164 research outputs found
Classical big-bounce cosmology: dynamical analysis of a homogeneous and irrotational Weyssenhoff fluid
A dynamical analysis of an effective homogeneous and irrotational Weyssenhoff
fluid in general relativity is performed using the 1+3 covariant approach that
enables the dynamics of the fluid to be determined without assuming any
particular form for the space-time metric. The spin contributions to the field
equations produce a bounce that averts an initial singularity, provided that
the spin density exceeds the rate of shear. At later times, when the spin
contribution can be neglected, a Weyssenhoff fluid reduces to a standard
cosmological fluid in general relativity. Numerical solutions for the time
evolution of the generalised scale factor in spatially-curved models are
presented, some of which exhibit eternal oscillatory behaviour without any
singularities. In spatially-flat models, analytical solutions for particular
values of the equation-of-state parameter are derived. Although the scale
factor of a Weyssenhoff fluid generically has a positive temporal curvature
near a bounce, it requires unreasonable fine tuning of the equation-of-state
parameter to produce a sufficiently extended period of inflation to fit the
current observational data.Comment: 34 pages, 18 figure
Analyse par géomatique des bilans et des flux d'azote et de phosphore dans un bassin versant agricole: le cas de la rivière Boyer.
L'essor et la concentration géographique des productions animales, dans certains bassins versant
à vocation agricole, génèrent d'imposants volumes de fumiers et de lisiers qui dépassent souvent
la superficie de sol disponible pour les valoriser. On assiste alors à une surfertilisation des cultures,
qui provoque l'accroissement de la teneur en phosphore du sol, le risque de transfert vers
l'environnement ainsi que le lessivage des nitrates. Ces nutriments, lorsqu'ils dépassent un certain
niveau, sont responsables de l'eutrophisation des cours d'eau. Cette problématique de pollution
diffuse est actuellement largement responsable de la détérioration de la qualité des eaux dans
plusieurs régions du Québec.
Ce projet de maîtrise a pour objectif d'analyser les bilans et les flux d'azote et de phosphore dans
le bassin versant de la rivière Boyer, un bassin où la pollution diffuse a été constatée. Ceci permet
d'estimer les stocks d'éléments nutritifs présents et de définir les zones critiques où les pertes vers
l'environnement sont les plus susceptibles de se produire. Pour ce faire, nous devons considérer
plusieurs données spatiales des différents facteurs qui interviennent, telles que l'occupation du sol,
la topographie, la pédologie, les conditions climatiques, les différentes pratiques culturales, etc.
Ces informations sont intégrées en exploitant les propriétés des Systèmes d'Information
Géographique (SIG) qui tiennent compte de la localisation spatiale des données considérées. Dans
le cadre du présent travail, nous exploitons le SIG IDRISI.
Afin d'obtenir une représentation réaliste du mode de fonctionnement du bassin versant à l'étude,
un bilan des apports et des exportations d'azote et de phosphore et des modèles de pertes
annuelles d'azote et de phosphore sont établis au sein du SIG. Les pertes annuelles de phosphore
particulaire et soluble peuvent être estimées en intégrant un modèle de perte de sol au SIG. Les
pertes annuelles d'azote, par lessivage et ruissellement superficiel, sont estimées à l'aide d'un
modèle intégré au SIG, qui est construit à partir de coefficients de pertes tirés de la littérature.
Les charges annuelles de phosphore et d'azote sont cumulées sur l'ensemble du bassin versant.
Le cumul se fait suivant les directions d'écoulement, qui sont déterminées grâce à un algorithme
de drainage interfacé au logiciel IDRISI. Cet algorithme utilise un modèle numérique d'altitude
(MNA) pour générer les directions d'écoulement suivant la plus forte pente. Une comparaison
entre les valeurs des concentrations d'azote et de phosphore, issues des simulations et des valeurs
mesurées en 28 points d'échantillonnage du bassin, permet de discuter des modèles développés
et d'établir des relations avec certaines caractéristiques du territoire
Weyssenhoff fluid dynamics in general relativity using a 1+3 covariant approach
The Weyssenhoff fluid is a perfect fluid with spin where the spin of the
matter fields is the source of torsion in an Einstein-Cartan framework. Obukhov
and Korotky showed that this fluid can be described as an effective fluid with
spin in general relativity. A dynamical analysis of such a fluid is performed
in a gauge invariant manner using the 1+3 covariant approach. This yields the
propagation and constraint equations for the set of dynamical variables. A
verification of these equations is performed for the special case of
irrotational flow with zero peculiar acceleration by evolving the constraints.Comment: 20 page
The ideal relativistic rotating gas as a perfect fluid with spin
We show that the ideal relativistic spinning gas at complete thermodynamical
equilibrium is a fluid with a non-vanishing spin density tensor \sigma_\mu \nu.
