4,412 research outputs found
Rotating and counterrotating relativistic thin disks as sources of stationary electrovacuum spacetimes
A detailed study is presented of the counterrotating model (CRM) for
electrovacuum stationary axially symmetric relativistic thin disks of infinite
extension without radial stress, in the case when the eigenvalues of the
energy-momentum tensor of the disk are real quantities, so that there is not
heat flow. We find a general constraint over the counterrotating tangential
velocities needed to cast the surface energy-momentum tensor of the disk as the
superposition of two counterrotating charged dust fluids. We then show that, in
some cases, this constraint can be satisfied if we take the two counterrotating
tangential velocities as equal and opposite or by taking the two
counterrotating streams as circulating along electro-geodesics. However, we
show that, in general, it is not possible to take the two counterrotating
fluids as circulating along electro-geodesics nor take the two counterrotating
tangential velocities as equal and opposite. A simple family of models of
counterrotating charged disks based on the Kerr-Newman solution are considered
where we obtain some disks with a CRM well behaved. We also show that the disks
constructed from the Kerr-Newman solution can be interpreted, for all the
values of parameters, as a matter distribution with currents and purely
azimuthal pressure without heat flow. The models are constructed using the
well-known "displace, cut and reflect" method extended to solutions of vacuum
Einstein-Maxwell equations. We obtain, in all the cases, counterrotating
Kerr-Newman disks that are in agreement with all the energy conditions.Comment: 22 pages, 7 figures, Late
Geometrical resonance in spatiotemporal systems
We generalize the concept of geometrical resonance to perturbed sine-Gordon,
Nonlinear Schrödinger and Complex Ginzburg-Landau equations. Using this
theory we can control different dynamical patterns. For instance, we can
stabilize breathers and oscillatory patterns of large amplitudes successfully
avoiding chaos. On the other hand, this method can be used to suppress
spatiotemporal chaos and turbulence in systems where these phenomena are
already present. This method can be generalized to even more general
spatiotemporal systems.Comment: 2 .epl files. Accepted for publication in Europhysics Letter
Impacto del uso de combustibles alternativos sobre la reactividad del clínker
The use of alternative fuels in the cement industry has increased its relevance in the past decades due to their ecological and economic benefits. In concert with the efforts to increase its use, several studies have focused on their potential impact with respect to clinker reactivity and how they could affect the expected physical and mechanical properties. This work studied the effects of five alternative fuels on the reactivity of eight industrial clinker samples, considering several analytical techniques. Differences were identified among the clinker samples after replacing the alternative fuels, mainly with simultaneous eliminations, as in samples S4, S5 and S8. The modifications were related to the polymorph, size and reactivity of tricalcium silicate and to the clinker profile during the hydration process, due to the SO3 consumption rate. These changes were expressed in the higher compressive strengths in comparison with the original reference clinker.El uso de combustibles alternativos en la industria cementera ha ido en incremento en las últimas décadas debido a su beneficio ecológico y económico. En línea con los esfuerzos enfocados en incrementar su uso, se han realizado distintos estudios enfocados en determinar su impacto potencial en la reactividad del clínker y en cómo pueden afectar sus propiedades físicas y mecánicas. Este trabajo estudia el efecto de cinco combustibles alternos sobre la reactividad de ocho muestras industriales de clínker, considerando diferentes técnicas analíticas. Se identificaron diferencias principalmente cuando se realizaron eliminaciones simultáneas de varios combustibles, como en las muestras S4, S5 y S8. Las modificaciones se basan en un cambio en el polimorfo, tamaño y reactividad del silicato tricálcico y del perfil de hidratación de los clínkeres, relacionado con el ritmo del consumo del SO3. Las modificaciones se reflejaron en un incremento en la resistencia a la compresión
Analysis of non-regulated vehicular emissions by extractive FTIR spectrometry: tests on a hybrid car in Mexico City
International audienceA methodology to acquire valuable information on the chemical composition and evolution of vehicular emissions is presented. The analysis of the gases is performed by passing a constant flow of a sample gas from the tail-pipe into a 10 L multi-pass cell. The absorption spectra within the cell are obtained using an FTIR spectrometer at 0.5 cm-1 resolution along a 13.1 m optical path. Additionally, the total flow from the exhaust is continuously measured from a differential pressure sensor on a Pitot tube installed at the exit of the exhaust. This configuration aims to obtain a good speciation capability by coadding spectra during 30 s and reporting the emission (in g/km) of key and non-regulated pollutants, such as CO2, CO, NO, SO2, NH3, HCHO, NMHC, during predetermined driving routines. The advantages and disadvantages of increasing the acquisition frequency, as well as the effect of other parameters such as spectral resolution, cell volume and flow rate, are discussed. With the aim of testing and evaluating the proposed technique, experiments were performed on a dynamometer running FTP-75 and typical driving cycles of the Mexico City Metropolitan Area (MCMA) on a Toyota Prius hybrid vehicle. This car is an example of recent automotive technology to reach the market dedicated to