5,932 research outputs found
Functional implications of the intertarsal joint shape in a terrestrial ( Coturnix coturnix ) versus a semi-aquatic bird ( Callonetta leucophrys )
International audienceAs birds have a diversity of locomotor behaviors, their skeleton is subjected to a variety of mechanical constraints (gravitational, aerodynamic and sometimes hydrodynamic forces). Yet, only minor modifications in post-cranial skeleton shape are observed across the diversity of avian species in comparison with other vertebrates. The goal of this study was to explore potential morphological adjustments that allow locomotion in different habitats in Anatidae. Specifically, we compared a strictly terrestrial bird, the common quail Coturnix coturnix, and a semi-aquatic bird, the ringed teal Callonetta leucophrys, to explore whether their anatomy reflects the constraints of locomotion in different habitats (water vs. land). We compared the tibiotarsus and the tarsometatarsus shape between the two species using a geometric morphometric approach. Our data illustrate distinct differences between species with a more medially oriented intertarsal joint in the ringed teal than in the common quail, which may be linked to the kinematics of walking and paddling. This study lays the foundations to understand the functional requirements for moving in both terrestrial and aquatic environments in Anatidae, and suggests morphological characteristics of the bird hindlimb skeleton that may help to predict the motions it is capable of
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Inexact Newton Dogleg Methods
The dogleg method is a classical trust-region technique for globalizing Newton\u27s method. While it is widely used in optimization, including large-scale optimization via truncated-Newton approaches, its implementation in general inexact Newton methods for systems of nonlinear equations can be problematic. In this paper, we first outline a very general dogleg method suitable for the general inexact Newton context and provide a global convergence analysis for it. We then discuss certain issues that may arise with the standard dogleg implementational strategy and propose modified strategies that address them. Newton-Krylov methods have provided important motivation for this work, and we conclude with a report on numerical experiments involving a Newton-GMRES dogleg method applied to benchmark CFD problems
Optimal General Matchings
Given a graph and for each vertex a subset of the
set , where denotes the degree of vertex
in the graph , a -factor of is any set such that
for each vertex , where denotes the number of
edges of incident to . The general factor problem asks the existence of
a -factor in a given graph. A set is said to have a {\em gap of
length} if there exists a natural number such that and . Without any restrictions the
general factor problem is NP-complete. However, if no set contains a gap
of length greater than , then the problem can be solved in polynomial time
and Cornuejols \cite{Cor} presented an algorithm for finding a -factor, if
it exists. In this paper we consider a weighted version of the general factor
problem, in which each edge has a nonnegative weight and we are interested in
finding a -factor of maximum (or minimum) weight. In particular, this
version comprises the minimum/maximum cardinality variant of the general factor
problem, where we want to find a -factor having a minimum/maximum number of
edges.
We present an algorithm for the maximum/minimum weight -factor for the
case when no set contains a gap of length greater than . This also
yields the first polynomial time algorithm for the maximum/minimum cardinality
-factor for this case
Construction and Performance of a Micro-Pattern Stereo Detector with Two Gas Electron Multipliers
The construction of a micro-pattern gas detector of dimensions 40x10 cm**2 is
described. Two gas electron multiplier foils (GEM) provide the internal
amplification stages. A two-layer readout structure was used, manufactured in
the same technology as the GEM foils. The strips of each layer cross at an
effective crossing angle of 6.7 degrees and have a 406 um pitch. The
performance of the detector has been evaluated in a muon beam at CERN using a
silicon telescope as reference system. The position resolutions of two
orthogonal coordinates are measured to be 50 um and 1 mm, respectively. The
muon detection efficiency for two-dimensional space points reaches 96%.Comment: 21 pages, 17 figure
Effect of the disorder in graphene grain boundaries: A wave packet dynamics study
Chemical vapor deposition (CVD) on Cu foil is one of the most promising methods to produce graphene samples despite of introducing numerous grain boundaries into the perfect graphene lattice. A rich variety of GB structures can be realized experimentally by controlling the parameters in the CVD method. Grain boundaries contain non-hexagonal carbon rings (4, 5, 7, 8 membered rings) and vacancies in various ratios and arrangements. Using wave packet dynamic (WPD) simulations and tight-binding electronic structure calculations, we have studied the effect of the structure of GBs on the transport properties. Three model GBs with increasing disorder were created in the computer: a periodic 5-7 GB, a "serpentine" GB, and a disordered GB containing 4, 8 membered rings and vacancies. It was found that for small energies (E = EF ± 1 eV) the transmission decreases with increasing disorder. Four membered rings and vacancies are identified as the principal scattering centers. Revealing the connection between the properties of GBs and the CVD growth method may open new opportunities in the graphene based nanoelectronics. © 2013 Elsevier B.V. All rights reserved
