203 research outputs found
Non-axisymmetric relativistic Bondi-Hoyle accretion onto a Schwarzschild black hole
We present the results of an exhaustive numerical study of fully relativistic
non-axisymmetric Bondi-Hoyle accretion onto a moving Schwarzschild black hole.
We have solved the equations of general relativistic hydrodynamics with a
high-resolution shock-capturing numerical scheme based on a linearized Riemann
solver. The numerical code was previously used to study axisymmetric flow
configurations past a Schwarzschild hole. We have analyzed and discussed the
flow morphology for a sample of asymptotically high Mach number models. The
results of this work reveal that initially asymptotic uniform flows always
accrete onto the hole in a stationary way which closely resembles the previous
axisymmetric patterns. This is in contrast with some Newtonian numerical
studies where violent flip-flop instabilities were found. As discussed in the
text, the reason can be found in the initial conditions used in the
relativistic regime, as they can not exactly duplicate the previous Newtonian
setups where the instability appeared. The dependence of the final solution
with the inner boundary condition as well as with the grid resolution has also
been studied. Finally, we have computed the accretion rates of mass and linear
and angular momentum.Comment: 21 pages, 13 figures, Latex, MNRAS (in press
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
"Mariage des Maillages": A new numerical approach for 3D relativistic core collapse simulations
We present a new 3D general relativistic hydrodynamics code for simulations
of stellar core collapse to a neutron star, as well as pulsations and
instabilities of rotating relativistic stars. It uses spectral methods for
solving the metric equations, assuming the conformal flatness approximation for
the three-metric. The matter equations are solved by high-resolution
shock-capturing schemes. We demonstrate that the combination of a finite
difference grid and a spectral grid can be successfully accomplished. This
"Mariage des Maillages" (French for grid wedding) approach results in high
accuracy of the metric solver and allows for fully 3D applications using
computationally affordable resources, and ensures long term numerical stability
of the evolution. We compare our new approach to two other, finite difference
based, methods to solve the metric equations. A variety of tests in 2D and 3D
is presented, involving highly perturbed neutron star spacetimes and
(axisymmetric) stellar core collapse, demonstrating the ability to handle
spacetimes with and without symmetries in strong gravity. These tests are also
employed to assess gravitational waveform extraction, which is based on the
quadrupole formula.Comment: 29 pages, 16 figures; added more information about convergence tests
and grid setu
Alternatives to infrastructure privatization revisited : public enterprise reform from the1960s to the 1980s
Frustration with the performance of State-owned enterprises (SOEs) has led to two rounds of reform: the first round, from the 1960s through the 1980s, attempted to improve SOE performance while maintaining public ownership while the second, beginning in the late 1980s, viewed privatization as the answer. Interest in the earlier round of reform has increased recently as controversy has slowed or halted privatization in many countries, especially for SOEs providing infrastructure services that are basic to everyday life and are thought to have elements of monopoly. This paper reexamines the earlier round of reforms, focusing particularly on efforts to increase the firms'capacity with infusions of human and physical capital, to strengthen managerial incentives through performance contracts and corporatization and to alter the mix of political and economic forces that impinge on the firm by strengthening the involvement of taxpayers, customers or private investors. The review suggests that these earlier approaches generated only modest success but that some of them, selectively applied, may be helpful in improving the performance of infrastructure firms that remain in public hands.Debt Markets,Emerging Markets,Microfinance,Private Participation in Infrastructure,E-Business
What are the prospects for privatizing infrastructure? Lessons from U. S. roads and solid waste
Infrastructure (Economics) ; Roads
A new spherically symmetric general relativistic hydrodynamical code
In this paper we present a full general relativistic one-dimensional
hydro-code which incorporates a modern high-resolution shock-capturing
algorithm, with an approximate Riemann solver, for the correct modelling of
formation and propagation of strong shocks. The efficiency of this code in
treating strong shocks is demonstrated by some numerical experiments. The
interest of this technique in several astrophysical scenarios is discussed.Comment: 27 pages, 26 figures, accepted Ap
Construct, Merge, Solve & Adapt A new general algorithm for combinatorial optimization
[EN]This paper describes a general hybrid metaheuristic for combinatorial optimization labelled Construct,Merge, Solve & Adapt. The proposed algorithm is a specific instantiation of a framework known from theliterature as Generate-And-Solve, which is based on the following general idea. First, generate a reducedsub-instance of the original problem instance, in a way such that a solution to the sub-instance is also asolution to the original problem instance. Second, apply an exact solver to the reduced sub-instance inorder to obtain a (possibly) high quality solution to the original problem instance. And third, make use ofthe results of the exact solver as feedback for the next algorithm iteration. The minimum common stringpartition problem and the minimum covering arborescence problem are chosen as test cases in order todemonstrate the application of the proposed algorithm. The obtained results show that the algorithm iscompetitive with the exact solver for small to medium size problem instances, while it significantlyoutperforms the exact solver for larger problem instancesC. Blum was supported by project TIN2012-37930-02 of the Spanish Government. In addition, support is acknowledged from IKERBASQUE (Basque Foundation for Science). J.A. Lozano was partially supported by the IT609-13 program (Basque Government) and project TIN2013-41272P (Spanish Ministry of Science and Innovation)Peer reviewe
- …