8,057 research outputs found
Enhanced transmission of slit arrays in an extremely thin metallic film
Horizontal resonances of slit arrays are studied. They can lead to an
enhanced transmission that cannot be explained using the single-mode
approximation. A new type of cavity resonance is found when the slits are
narrow for a wavelength very close to the period. It can be excited for very
low thicknesses. Optimization shows these structures could constitute
interesting monochromatic filters
Dynamic Provenance for SPARQL Update
While the Semantic Web currently can exhibit provenance information by using
the W3C PROV standards, there is a "missing link" in connecting PROV to storing
and querying for dynamic changes to RDF graphs using SPARQL. Solving this
problem would be required for such clear use-cases as the creation of version
control systems for RDF. While some provenance models and annotation techniques
for storing and querying provenance data originally developed with databases or
workflows in mind transfer readily to RDF and SPARQL, these techniques do not
readily adapt to describing changes in dynamic RDF datasets over time. In this
paper we explore how to adapt the dynamic copy-paste provenance model of
Buneman et al. [2] to RDF datasets that change over time in response to SPARQL
updates, how to represent the resulting provenance records themselves as RDF in
a manner compatible with W3C PROV, and how the provenance information can be
defined by reinterpreting SPARQL updates. The primary contribution of this
paper is a semantic framework that enables the semantics of SPARQL Update to be
used as the basis for a 'cut-and-paste' provenance model in a principled
manner.Comment: Pre-publication version of ISWC 2014 pape
Fish evacuate smoothly respecting a social bubble
Crowd movements are observed among different species and on different scales,
from insects to mammals, as well as in non-cognitive systems, such as motile
cells. When forced to escape through a narrow opening, most terrestrial animals
behave like granular materials and clogging events decrease the efficiency of
the evacuation. Here, we explore the evacuation behavior of macroscopic,
aquatic agents, neon fish, and challenge their gregarious behavior by forcing
the school through a constricted passage. Using a statistical analysis method
developed for granular matter and applied to crowd evacuation, our results
clearly show that, unlike crowds of people or herds of sheep, no clogging
occurs at the bottleneck. The fish do not collide and wait for a minimum
waiting time between two successive exits, while respecting a social distance.
When the constriction becomes similar to or smaller than their social distance,
the individual domains defined by this cognitive distance are deformed and fish
density increases. We show that the current of escaping fish behaves like a set
of deformable 2D-bubbles, their 2D domain, passing through a constriction.
Schools of fish show that, by respecting social rules, a crowd of individuals
can evacuate without clogging, even in an emergency situation.Comment: 7 pages, 4 figure
The Breakdown of Kinetic Theory in Granular Shear Flows
We examine two basic assumptions of kinetic theory-- binary collisions and
molecular chaos-- using numerical simulations of sheared granular materials. We
investigate a wide range of densities and restitution coefficients and
demonstrate that kinetic theory breaks down at large density and small
restitution coefficients. In the regimes where kinetic theory fails, there is
an associated emergence of clusters of spatially correlated grains
Optimal estimation for Large-Eddy Simulation of turbulence and application to the analysis of subgrid models
The tools of optimal estimation are applied to the study of subgrid models
for Large-Eddy Simulation of turbulence. The concept of optimal estimator is
introduced and its properties are analyzed in the context of applications to a
priori tests of subgrid models. Attention is focused on the Cook and Riley
model in the case of a scalar field in isotropic turbulence. Using DNS data,
the relevance of the beta assumption is estimated by computing (i) generalized
optimal estimators and (ii) the error brought by this assumption alone. Optimal
estimators are computed for the subgrid variance using various sets of
variables and various techniques (histograms and neural networks). It is shown
that optimal estimators allow a thorough exploration of models. Neural networks
are proved to be relevant and very efficient in this framework, and further
usages are suggested
Quasi-rigidity: some uniqueness issues
Quasi-rigidity means that one builds a theory for assemblies of grains under
a slowly changing external load by using the deformation of those grains as a
small parameter. Is quasi-rigidity a complete theory for these granular
assemblies? Does it provide unique predictions of the assembly's behavior, or
must some other process be invoked to decide between several possibilities? We
provide evidence that quasi-rigidity is a complete theory by showing that two
possible sources of indeterminacy do not exist for the case of disk shaped
grains. One possible source of indeterminacy arises from zero-frequency modes
