294 research outputs found
Revisiting Rotational Perturbations and the Microwave Background
We consider general-relativistic rotational perturbations in homogeneous and
isotropic Friedman - Robertson - Walker (FRW) cosmologies. Taking linear
perturbations of FRW models, the general solution of the field equations
contains tensorial, vectorial and scalar functions. The vectorial terms are in
connection with rotations in the given model and due to the Sachs - Wolfe
effect they produce contributions to the temperature fluctuations of the cosmic
microwave background radiation (CMBR). In present paper we obtain the analytic
time dependence of these contributions in a spatially flat, FRW model with
pressureless ideal fluid, in the presence and the absence of a cosmological
constant. We find that the solution can be separated into an integrable and a
non-integrable part as is the situation in the case of scalar perturbations.
Analyzing the solutions and using the results of present observations we
estimate the order of magnitude of the angular velocity corresponding to the
rotation tensor at the time of decoupling and today.Comment: accepted for publication in Int. J. Mod. Phys.
New Relativistic Effects in the Dynamics of Nonlinear Hydrodynamical Waves
In Newtonian and relativistic hydrodynamics the Riemann problem consists of
calculating the evolution of a fluid which is initially characterized by two
states having different values of uniform rest-mass density, pressure and
velocity. When the fluid is allowed to relax, one of three possible
wave-patterns is produced, corresponding to the propagation in opposite
directions of two nonlinear hydrodynamical waves. New effects emerge in a
special relativistic Riemann problem when velocities tangential to the initial
discontinuity surface are present. We show that a smooth transition from one
wave-pattern to another can be produced by varying the initial tangential
velocities while otherwise maintaining the initial states unmodified. These
special relativistic effects are produced by the coupling through the
relativistic Lorentz factors and do not have a Newtonian counterpart.Comment: 4 pages, 5 figure
Nonequilibrium corrections in the pressure tensor due to an energy flux
The physical interpretation of the nonequilibrium corrections in the pressure
tensor for radiation submitted to an energy flux obtained in some previous
works is revisited. Such pressure tensor is shown to describe a moving
equilibrium system but not a real nonequilibrium situation.Comment: 4 pages, REVTeX, Brief Report to appear in PRE Dec 9
Resonant interaction between gravitational waves, electromagnetic waves and plasma flows
In magnetized plasmas gravitational and electromagnetic waves may interact
coherently and exchange energy between themselves and with plasma flows. We
derive the wave interaction equations for these processes in the case of waves
propagating perpendicular or parallel to the plasma background magnetic field.
In the latter case, the electromagnetic waves are taken to be circularly
polarized waves of arbitrary amplitude. We allow for a background drift flow of
the plasma components which increases the number of possible evolution
scenarios. The interaction equations are solved analytically and the
characteristic time scales for conversion between gravitational and
electromagnetic waves are found. In particular, it is shown that in the
presence of a drift flow there are explosive instabilities resulting in the
generation of gravitational and electromagnetic waves. Conversely, we show that
energetic waves can interact to accelerate particles and thereby \emph{produce}
a drift flow. The relevance of these results for astrophysical and cosmological
plasmas is discussed.Comment: 12 pages, 1 figure, typos corrected and numerical example adde
An HLLC Solver for Relativistic Flows -- II. Magnetohydrodynamics
An approximate Riemann solver for the equations of relativistic
magnetohydrodynamics (RMHD) is derived. The HLLC solver, originally developed
by Toro, Spruce and Spears, generalizes the algorithm described in a previous
paper (Mignone & Bodo 2004) to the case where magnetic fields are present. The
solution to the Riemann problem is approximated by two constant states bounded
by two fast shocks and separated by a tangential wave. The scheme is
Jacobian-free, in the sense that it avoids the expensive characteristic
decomposition of the RMHD equations and it improves over the HLL scheme by
restoring the missing contact wave.
Multidimensional integration proceeds via the single step, corner transport
upwind (CTU) method of Colella, combined with the contrained tranport (CT)
algorithm to preserve divergence-free magnetic fields. The resulting numerical
scheme is simple to implement, efficient and suitable for a general equation of
state. The robustness of the new algorithm is validated against one and two
dimensional numerical test problems.Comment: 17 pages, 12 figure
Building Fuzzy Elevation Maps from a Ground-based 3D Laser Scan for Outdoor Mobile Robots
Mandow, A; Cantador, T.J.; Reina, A.J.; Martínez, J.L.; Morales, J.; García-Cerezo, A. "Building Fuzzy Elevation Maps from a Ground-based 3D Laser Scan for Outdoor Mobile Robots," Robot2015: Second Iberian Robotics Conference, Advances in Robotics, (2016) Advances in Intelligent Systems and Computing, vol. 418. This is a self-archiving copy of the author’s accepted manuscript. The final publication is available at Springer via
http://link.springer.com/book/10.1007/978-3-319-27149-1.The paper addresses terrain modeling for mobile robots with fuzzy elevation maps by improving computational
speed and performance over previous work on fuzzy terrain identification from a three-dimensional (3D) scan. To this end,
spherical sub-sampling of the raw scan is proposed to select training data that does not filter out salient obstacles. Besides,
rule structure is systematically defined by considering triangular sets with an unevenly distributed standard fuzzy partition
and zero order Sugeno-type consequents. This structure, which favors a faster training time and reduces the number of rule
