246 research outputs found
Hyperbolic character of the angular moment equations of radiative transfer and numerical methods
We study the mathematical character of the angular moment equations of
radiative transfer in spherical symmetry and conclude that the system is
hyperbolic for general forms of the closure relation found in the literature.
Hyperbolicity and causality preservation lead to mathematical conditions
allowing to establish a useful characterization of the closure relations. We
apply numerical methods specifically designed to solve hyperbolic systems of
conservation laws (the so-called Godunov-type methods), to calculate numerical
solutions of the radiation transport equations in a static background. The
feasibility of the method in any kind of regime, from diffusion to
free-streaming, is demonstrated by a number of numerical tests and the effect
of the choice of the closure relation on the results is discussed.Comment: 37 pags, 12 figures, accepted for publication in MNRA
Evidence for Heating of Neutron Stars by Magnetic Field Decay
We show the existence of a strong trend between neutron star surface
temperature and the dipolar component of the magnetic field extending through
three orders of field magnitude, a range that includes magnetars, radio-quiet
isolated neutron stars, and many ordinary radio pulsars. We suggest that this
trend can be explained by the decay of currents in the crust over a time scale
of few Myr. We estimate the minimum temperature that a NS with a given magnetic
field can reach in this interpretation.Comment: 4 pages, 1 figures, version accepted for publication in Phys. Rev.
Let
Obtención de las soluciones periódicas de un oscilador no lineal mediante un método rápido de Galerkin
Este artÃculo está enfocado a la determinación de las soluciones periódicas de los osciladores no lineales asà como al análisis cualitativo de su estabilidad. Estos osciladores están modelizados
por la ecuación diferencial
3
Z ( t ) + kk(t) + E ajzj = g(t)
j=1
siendo g(t) una fuerza T-periódica.
En este trabajo desarrollamos un algoritmo basado en el método de Galerkin que utiliza
la transformada rápida de Fourier (FFT) para calcular las soluciones periódicas de la ecuación anterior. Además, incluimos un algoritmo combinado en un apéndice, con convergencia rápida, para resolver las ecuaciones algebraicas no lineales obtenidas por dicho método. Finalmente, validamos esta metodologÃa aplicando el algoritmo pa,ra obtener las soluciones periódicas de un oscilador de Duffing con comportamiento caótico.This paper is focused to the determination of the harmonic solutions of the non linear
oscillators modelled by the following differential equation
3
Z (t) + kk (t) + E a, r' = g (t)
j=l
being g(t) a T-periodic driving force. In this work we develop an algorithm based on the Galerkin method, using the Fast Fourier
Transform (FFT) to calculate the harmonic solutions of previous equation. Furthermore, we include a combined algorithm with fast convergence to solve the non linear algebraic equations obtained in the Galerkin Fast Algorithm. Finally, we validate this methodology applying the algorithm to obtain the harmonic solutions of a Duffing oscillator with a chaotic behavior.Peer Reviewe
Prevalence of dens invaginatus assessed by CBCT : systematic review and meta-analysis
Dens invaginatus is a developmental dental anomaly resulting from an invagination of dental tissues folding from the outer surface towards dental pulp. The aim of this systematic review and meta-analysis was to determine the prevalence of dens invaginatu
Numerical 3+1 general relativistic magnetohydrodynamics: a local characteristic approach
We present a general procedure to solve numerically the general relativistic
magnetohydrodynamics (GRMHD) equations within the framework of the 3+1
formalism. The work reported here extends our previous investigation in general
relativistic hydrodynamics (Banyuls et al. 1997) where magnetic fields were not
considered. The GRMHD equations are written in conservative form to exploit
their hyperbolic character in the solution procedure. All theoretical
ingredients necessary to build up high-resolution shock-capturing schemes based
on the solution of local Riemann problems (i.e. Godunov-type schemes) are
described. In particular, we use a renormalized set of regular eigenvectors of
the flux Jacobians of the relativistic magnetohydrodynamics equations. In
addition, the paper describes a procedure based on the equivalence principle of
general relativity that allows the use of Riemann solvers designed for special
relativistic magnetohydrodynamics in GRMHD. Our formulation and numerical
methodology are assessed by performing various test simulations recently
considered by different authors. These include magnetized shock tubes,
