes with increasing PEO molecular weight. The tendency agrees well with the calculated
filaments’ Deborah number, which increases with PEO molecular weight. The approach presented herein describes a highthroughput
and efficient method for the massive production of viscous filaments. These may be transformed into fibers by an
on-line drying step.Ministerio de Economía y Competitividad DPI2016-78887-C3-1-
Continuous Time Random Walks in periodic systems: fluid limit and fractional differential equations on the circle
In this article, the continuous time random walk on the circle is studied. We
derive the corresponding generalized master equation and discuss the effects of
topology, especially important when Levy flights are allowed. Then, we work out
the fluid limit equation, formulated in terms of the periodic version of the
fractional Riemann-Liouville operators, for which we provide explicit
expressions. Finally, we compute the propagator in some simple cases. The
analysis presented herein should be relevant when investigating anomalous
transport phenomena in systems with periodic dimensions.Comment: 14 pages, 1 figure. References added. Published versio
Risk factors for Taenia saginata cysticercus infection in cattle in the United Kingdom: a farm-level case-control study and assessment of the role of movement history, age and sex
Fractional generalization of Fick's law: a microscopic approach
In the study of transport in inhomogeneous systems it is common to construct
transport equations invoking the inhomogeneous Fick law. The validity of this
approach requires that at least two ingredients be present in the system.
First, finite characteristic length and time scales associated to the dominant
transport process must exist. Secondly, the transport mechanism must satisfy a
microscopic symmetry: global reversibility. Global reversibility is often
satisfied in nature. However, many complex systems exhibit a lack of finite
characteristic scales. In this Letter we show how to construct a generalization
of the inhomogeneous Fick law that does not require the existence of
characteristic scales while still satisfying global reversibility.Comment: 4 pages. Published versio
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