parameters, also serves to compute a fuzzy reliability mask for the continuous fuzzy surface. The paper offers a case study
using a Hokuyo-based 3D rangefinder to model terrain with and without outstanding obstacles. Performance regarding error
and model size is compared favorably with respect to a solution that uses quadric-based surface simplification (QSlim).This work was partially supported by the Spanish CICYT project DPI 2011-22443, the Andalusian project PE-2010 TEP-6101, and Universidad de Málaga-Andalucía Tech
Nonlinear coupled Alfv\'{e}n and gravitational waves
In this paper we consider nonlinear interaction between gravitational and
electromagnetic waves in a strongly magnetized plasma. More specifically, we
investigate the propagation of gravitational waves with the direction of
propagation perpendicular to a background magnetic field, and the coupling to
compressional Alfv\'{e}n waves. The gravitational waves are considered in the
high frequency limit and the plasma is modelled by a multifluid description. We
make a self-consistent, weakly nonlinear analysis of the Einstein-Maxwell
system and derive a wave equation for the coupled gravitational and
electromagnetic wave modes. A WKB-approximation is then applied and as a result
we obtain the nonlinear Schr\"{o}dinger equation for the slowly varying wave
amplitudes. The analysis is extended to 3D wave pulses, and we discuss the
applications to radiation generated from pulsar binary mergers. It turns out
that the electromagnetic radiation from a binary merger should experience a
focusing effect, that in principle could be detected.Comment: 20 pages, revtex4, accepted in PR
Deep venous thrombosis and abortion: an unusual clinical manifestation of severe form of pectus excavatum
Pectus excavatum is a chest wall malformation with a strong psychological and aesthetic impact. Rarely, pectus excavatum patients can show respiratory or cardiac symptoms occurring mainly during physical exertion. We report a case of a 34-year-old pregnant woman with a severe degree of pectus excavatum who developed serious cardiovascular disease resulting in spontaneous twin abortion at the twenty-first week of gestation. Cardiovascular disease was resolved after open surgical correction of pectus excavatum. This case shows how a tardive diagnosis and a delayed surgical approach for pectus excavatum can lead to severe consequences
Unconstrained Hamiltonian formulation of General Relativity with thermo-elastic sources
A new formulation of the Hamiltonian dynamics of the gravitational field
interacting with(non-dissipative) thermo-elastic matter is discussed. It is
based on a gauge condition which allows us to encode the six degrees of freedom
of the ``gravity + matter''-system (two gravitational and four
thermo-mechanical ones), together with their conjugate momenta, in the
Riemannian metric q_{ij} and its conjugate ADM momentum P^{ij}. These variables
are not subject to constraints. We prove that the Hamiltonian of this system is
equal to the total matter entropy. It generates uniquely the dynamics once
expressed as a function of the canonical variables. Any function U obtained in
this way must fulfil a system of three, first order, partial differential
equations of the Hamilton-Jacobi type in the variables (q_{ij},P^{ij}). These
equations are universal and do not depend upon the properties of the material:
its equation of state enters only as a boundary condition. The well posedness
of this problem is proved. Finally, we prove that for vanishing matter density,
the value of U goes to infinity almost everywhere and remains bounded only on
the vacuum constraints. Therefore the constrained, vacuum Hamiltonian (zero on
constraints and infinity elsewhere) can be obtained as the limit of a ``deep
potential well'' corresponding to non-vanishing matter. This unconstrained
description of Hamiltonian General Relativity can be useful in numerical
calculations as well as in the canonical approach to Quantum Gravity.Comment: 29 pages, TeX forma
Preliminary experience with 4K ultra-high definition endoscope: analysis of pros and cons in skull base surgery
Negli ultimi venti anni la chirurgia endoscopica del basicranio ha osservato continui sviluppi tecnici e tecnologici. Lendoscopia 3D e l alta definizione (HD) 4K hanno fornito grandi vantaggi in termini di visualizzazione e di risoluzione spaziale. L ultra HD 4K, recentemente introdotta nella pratica clinica, determinerà i prossimi passi soprattutto nella chirurgica endoscopica del basicranio. I pazienti sono stati operati attraverso un approccio transnasale transfenoidale endoscopico, utilizzando un endoscopio Olympus NBI 4K UHD con ottica 4 mm 0 ° Ultra Telescope, lampada allo xeno 300 W (CLV-S400) predisposto per la tecnologia narrow band imaging (NBI) collegato con una videocamera ad un alta qualità unità di controllo (OTV-S400 - VISERA 4K UHD) (Olympus, Tokyo, Giappone). Due schermi, un 31 Monitor - (LMD-X310S) e quello principale ultra-HD 55 a pollici ottimizzati per la riproduzione immagini UHD (LMD-X550S). In casi selezionati abbiamo usato un sistema di navigazione (Stealthstation S7, Medtronic, Minneapolis, MN, Stati Uniti). Abbiamo valutato 22 adenomi ipofisari (86,3% macroadenomi; 13,7% microadenomi). Il 50% non erano secernenti (NS), 22,8% GH, 18,2% ACTH, 9% PRLsecernenti. 3/22 erano recidive. Nel 91% dei casi abbiamo raggiunto la rimozione totale, mentre nel 9% la resezione subtotale. Un followup medio di 187 giorni, durata media del ricovero era 3,09 ± 0,61 giorni. Tempo chirurgico 128,18 ± 30,74 minuti. Abbiamo avuto solo 1 caso di fistola intraoperatoria a basso flusso senza ulteriori complicazioni nel follow up. Il 100% dei casi non ha richiesto emotrasfusione. La visualizzazione e lalta risoluzione del campo operatorio hanno fornito una vista dettagliata di tutte le strutture anatomiche e patologie e permesso il miglioramento della sicurezza e lefficacia della procedura chirurgica. Il tempo operatorio è stato simile a quello dellendoscopio HD standard 2D e 3D, come la fatica fisica era paragonabile ad altri in termini di ergonomicità e peso
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