19,341 research outputs found
The impact of kinetic effects on the properties of relativistic electron-positron shocks
We assess the impact of non-thermally shock-accelerated particles on the
magnetohydrodynamic (MHD) jump conditions of relativistic shocks. The adiabatic
constant is calculated directly from first principle particle-in-cell
simulation data, enabling a semi-kinetic approach to improve the standard fluid
model and allowing for an identification of the key parameters that define the
shock structure. We find that the evolving upstream parameters have a stronger
impact than the corrections due to non-thermal particles. We find that the
decrease of the upstream bulk speed yields deviations from the standard MHD
model up to 10%. Furthermore, we obtain a quantitative definition of the shock
transition region from our analysis. For Weibel-mediated shocks the inclusion
of a magnetic field in the MHD conservation equations is addressed for the
first time
Exploring the nature of collisionless shocks under laboratory conditions
Collisionless shocks are pervasive in astrophysics and they are critical to
understand cosmic ray acceleration. Laboratory experiments with intense lasers
are now opening the way to explore and characterise the underlying
microphysics, which determine the acceleration process of collisionless shocks.
We determine the shock character - electrostatic or electromagnetic - based on
the stability of electrostatic shocks to transverse electromagnetic
fluctuations as a function of the electron temperature and flow velocity of the
plasma components, and we compare the analytical model with particle-in-cell
simulations. By making the connection with the laser parameters driving the
plasma flows, we demonstrate that shocks with different and distinct underlying
microphysics can be explored in the laboratory with state-of-the-art laser
systems
Four-dimensional gravity on supersymmetric dilatonic domain walls
We investigate the localization of four-dimensional metastable gravity in
supersymmetric dilatonic domain walls through massive modes by considering
several scenarios in the model. We compute corrections to the Newtonian
potential for small and long distances compared with a crossover scale given in
terms of the dilatonic coupling. 4D gravity behavior is developed on the brane
for distance very much below the crossover scale, while for distance much
larger, the 5D gravity is recovered. Whereas in the former regime gravity is
always attractive, in the latter regime due to non-normalizable unstable
massive graviton modes present on the spectrum, in some special cases, gravity
appears to be repulsive and signalizes a gravitational confining phase which is
able to produce an inflationary phase of the Universe.Comment: 11 pages, 4 figures, Latex. Version to appear in PL
Cyclical Effects of Bank Capital Buffers with Imperfect Credit Markets: international evidence
This paper analyzes the cyclical effects of bank capital buffers using an international sample of 2,361 banks from 92 countries over the 1990-2007 period. We find that capital buffers reduce the bank credit supply but – through what could be “monitoring or signaling effects” – have also an expansionary effect on economic activity by reducing lending and deposit rate spreads. This influence on lending and deposit rate spreads is more pronunced in developing countries and during downturns. The results suggest that capital buffers have a counter-cyclical effect in these countries. Our data do not suggest differences in the cyclical effects of capital buffers between Basel I and Basel II.
Controlled Shock Shells and Intracluster Fusion Reactions in the Explosion of Large Clusters
The ion phase-space dynamics in the Coulomb explosion of very large ( atoms) deuterium clusters can be tailored using two consecutive
laser pulses with different intensities and an appropriate time delay. For
suitable sets of laser parameters (intensities and delay), large-scale shock
shells form during the explosion, thus highly increasing the probability of
fusion reactions within the single exploding clusters. In order to analyze the
ion dynamics and evaluate the intracluster reaction rate, a one-dimensional
theory is used, which approximately accounts for the electron expulsion from
the clusters. It is found that, for very large clusters (initial radius
100 nm), and optimal laser parameters, the intracluster fusion yield becomes
comparable to the intercluster fusion yield. The validity of the results is
confirmed with three-dimensional particle-in-cell simulations.Comment: 25 pages, 11 figures, to appear in Physical Review
Concentração plasmática de progesterona (P4) de cabras em lactação tratadas com hCG durante a estação de acasalamento natural.
An Introduction to Ontologies and Ontology Engineering
In the last decades, the use of ontologies in information systems has become more and more popular in various fields, such as web technologies, database integration, multi agent systems, natural language processing, etc. Artificial intelligent researchers have initially borrowed the word “ontology” from Philosophy, then the word spread in many scientific domain and ontologies are now used in several developments. The main goal of this chapter is to answer generic questions about ontologies, such as: Which are the different kinds of ontologies? What is the purpose of the use of ontologies in an application? Which methods can I use to build an ontology
Avaliação experimental do efeito de "strain shielding" no fémur distal induzido pela utilização de hastes na revisão da artroplastia total do joelho
A utilização de hastes na revisão da artroplastia total do joelho é um procedimento clínico
comum quando a qualidade do osso na superfície condilar é de baixa qualidade ou então
apresenta defeitos significativos. A haste é usada para aumentar a estabilidade do
componente condilar necessária a longevidade da artroplastia. No entanto, a utilização de
hastes é referida como causa para o aumento do efeito de stress/strain-shielding no osso,
podendo este conduzir a um efeito de reabsorção óssea. Os resultados experimentais obtidos
evidenciaram que a utilização de hastes altera o estado de deformação do osso relativamente
ao estado fisiológico, sendo estas mais desfavoráveis para o caso da utilização de hastes do
tipo press-fit. A utilização de hastes promove o efeito de strain-shielding que potencia o efeito
de reabsorção óssea podendo originar a instabilidade do implante a longo prazo. A questão
que se coloca é saber até que ponto a eventual reabsorção óssea é suficiente para provocar a
instabilidade da haste e consequentemente a mesma da prótese a longo prazo. A fixação
biomecânica (ou biológica) conseguida nos primeiros anos da artroplastia pode ser
suficiente para evitar os efeitos nefastos da reabsorção óssea
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