Theoretical analysis of the role of chromatin interactions in long-range action of enhancers and insulators
Long-distance regulatory interactions between enhancers and their target
genes are commonplace in higher eukaryotes. Interposed boundaries or insulators
are able to block these long distance regulatory interactions. The mechanistic
basis for insulator activity and how it relates to enhancer
action-at-a-distance remains unclear. Here we explore the idea that topological
loops could simultaneously account for regulatory interactions of distal
enhancers and the insulating activity of boundary elements. We show that while
loop formation is not in itself sufficient to explain action at a distance,
incorporating transient non-specific and moderate attractive interactions
between the chromatin fibers strongly enhances long-distance regulatory
interactions and is sufficient to generate a euchromatin-like state. Under
these same conditions, the subdivision of the loop into two topologically
independent loops by insulators inhibits inter-domain interactions. The
underlying cause of this effect is a suppression of crossings in the contact
map at intermediate distances. Thus our model simultaneously accounts for
regulatory interactions at a distance and the insulator activity of boundary
elements. This unified model of the regulatory roles of chromatin loops makes
several testable predictions that could be confronted with \emph{in vitro}
experiments, as well as genomic chromatin conformation capture and fluorescent
microscopic approaches.Comment: 10 pages, originally submitted to an (undisclosed) journal in May
201
Cones, pringles, and grain boundary landscapes in graphene topology
A polycrystalline graphene consists of perfect domains tilted at angle
{\alpha} to each other and separated by the grain boundaries (GB). These nearly
one-dimensional regions consist in turn of elementary topological defects,
5-pentagons and 7-heptagons, often paired up into 5-7 dislocations. Energy
G({\alpha}) of GB computed for all range 0<={\alpha}<=Pi/3, shows a slightly
asymmetric behavior, reaching ~5 eV/nm in the middle, where the 5's and 7's
qualitatively reorganize in transition from nearly armchair to zigzag
interfaces. Analysis shows that 2-dimensional nature permits the off-plane
relaxation, unavailable in 3-dimensional materials, qualitatively reducing the
energy of defects on one hand while forming stable 3D-landsapes on the other.
Interestingly, while the GB display small off-plane elevation, the random
distributions of 5's and 7's create roughness which scales inversely with
defect concentration, h ~ n^(-1/2)Comment: 9 pages, 4 figure
Modeling of the Radiation Doses during Dismantling of RBMK-1500 Reactor Pressurized Tanks from Emergency Core Cooling System
Decommissioning of the Ignalina Nuclear Power Plant involves multiple problems. One of them is personnel radiation safety during the performance of dismantling activities. In this paper, modeling results of radiation doses during the dismantling of the pressurized tank from the emergency core cooling system (ECCS PT) of RBMK-1500 reactor are presented. The radiological surveys indicate that the inner surface of the ECCS PT is contaminated with radioactive products of corrosion and sediments due to the radioactive water. The effective doses to the workers have been modeled for different strategies of ECCS PT dismantling. In order to select the optimal personnel radiation safety, the modeling has been performed by the means of computer code âVISIPLAN 3D ALARA Planning toolâ developed by SCK CEN (Belgium). The impacts of dismantling tools, shielding types, and extract ventilation flow rate on effective doses during the dismantling of ECCS PT have been analyzed. The total effective personnel doses have been obtained by summarizing the effective personnel doses from various sources of exposure, that is, direct radiation from radioactive equipment, internal radiation due to inhalation of radioactive aerosols, and direct radiation from radioactive aerosols arising during hot cutting in premises. The uncertainty of the collective doses is also presented in this paper
Electronic transport in polycrystalline graphene
Most materials in available macroscopic quantities are polycrystalline.
Graphene, a recently discovered two-dimensional form of carbon with strong
potential for replacing silicon in future electronics, is no exception. There
is growing evidence of the polycrystalline nature of graphene samples obtained
using various techniques. Grain boundaries, intrinsic topological defects of
polycrystalline materials, are expected to dramatically alter the electronic
transport in graphene. Here, we develop a theory of charge carrier transmission
through grain boundaries composed of a periodic array of dislocations in
graphene based on the momentum conservation principle. Depending on the grain
boundary structure we find two distinct transport behaviours - either high
transparency, or perfect reflection of charge carriers over remarkably large
energy ranges. First-principles quantum transport calculations are used to
verify and further investigate this striking behaviour. Our study sheds light
on the transport properties of large-area graphene samples. Furthermore,
purposeful engineering of periodic grain boundaries with tunable transport gaps
would allow for controlling charge currents without the need of introducing
bulk band gaps in otherwise semimetallic graphene. The proposed approach can be
regarded as a means towards building practical graphene electronics.Comment: accepted in Nature Material
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