present in the packing. This problem can be solved by considering the
conditions required to obtain force equilibrium. A second possible source of
indeterminacy is the necessity to choose the status (sliding or non-sliding) at
each contact. We show that only one choice is permitted, if contacts slide only
when required by Coulomb friction.Comment: 14 pages, 3 figures, submitted to Phys Rev E (introduction and
conclusion revised
Prospecting Period Measurements with LSST - Low Mass X-ray Binaries as a Test Case
The Large Synoptic Survey Telescope (LSST) will provide for unbiased sampling
of variability properties of objects with mag 24. This should allow for
those objects whose variations reveal their orbital periods (), such
as low mass X-ray binaries (LMXBs) and related objects, to be examined in much
greater detail and with uniform systematic sampling. However, the baseline LSST
observing strategy has temporal sampling that is not optimised for such work in
the Galaxy. Here we assess four candidate observing strategies for measurement
of in the range 10 minutes to 50 days. We simulate multi-filter
quiescent LMXB lightcurves including ellipsoidal modulation and stochastic
flaring, and then sample these using LSST's operations simulator (OpSim) over
the (mag, ) parameter space, and over five sightlines sampling a range
of possible reddening values. The percentage of simulated parameter space with
correctly returned periods ranges from 23 %, for the current baseline
strategy, to 70 % for the two simulated specialist strategies. Convolving
these results with a distribution, a modelled Galactic spatial
distribution and reddening maps, we conservatively estimate that the most
recent version of the LSST baseline strategy will allow determination
for 18 % of the Milky Way's LMXB population, whereas strategies that do
not reduce observations of the Galactic Plane can improve this dramatically to
32 %. This increase would allow characterisation of the full binary
population by breaking degeneracies between suggested distributions
in the literature. Our results can be used in the ongoing assessment of the
effectiveness of various potential cadencing strategies.Comment: Replacement after addressing minor corrections from the referee -
mainly improvements in clarificatio
Optical performance of the JWST MIRI flight model: characterization of the point spread function at high-resolution
The Mid Infra Red Instrument (MIRI) is one of the four instruments onboard
the James Webb Space Telescope (JWST), providing imaging, coronagraphy and
spectroscopy over the 5-28 microns band. To verify the optical performance of
the instrument, extensive tests were performed at CEA on the flight model (FM)
of the Mid-InfraRed IMager (MIRIM) at cryogenic temperatures and in the
infrared. This paper reports on the point spread function (PSF) measurements at
5.6 microns, the shortest operating wavelength for imaging. At 5.6 microns the
PSF is not Nyquist-sampled, so we use am original technique that combines a
microscanning measurement strategy with a deconvolution algorithm to obtain an
over-resolved MIRIM PSF. The microscanning consists in a sub-pixel scan of a
point source on the focal plane. A data inversion method is used to reconstruct
PSF images that are over-resolved by a factor of 7 compared to the native
resolution of MIRI. We show that the FWHM of the high-resolution PSFs were
5-10% wider than that obtained with Zemax simulations. The main cause was
identified as an out-of-specification tilt of the M4 mirror. After correction,
two additional test campaigns were carried out, and we show that the shape of
the PSF is conform to expectations. The FWHM of the PSFs are 0.18-0.20 arcsec,
in agreement with simulations. 56.1-59.2% of the total encircled energy
(normalized to a 5 arcsec radius) is contained within the first dark Airy ring,
over the whole field of view. At longer wavelengths (7.7-25.5 microns), this
percentage is 57-68%. MIRIM is thus compliant with the optical quality
requirements. This characterization of the MIRIM PSF, as well as the
deconvolution method presented here, are of particular importance, not only for
the verification of the optical quality and the MIRI calibration, but also for
scientific applications.Comment: 13 pages, submitted to SPIE Proceedings vol. 7731, Space Telescopes
and Instrumentation 2010: Optical, Infrared, and Millimeter Wav
Quark-lepton mass unification at TeV scales
A scenario combining a model of early (TeV) unification of quarks and leptons
with the physics of large extra dimensions provides a natural mechanism linking
quark and lepton masses at TeV scale. This has been dubbed as early
quark-lepton mass unification by one of us (PQH) in one of the two models of
early quark-lepton unification, which are consistent with data, namely SU(4)_PS
\otimes SU(2)_L \otimes SU(2)_R \otimes SU(2)_H. In particular, it focused on
the issue of naturally light Dirac neutrino. The present paper will focus on
similar issues in the other model, namely SU(4)_PS \otimes SU(3)_L \otimes
SU(3)_H.Comment: Accepted for publication in PRD: The new version is in agreement with
the accepted manuscrip
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