9,198 research outputs found
Homogenization for advection-diffusion in a perforated domain
The volume of a Wiener sausage constructed from a diffusion process with periodic, mean-zero, divergence-free velocity field, in dimension 3 or more, is shown to have a non-random and positive asymptotic rate of growth. This is used to establish the existence of a homogenized limit for such a diffusion when subject to Dirichlet conditions on the boundaries of a sparse and independent array of obstacles. There is a constant effective long-time loss rate at the obstacles. The dependence of this rate on the form and intensity of the obstacles and on the velocity field is investigated. A Monte Carlo algorithm for the computation of the volume growth rate of the sausage is introduced and some numerical results are presented for the Taylor–Green velocity field
Solar Irradiance Variability is Caused by the Magnetic Activity on the Solar Surface
The variation in the radiative output of the Sun, described in terms of solar
irradiance, is important to climatology. A common assumption is that solar
irradiance variability is driven by its surface magnetism. Verifying this
assumption has, however, been hampered by the fact that models of solar
irradiance variability based on solar surface magnetism have to be calibrated
to observed variability. Making use of realistic three-dimensional
magnetohydrodynamic simulations of the solar atmosphere and state-of-the-art
solar magnetograms from the Solar Dynamics Observatory, we present a model of
total solar irradiance (TSI) that does not require any such calibration. In
doing so, the modeled irradiance variability is entirely independent of the
observational record. (The absolute level is calibrated to the TSI record from
the Total Irradiance Monitor.) The model replicates 95% of the observed
variability between April 2010 and July 2016, leaving little scope for
alternative drivers of solar irradiance variability at least over the time
scales examined (days to years).Comment: Supplementary Materials;
https://journals.aps.org/prl/supplemental/10.1103/PhysRevLett.119.091102/supplementary_material_170801.pd
Astronomical Data Management
We present a summary of the major contributions to the Special Session on
Data Management held at the IAU General Assembly in Prague in 2006. While
recent years have seen enormous improvements in access to astronomical data,
and the Virtual Observatory aims to provide astronomers with seamless access to
on-line resources, more attention needs to be paid to ensuring the quality and
completeness of those resources. For example, data produced by telescopes are
not always made available to the astronomical community, and new instruments
are sometimes designed and built with insufficient planning for data
management, while older but valuable legacy data often remain undigitised. Data
and results published in journals do not always appear in the data centres, and
astronomers in developing countries sometimes have inadequate access to on-line
resources. To address these issues, an 'Astronomers Data Manifesto' has been
formulated with the aim of initiating a discussion that will lead to the
development of a 'code of best practice' in astronomical data management.Comment: Proceedings of Special Session SPS6 (Astronomical Data Management) at
the IAU GA 2006. To appear in Highlights of Astronomy, Volume 14, ed. K.A.
van der Huch
Development of welding techniques and filler metals for high strength aluminum alloys Annual summary report, 27 Jun. 1964 - 27 Jun. 1965
Welding techniques and filler metals for high strength aluminum alloy
Abundances of metal-weak thick-disc candidates
High resolution spectra of 5 candidate metal-weak thick-disc stars suggested
by Beers & Sommer-Larsen (1995) are analyzed to determine their chemical
abundances. The low abundance of all the objects has been confirmed with
metallicity reaching [Fe/H]=-2.9. However, for three objects, the astrometric
data from the Hipparcos catalogue suggests they are true halo members. The
remaining two, for which proper-motion data are not available, may have
disc-like kinematics. It is therefore clear that it is useful to address
properties of putative metal-weak thick-disc stars only if they possess full
kinematic data. For CS 22894-19 the abundance pattern similar to those of
typical halo stars is found, suggesting that chemical composition is not a
useful discriminant between thick-disc and halo stars. CS 29529-12 is found to
be C enhanced with [C/Fe]=+1.0; other chemical peculiarities involve the s
process elements: [Sr/Fe]=-0.65 and [Ba/Fe]=+0.62, leading to a high [Ba/Sr]
considerably larger than what is found in more metal-rich carbon-rich stars,
but similar to LP 706-7 and LP 625-44 discussed by Norris et al (1997a).
Hipparcos data have been used to calculate the space velocities of 25 candidate
metal-weak thick-disc stars, thus allowing us to identify 3 bona fide members,
which support the existence of a metal-poor tail of the thick-disc, at variance
with a claim to the contrary by Ryan & Lambert (1995).Comment: to be published in MNRA
Yoga in school-aged children: Impact on children's mental health and well-being during the COVID-19 pandemic
Health Technology Assessment (HTA) Capability Framework
Health Technology Assessment (HTA) Capability Framewor
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