12,533 research outputs found
A note on shell models for MHD Turbulence
We investigate the time evolution of two different (GOY-like) shell models
which have been recently proposed to describe the gross features of MHD
turbulence. We see that, even if they are formally of the same type sharing
with MHD equations quadratic couplings and similar conserved quantities,
fundamental differences exist which are related to the ideal invariants.Comment: 6 pages, 5 figures.eps, to appear in Europhysics Letter
Multiscale fluid--particle thermal interaction in isotropic turbulence
We use direct numerical simulations to investigate the interaction between
the temperature field of a fluid and the temperature of small particles
suspended in the flow, employing both one and two-way thermal coupling, in a
statistically stationary, isotropic turbulent flow. Using statistical analysis,
we investigate this variegated interaction at the different scales of the flow.
We find that the variance of the fluid temperature gradients decreases as the
thermal response time of the suspended particles is increased. The probability
density function (PDF) of the fluid temperature gradients scales with its
variance, while the PDF of the rate of change of the particle temperature,
whose variance is associated with the thermal dissipation due to the particles,
does not scale in such a self-similar way. The modification of the fluid
temperature field due to the particles is examined by computing the particle
concentration and particle heat fluxes conditioned on the magnitude of the
local fluid temperature gradient. These statistics highlight that the particles
cluster on the fluid temperature fronts, and the important role played by the
alignments of the particle velocity and the local fluid temperature gradient.
The temperature structure functions, which characterize the temperature
fluctuations across the scales of the flow, clearly show that the fluctuations
of the fluid temperature increments are monotonically suppressed in the two-way
coupled regime as the particle thermal response time is increased. Thermal
caustics dominate the particle temperature increments at small scales, that is,
particles that come into contact are likely to have very large differences in
their temperature. This is caused by the nonlocal thermal dynamics of the
particles..
Measuring the LISA test mass magnetic proprieties with a torsion pendulum
Achieving the low frequency LISA sensitivity requires that the test masses
acting as the interferometer end mirrors are free-falling with an unprecedented
small degree of deviation. Magnetic disturbances, originating in the
interaction of the test mass with the environmental magnetic field, can
significantly deteriorate the LISA performance and can be parameterized through
the test mass remnant dipole moment and the magnetic susceptibility
. While the LISA test flight precursor LTP will investigate these effects
during the preliminary phases of the mission, the very stringent requirements
on the test mass magnetic cleanliness make ground-based characterization of its
magnetic proprieties paramount. We propose a torsion pendulum technique to
accurately measure on ground the magnetic proprieties of the LISA/LTP test
masses.Comment: 6 pages, 3 figure
Microstructured superhydrorepellent surfaces: Effect of drop pressure on fakir-state stability and apparent contact angles
In this paper we present a generalized Cassi-Baxter equation to take into
account the effect of drop pressure on the apparent contact angle theta_{app}.
Also we determine the limiting pressure p_{W} which causes the impalement
transition to the Wenzel state and the pull-off pressure p_{out} at which the
drop detaches from the substrate. The calculations have been carried out for
axial-symmetric pillars of three different shapes: conical, hemispherical
topped and flat topped cylindrical pillars. Calculations show that, assuming
the same pillar spacing, conical pillars may be more incline to undergo an
impalement transition to the Wenzel state, but, on the other hand, they are
characterized by a vanishing pull-off pressure which causes the drop not to
adhere to the substrate and therefore to detach very easily. We infer that this
property should strongly reduce the contact angle hysteresis as experimentally
osberved in Ref. \cite{Martines-Conical-Shape}. It is possible to combine large
resistance to impalement transition (i.e. large value of p_{W}) and small (or
even vanishing) detaching pressure p_{out} by employing cylindrical pillars
with conical tips. We also show that depending on the particular pillar
geometry, the effect of drop pressure on the apparent contact angle theta_{app}
may be more or less significant. In particular we show that in case of conical
pillars increasing the drop pressure causes a significant decrease of
theta_{app} in agreement with some experimental investigations
\cite{LafunaTransitio}, whereas theta_{app} slightly increases for
hemispherical or flat topped cylindrical pillars.Comment: 21 pages, 13 figure
Mixture or mosaic? Genetic patterns in UK grey squirrels support a human-mediated ‘long-jump’ invasion mechanism
Aim Clarifying whether multiple introductions of a species remain relatively isolated or merge and interbreed is essential for understanding the dynamics of invasion processes. Multiple introductions from different sources can result in a mixture of genetically distinct populations, increasing the total genetic diversity. This mixing can resolve the ‘genetic paradox’, whereby in spite of the relatively small numbers of introduced individuals, the augmented diversity due to this mixing increases adaptability and the ability of the species to spread in new environments. Here, we aim to assess whether the expansion of a successful invader, the Eastern grey squirrel, was partly driven by the merger of multiple introductions and the effects of such a merger on diversity. Location UK, Ireland. Methods We analysed the genetic variation at 12 microsatellite loci of 381 individuals sampled from one historical and 14 modern populations of grey squirrels. Results Our data revealed that current UK population structure resembles a mosaic, with minimal interpopulation mixing and each element reflecting the genetic make-up of historic introductions. The genetic diversity of each examined population was lower than a US population or a historical UK population. Numbers of releases in a county did not correlate with county-level genetic diversity. Inbreeding coefficients remain high, and effective population sizes remain small. Main conclusions Our results support the conclusion that rapid and large-scale expansion in this species in the UK was not driven by a genetic mixing of multiple introduced populations with a single expansion front, but was promoted by repeated translocations of small propagules. Our results have implications for the management of grey squirrels and other invasive species and also demonstrate how invaders can overcome the genetic paradox, if spread is facilitated by human-mediated dispersal
Comment on "Multidisciplinary investigation on a lava fountain preceding a flank eruption: The 10 May 2008 Etna case" by A. Bonaccorso et al.
The paper "Multidisciplinary investigation on a lava fountain preceding a flank eruption: The 10 May 2008 Etna case", by Bonaccorso et al. (2011), presents a multi-parameter dataset encompassing the eruptive episode featured in the title. Through the dataset at their disposal, the authors tried to set constraints on the coupled phenomena which governed the paroxysmal event and subsequent flank eruption. Even though the joint analysis of different data offers considerable potential to extract additional information on the dynamics behind the observed phenomena, the most obvious implication is the risk of not treating all the available information with due care, which may lead to misinterpretation of the data.
In the following, we discuss issues concerning the analysis and interpretation of gravity and tilt data in Bonaccorso et al. (2011) and show why, in our opinion, the conclusion that "all the data concur in indicating that the 10 May lava fountain was generated by the fragmentation of a foam layer trapped at the top of a shallow reservoir" is not soundly based
- …