6,112 research outputs found
Invited papers from the international meeting on 'New Frontiers in Numerical Relativity' (Albert Einstein Institute, Potsdam, Germany, 17-21 July 2006)
Traditionally, frontiers represent a treacherous terrain to venture into, where hidden obstacles are present and uncharted territories lie ahead. At the same time, frontiers are also a place where new perspectives can be appreciated and have often been the cradle of new and thriving developments. With this in mind and inspired by this spirit, the Numerical Relativity Group at the Albert Einstein Institute (AEI) organized a `New Frontiers in Numerical Relativity' meeting on 17–21 July 2006 at the AEI campus in Potsdam, Germany
Prediction of ductile fracture in anisotropic steels for pipeline applications
Large diameter steel pipelines for gas transportation may experience extreme overloads due to external actions such as soil sliding, faults movements, third part interactions. In these scenarios the material undergoes severe plastic strains which locally may reach the fracture limits. Due to the manufacturing process, the steels used in such applications have an anisotropic behavior both for plasticity and fracture. In this paper two steel grades have been characterized in view of anisotropic plastic fracture. Fracture tests have been planned to characterize the fracture behavior under different stress states and in different directions to define the anisotropic sensitivity. Finite element modelling, incorporating an anisotropic plasticity formulation, has been used to calculate the local fracture parameters in the specimens and to define the complete ductile fracture locus. An uncoupled damage evolution law has been finally used to evaluate the fracture limits on real pipelines failed in full scale laboratory tests. The strain to fracture prediction has been verified by local strain measurements on the fractured pipes. The model robustness has been also verified on global parameter predictions, such us the burst pressur
A Complete Statistical Analysis for the Quadrupole Amplitude in an Ellipsoidal Universe
A model of Universe with a small eccentricity due to the presence of a
magnetic field at the decoupling time (i.e. an Ellipsoidal Universe) has been
recently proposed for the solution of the low quadrupole anomaly of the angular
power spectrum of cosmic microwave background anisotropies. We present a
complete statistical analysis of that model showing that the probability of
increasing of the amplitude of the quadrupole is larger than the probability of
decreasing in the whole parameters' space.Comment: 5 pages, 3 figure
Dynamics of Ferromagnetic Walls: Gravitational Properties
We discuss a new mechanism which allows domain walls produced during the
primordial electroweak phase transition. We show that the effective surface
tension of these domain walls can be made vanishingly small due to a peculiar
magnetic condensation induced by fermion zero modes localized on the wall. We
find that in the perfect gas approximation the domain wall network behaves like
a radiation gas. We consider the recent high-red shift supernova data and we
find that the corresponding Hubble diagram is compatible with the presence in
the Universe of a ideal gas of ferromagnetic domain walls. We show that our
domain wall gas induces a completely negligible contribution to the large-scale
anisotropy of the microwave background radiation.Comment: Replaced with revised version, accepted for publication in IJMP
Testing the Isotropy of the Universe with Type Ia Supernovae
We analyze the magnitude-redshift data of type Ia supernovae included in the
Union and Union2 compilations in the framework of an anisotropic Bianchi type I
cosmological model and in the presence of a dark energy fluid with anisotropic
equation of state. We find that the amount of deviation from isotropy of the
equation of state of dark energy, the skewness \delta, and the present level of
anisotropy of the large-scale geometry of the Universe, the actual shear
\Sigma_0, are constrained in the ranges -0.16 < \delta < 0.12 and -0.012 <
\Sigma_0 < 0.012 (1\sigma C.L.) by Union2 data. Supernova data are then
compatible with a standard isotropic universe (\delta = \Sigma_0 = 0), but a
large level of anisotropy, both in the geometry of the Universe and in the
equation of state of dark energy, is allowed.Comment: 12 pages, 7 figures, 2 tables. Union2 analysis added. New references
added. To appear in Phys. Rev.
The importance of precession in modelling the direction of the final spin from a black-hole merger
The prediction of the spin of the black hole resulting from the merger of a
generic black-hole binary system is of great importance to study the
cosmological evolution of supermassive black holes. Several attempts have been
recently made to model the spin via simple expressions exploiting the results
of numerical-relativity simulations. Here, I first review the derivation of a
formula, proposed in Barausse & Rezzolla, Apj 704 L40, which accurately
predicts the final spin magnitude and direction when applied to binaries with
separations of hundred or thousands of gravitational radii. This makes my
formula particularly suitable for cosmological merger-trees and N-body
simulations, which provide the spins and angular momentum of the two black
holes when their separation is of thousands of gravitational radii. More
importantly, I investigate the physical reason behind the good agreement
between my formula and numerical relativity simulations, and nail it down to
the fact that my formula takes into account the post-Newtonian precession of
the spins and angular momentum in a consistent manner.Comment: 6 pages, 2 figures. Panel added to fig 2, discussion extended to
comply with referee's comments. Version accepted for publication as
proceeding of the 8th Amaldi International Conference on Gravitational Waves,
NYC, 21-26 June 200
The Lazarus project: A pragmatic approach to binary black hole evolutions
We present a detailed description of techniques developed to combine 3D
numerical simulations and, subsequently, a single black hole close-limit
approximation. This method has made it possible to compute the first complete
waveforms covering the post-orbital dynamics of a binary black hole system with
the numerical simulation covering the essential non-linear interaction before
the close limit becomes applicable for the late time dynamics. To determine
when close-limit perturbation theory is applicable we apply a combination of
invariant a priori estimates and a posteriori consistency checks of the
robustness of our results against exchange of linear and non-linear treatments
near the interface. Once the numerically modeled binary system reaches a regime
that can be treated as perturbations of the Kerr spacetime, we must
approximately relate the numerical coordinates to the perturbative background
coordinates. We also perform a rotation of a numerically defined tetrad to
asymptotically reproduce the tetrad required in the perturbative treatment. We
can then produce numerical Cauchy data for the close-limit evolution in the
form of the Weyl scalar and its time derivative
with both objects being first order coordinate and tetrad invariant. The
Teukolsky equation in Boyer-Lindquist coordinates is adopted to further
continue the evolution. To illustrate the application of these techniques we
evolve a single Kerr hole and compute the spurious radiation as a measure of
the error of the whole procedure. We also briefly discuss the extension of the
project to make use of improved full numerical evolutions and outline the
approach to a full understanding of astrophysical black hole binary systems
which we can now pursue.Comment: New typos found in the version appeared in PRD. (Mostly found and
collected by Bernard Kelly
Lorentz Symmetry Violation and Galactic Magnetism
We analyze the generation of primordial magnetic fields during de Sitter
inflation in a Lorentz-violating theory of Electrodynamics containing a
Chern-Simons term which couples the photon to an external four-vector. We find
that, for appropriate magnitude of the four-vector, the generated field is
maximally helical and, through an inverse cascade caused by turbulence of
primeval plasma, reaches at the time of protogalactic collapse an intensity and
correlation length such as to directly explain galactic magnetism.Comment: 5 pages, minor revisions, version published in Phys. Lett.
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