reduce emissions and therefore pressing the need of low detection techniques. This study shows the potential of the proposed technique to measure and report in real time the emissions of a large variety of pollutants, even from a super ultra-low emission vehicle (SULEV). The emissions of HC's, NOx, CO and CO2 obtained here are similar to experiments performed in other locations with the same vehicle model. Some differences suggest that an inefficient combustion process and type of gasoline used in the MCMA may be partly responsible for lower CO2 and higher CO and NO emission factors. Also, a fast reduction of NO emission to very low values is observed after cold ignition, giving rise to moderate N2O and eventually NH3 emissions. The proposed technique provides a tool for future studies comparing in detail the emissions of different technologies using alternative fuels and emission control systems
Tecnical Note: Analysis of non-regulated vehicular emissions by extractive FTIR spectrometry: tests on a hybrid car in Mexico City
International audienceA methodology to acquire valuable information on the chemical composition and evolution of vehicular emissions is presented. The analysis of the gases is performed by passing a constant flow of a sample gas from the tail-pipe into a 10 L multi-pass cell. The absorption spectra within the cell are obtained using an FTIR spectrometer at 0.5 cm?1 resolution along a 13.1 m optical path. Additionally, the total flow from the exhaust is continuously measured from a differential pressure sensor on a \textit{Pitot} tube installed at the exit of the exhaust. This configuration aims to obtain a good speciation capability by coadding spectra during 30 s and reporting the emission (in g/km) of both criteria and non-regulated pollutants, such as CO2, CO, NO, SO2, NH3, HCHO and some NMHC, during predetermined driving cycles. The advantages and disadvantages of increasing the measurement frequency, as well as the effect of other parameters such as spectral resolution, cell volume and flow rate, are discussed. To test and evaluate the proposed technique, experiments were performed on a dynamometer running FTP-75 and typical driving cycles for the Mexico City Metropolitan Area (MCMA) on a Toyota Prius hybrid vehicle. This car is an example of recent marketed automotive technology dedicated to reduced emissions, increasing the need for sensitive detection techniques. This study shows the potential of the proposed technique to measure and report in real time the emissions of a large variety of pollutants, even from a super ultra-low emission vehicle (SULEV). The emissions of HC's, NOx, CO and CO2 obtained here were compared to experiments performed in other locations with the same model vehicle. The proposed technique provides a tool for future studies comparing in detail the emissions of vehicles using alternative fuels and emission control systems
Klein paradox between transmitted and reflected Dirac waves on Bour surfaces
It is supposed the existence of a curved graphene sheet with the geometry of
a Bour surface , such as the catenoid (or helicoid), , and the
classical Enneper surface, , among others. In particular, in this work,
the propagation of the electronic degrees of freedom on these surfaces is
studied based on the Dirac equation. As a consequence of the polar geometry of
, it is found that the geometry of the surface causes the Dirac fermions
to move as if they would be subjected to an external potential coupled to a
spin-orbit term. The geometry-induced potential is interpreted as a barrier
potential, which is asymptotically zero. Furthermore, the behaviour of
asymptotic Dirac states and scattering states are studied through the
Lippmann-Schwinger formalism. It is found that for surfaces and
, the total transmission phenomenon is found for sufficiently large
values of energy, while for surfaces , with , it is shown that
there is an energy point where Klein's paradox is realized, while for
energy values it is found that the conductance of the hypothetical
material is completely suppressed, .Comment: 31 pages, 9 figure
3D numerical model of the Omega Nebula (M17): simulated thermal X-ray emission
We present 3D hydrodynamical simulations of the superbubble M17, also known
as the Omega nebula, carried out with the adaptive grid code yguazu'-a, which
includes radiative cooling. The superbubble is modelled considering the winds
of 11 individual stars from the open cluster inside the nebula (NGC 6618), for
which there are estimates of the mass loss rates and terminal velocities based
on their spectral types. These stars are located inside a dense interstellar
medium, and they are bounded by two dense molecular clouds.
We carried out three numerical models of this scenario, considering different
line of sight positions of the stars (the position in the plane of the sky is
known, thus fixed). Synthetic thermal X-ray emission maps are calculated from
the numerical models and compared with ROSAT observations of this astrophysical
object. Our models reproduce successfully both the observed X-ray morphology
and the total X-ray luminosity, without taking into account thermal conduction
effects.Comment: 8 pages, 6 figures, accepted for publication in MNRA
Necesidades, intereses, barreras y atributos de un profesor investigador
Este trabajo da cuenta del proceso que se realizó durante un año, acerca de encontrar los elementos que constituyen a un profesor investigador, como las necesidades, intereses barreras y atributos, para lo cual se realizaron tres estudios de caso a tres profesoras de matemáticas de educación básica y media de Bogotá, realizando observaciones y entrevistas para recolectar información que posteriormente se analizarla y llegar a algunas conclusiones acerca de dichos elementos
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