spherical accretion onto a Schwarzschild black hole, equatorial accretion onto
a Kerr black hole, and magnetized thick accretion disks around a black hole
prone to the magnetorotational instability.Comment: 18 pages, 8 figures, submitted to Ap
Comparative study of debris and smear layer removal with EDTA and Er,Cr:YSGG laser
Background: To evaluate in vitro , the ability in removing debris and Smear Layer of 17% EDTA and Er,Cr:YSGG laser. Material and Methods: 58 unirradicular teeth were instrumented with MTwo® and divided into 3 groups according to irrigation protocol: 17%EDTA, laser and a combination of 17%EDTA and laser. All samples were analyzed in the apical and middle third with Scanning Electron Microscope. The Chi-cuadrado and McNemar tests were used to determine the statistical analysis and data processing and analysis was performed with the statistical package StatGraphics Centurion XVI. Results: Debris analysis showed statistical significant differences when compared EDTA vs laser and EDTA vs EDTA+laser in the middle third. The Smear Layer removal showed statistical significant differences in the middle third when compared EDTA vs laser and EDTA vs EDTA+laser. Conclusions: Laser showed a greater cleaning capacity than EDTA in the middle third; the cleanliness was even better when combined laser with EDTA, so the effect is accumulative
On numerical relativistic hydrodynamics and barotropic equations of state
The characteristic formulation of the relativistic hydrodynamic equations
(Donat et al 1998 J. Comput. Phys. 146 58), which has been implemented in many
relativistic hydro-codes that make use of Godunov-type methods, has to be
slightly modified in the case of evolving barotropic flows. For a barotropic
equation of state, a removable singularity appears in one of the eigenvectors.
The singularity can be avoided by means of a simple renormalization which makes
the system of eigenvectors well defined and complete. An alternative strategy
for the particular case of barotropic flows is discussed.Comment: 7 pages, no figures. Accepted for publication in Class. Quantum Gra
Resonant Kelvin-Helmholtz modes in sheared relativistic flows
Qualitatively new aspects of the (linear and non-linear) stability of sheared
relativistic (slab) jets are analyzed. The linear problem has been solved for a
wide range of jet models well inside the ultrarelativistic domain (flow Lorentz
factors up to 20; specific internal energies ). As a distinct
feature of our work, we have combined the analytical linear approach with
high-resolution relativistic hydrodynamical simulations, which has allowed us
i) to identify, in the linear regime, resonant modes specific to the
relativistic shear layer ii) to confirm the result of the linear analysis with
numerical simulations and, iii) more interestingly, to follow the instability
development through the non-linear regime. We find that very high-order
reflection modes with dominant growth rates can modify the global, long-term
stability of the relativistic flow. We discuss the dependence of these resonant
modes on the jet flow Lorentz factor and specific internal energy, and on the
shear layer thickness. The results could have potential applications in the
field of extragalactic relativistic jets.Comment: Accepted for publication in Physical Review E. For better quality
images, please check
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Tuning the Néel temperature in an antiferromagnet: the case of NixCo1−xO microstructures
We show that it is possible to tune the Néel temperature of nickel(II)-cobalt(II) oxide films by changing the Ni to Co ratio. We grow single crystalline micrometric triangular islands with tens of nanometers thickness on a Ru(0001) substrate using high temperature oxygen-assisted molecular beam epitaxy. Composition is controlled by adjusting the deposition rates of Co and Ni. The morphology, shape, crystal structure and composition are determined by low-energy electron microscopy and diffraction, and synchrotron-based x-ray absorption spectromicroscopy. The antiferromagnetic order is observed by x-ray magnetic linear dichroism. Antiferromagnetic domains up to micrometer width are observedThis work is supported by the Spanish Agencia Estatal de Investigación (MCIU/AEI/FEDER, EU)) through Projects Nos MAT2015-64110-C2-1-P, MAT2015-64110-C2-2-P, RTI2018-095303-B-C51, and RTI2018-095303-B-C53, by the European Commission through Project H2020 No. 720853 (Amphibian) and by the Comunidad de Madrid through Project. NANOMAGCOST-CM P2018/NMT-4321. These experiments were performed at the CIRCE beamline of the ALBA Synchrotron Light Facility. A.M. acknowledges funding via a CSIC-Alba agreemen
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