After having obtained the expression of the local spin-dependent phase space
density f(x,p)_(\sigma \tau) in the Boltzmann approximation, we derive the spin
density tensor and show that it is proportional to the acceleration tensor
Omega_\mu \nu constructed with the Frenet-Serret tetrad. We recover the proper
generalization of the fundamental thermodynamical relation, involving an
additional term -(1/2) \Omega_\mu \nu \sigma^\mu \nu. We also show that the
spin density tensor has a non-vanishing projection onto the four-velocity
field, i.e. t^\mu= sigma_\mu \nu u^\nu \ne 0, in contrast to the common
assumption t^\mu = 0, known as Frenkel condition, in the thus-far proposed
theories of relativistic fluids with spin. We briefly address the viewpoint of
the accelerated observer and inertial spin effects.Comment: Final published version in Annals of Physic
Traveling wave solutions in the Burridge-Knopoff model
The slider-block Burridge-Knopoff model with the Coulomb friction law is
studied as an excitable medium. It is shown that in the continuum limit the
system admits solutions in the form of the self-sustained shock waves traveling
with constant speed which depends only on the amount of the accumulated stress
in front of the wave. For a wide class of initial conditions the behavior of
the system is determined by these shock waves and the dynamics of the system
can be expressed in terms of their motion. The solutions in the form of the
periodic wave trains and sources of counter-propagating waves are analyzed. It
is argued that depending on the initial conditions the system will either tend
to synchronize or exhibit chaotic spatiotemporal behavior.Comment: 12 pages (ReVTeX), 7 figures (Postscript) to be published in Phys.
Rev.
Effect of electron irradiation on vortex dynamics in YBa_2Cu_3O_{7-x} single crystals
We report on drastic change of vortex dynamics with increase of quenched
disorder: for rather weak disorder we found a single vortex creep regime, which
we attribute to a Bragg-glass phase, while for enhanced disorder we found an
increase of both the depinning current and activation energy with magnetic
field, which we attribute to entangled vortex phase. We also found that
introduction of additional defects always increases the depinning current, but
it increases activation energy only for elastic vortex creep, while it
decreases activation energy for plastic vortex creep.Comment: 4 pages, 3 figures, submited to Phys. Rev.
Wavelet transform selection method for biological signal treatment
This paper presents the development and evaluation of an algorithm for compressing fetal electrocardiographic signals, taken superficially on the mother’s abdomen. This method for acquiring ECG signals produces a great volumen of information that makes it difficult for the records to be stored and transmitted. The proposed algorithm aims for lossless compression of the signal by applying Wavelet Packet Transform to keep errors below the unit, with compression rates over 20:1 and with conserved energy in reconstruction as comparison parameter. For algorithm validation, the signal files provided by PhysioBank DataBase are used
Design and optimization of index-guiding photonic crystal fiber gas sensor
Globalization is becoming an important issue for most businesses in the world. Since globalization changes business trends and shortens product life cycles, it requires companies to be more innovative in developing new ideas, products and processes. Clustering is one of ways to promote innovation by facilitating sharing information and ideas between firms, attracting buyers and suppliers, and providing opportunities for joint training. Many researches in developed countries found that the proximity between companies facilitated collaboration and provided a more conducive environment for R&D and knowledge sharing which can develop culture of entrepreneurship and innovation. Then, the success of clusters in developed countries has led many government and companies to establish new clusters.Since products from China have been dominated Indonesia's market share with lower price, it is very difficult for Indonesian Small and Medium Enterprises to compete with lower price also. Therefore, to face the competition, innovation is perhaps as an alternative strategy for Indonesian SMEs. In facts, more than 50% of small and medium enterprises in Indonesia are located in clusters and most of them are located in Java, Bali and Nusa Tenggara. Even though they located in cluster but their innovations still very low and judging from technology perspective, most of them have low level of technologies and still remain in the underdeveloped stage. Therefore, in this research, the author tries to find (1). To what extend do cluster Indonesia promote innovation, (2). To find the reasons why clusters in Indonesia has not been working well in promoting innovation and (3). To investigate what aspects can be improved by Indonesian SMEs to boost their innovation
On the anomalous large-scale flows in the Universe
Recent combined analyses of the CMB and galaxy cluster data reveal
unexpectedly large and anisotropic peculiar velocity fields at large scales. We
study cosmic models with included vorticity, acceleration and total angular
momentum of the Universe in order to understand the phenomenon. The Zeldovich
model is used to mimic the low redshift evolution of the angular momentum.
Solving coupled evolution equations of the second kind for density-contrast in
corrected Ellis-Bruni covariant and gauge-invariant formalism one can properly
normalize and evaluate integrated Sachs-Wolfe effect and peculiar velocity
field. The theoretical results compared to the observations favor a much larger
matter content of the Universe than that of the concordance model. Large-scale
flows appear anisotropic with dominant components placed in the plane
perpendicular to the axis of vorticity(rotation). The integrated Sachs-Wolfe
term has negative contribution to the CMB fluctuations for the negative
cosmological constant and it can explain the observed small power of the CMB TT
spectrum at large scales. The rate of the expansion of the Universe can be
substantially affected by the angular momentum if its magnitude is large
enough.Comment: 13 pages, 6 tables, 4 figures, 36 references; version to appear in
Eur. Phys. J.
Thermal spin torques in magnetic insulators
The damping of spin waves transmitted through a two-port magnonic device implemented on a yttrium iron garnet thin film is shown to be proportional to the temperature gradient imposed on the device. The sign of the damping depends on the relative orientation of the magnetic field, the wave vector, and the temperature gradient. The observations are accounted for qualitatively and quantitatively by using an extension of the variational principle that leads to the Landau-Lifshitz equation. All parameters of the model can be obtained by independent measurements
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