1,720,998 research outputs found
Resistive and multi-fluid RMHD on graphics processing units
No full textIn this work we present a proof of concept of CUDA-capable, resistive, multi-fluid models of relativistic magnetohydrodynamics (RMHD). Resistive and multi-fluid codes for simulating models of RMHD suffer from stiff source terms, so it is common to implement a set of semi-implicit time integrators to maintain numerical stability. We show, for the first time, that finite volume IMEX schemes for resistive and two-fluid models of RMHD can be accelerated by execution on graphics processing units, significantly reducing the demand set by these kinds of problems. We report parallel speed-ups of over 21× using double-precision floating-point accuracy, and highlight the optimization strategies required for these schemes, and how they differ from ideal RMHD models. The impact of these results is discussed in the context of the next-generation simulations of neutron star mergers.</p
Numerical simulations of interfaces in relativistic hydrodynamics
No full textWe consider models of relativistic matter containing sharp interfaces across which the matter model changes. These models will be relevant for neutron stars with crusts, phase transitions, or for viscous boundaries where the length scale is too short to be modelled smoothly. In particular we look at numerical techniques that allow us to evolve stable interfaces, for the interfaces to merge, and for strong waves and shocks to interact with the interfaces. We test these techniques for ideal hydrodynamics in special and general relativity for simple equations of state, finding that simple level set-based methods extend well to relativistic hydrodynamic
A variational approach to resistive relativistic plasmas
No full textWe develop an action principle to construct the field equations for a multi-fluid system containing charge-neutral fluids, plasmas, and dissipation (via resistive interactions), by combining the standard, Maxwell action and minimal coupling of the electromagnetic field with a recently developed action for relativistic dissipative fluids. We use a pull-back formalism from spacetime to abstract matter spaces to build unconstrained variations for both the charge-neutral fluids and currents making up the plasmas. Using basic linear algebra techniques, we show that a general 'relabeling' invariance exists for the abstract matter spaces. With the field equations in place, a phenomenological model for the resistivity is developed, using as constraints charge conservation and the Second Law of Thermodynamics. A minimal model for a system of electrons, protons, and heat is developed using the Onsager procedure for incorporating dissipation
Thermal aspects of neutron star mergers
No full textIn order to extract maximal information from neutron-star merger signals, both gravitational and electromagnetic, we need to ensure that our theoretical models/numerical simulations faithfully represent the extreme physics involved. This involves a range of issues, with the finite temperature effects regulating many of the relevant phenomena. As a step toward understanding these issues, we explore the conditions for β-equilibrium in neutron star matter for the densities and temperatures reached in a binary neutron star merger. Using the results from our out-of-equilibrium merger simulation, we consider how different notions of equilibrium may affect the merger dynamics, raising issues that arise when attempting to account for these conditions in future simulations. These issues are both computational and conceptual. We show that the effects lead to, in our case, a softening of the equation of state in some density regions, and to composition changes that affect processes that rely on deviation from equilibrium, such as bulk viscosity, both in terms of the magnitude and the equilibration timescales inherent to the relevant set of reactions. We also demonstrate that it is difficult to determine exactly which equilibrium conditions are relevant in which regions of the matter due to the dependence on neutrino absorption, further complicating the calculation of the reactions that work to restore the matter to equilibrium.</p
Impact of nuclear reactions on gravitational waves from neutron star mergers
No full textNuclear reactions may affect gravitational-wave signals from neutron-star mergers, but the impact is uncertain. To indicate the significance of this effect, we compare two numerical simulations representing intuitive extremes. In one case, reactions happen instantaneously. In the other case, they occur on timescales much slower than the evolutionary timescale. We show that, while the differences in the two gravitational-wave signals are small, the mismatch between them satisfies the condition for distinguishability using the Einstein Telescope noise curve, assuming that the neutron-star equation of state can be well constrained by experiments or by the postmerger signal of the event. This suggests that, to avoid systematic errors in equation of state parameters inferred from observed signals, we need to accurately implement nuclear reactions in future simulations.</p
Dynamics of primordial black hole formation
No full textWe examine numerically the formation of small black holes from primordial density fluctuations in a radiation-dominated spatially flat Friedmann–Robertson–Walker spacetime. Large amplitude fluctuations might be expected to form black holes, while smaller fluctuations will be washed out by the expansion of the universe. We have studied the interface between these two types of behaviour. Unlike earlier studies which suggested that there was no lower limit to the mass of a black hole, this work suggests that there is a minimum mass for a primordial black hole of the order of one ten thousandth of the mass contained within the horizon. We discuss the implications for critical collapse studies
Beyond ideal magnetohydrodynamics: resistive, reactive and relativistic plasmas
No full textWe develop a new framework for the modelling of charged fluid dynamics in general relativity. The model, which builds on a recently developed variational multi-fluid framework for dissipative fluids, accounts for relevant effects like the inertia of both charge currents and heat and, for mature systems, the decoupling of superfluid components. We discuss how the model compares to standard relativistic magnetohydronamics and consider the connection between the fluid dynamics, the microphysics and the underlying equation of state. As illustrations of the formalism, we consider three distinct two-fluid models describing (i) an Ohm's law for resistive charged flows, (ii) a relativistic heat equation, and (iii) an equation representing the momentum of a decoupled superfluid component. As a more complex example, we also formulate a three-fluid model which demonstrates the thermo-electric effect. The new framework allows us to model neutron stars (and related systems) at a hierarchy of increasingly complex levels, and should enable us to make progress on a range of exciting problems in astrophysics and cosmology
The physics of non-ideal general relativistic magnetohydrodynamics
No full textWe consider a framework for non-ideal magnetohydrodynamics in general relativity, paying particular attention to the physics involved. The discussion highlights the connection between the microphysics (associated with a given equation of state) and the global dynamics (from the point of view of numerical simulations), and includes a careful consideration of the assumptions that lead to ideal and resistive magnetohydrodynamics. We pay particular attention to the issue of local charge neutrality, which tends to be assumed but appears to be more involved than is generally appreciated. While we do not resolve all the involved issues, we highlight how some of the assumptions and simplifications may be tested by simulations. The final formulation prepares the ground for a new generation of models of relevant astrophysical scenarios
Covariant approach to relativistic large-eddy simulations: The fibration picture
No full textModels of turbulent flows require the resolution of a vast range of scales, from large eddies to small-scale features directly associated with dissipation. As the required resolution is not within reach of large scale numerical simulations, standard strategies involve a smoothing of the fluid dynamics, either through time averaging or spatial filtering. These strategies raise formal issues in general relativity, where the split between space and time is observer dependent. To make progress, we develop a new covariant framework for filtering/averaging based on the fibration of spacetime associated with fluid elements and the use of Fermi coordinates to facilitate a meaningful local analysis. We derive the resolved equations of motion, demonstrating how "effective"dissipative terms arise because of the coarse-graining, and paying particular attention to the thermodynamical interpretation of the resolved quantities. Finally, as the smoothing of the fluid dynamics inevitably leads to a closure problem, we propose a new closure scheme inspired by recent progress in the modeling of dissipative relativistic fluids, and crucially, demonstrate the linear stability of the proposed model.</p
Gravitational-wave emission from rotating gravitational collapse in three dimensions
Full text linkWe present the first three-dimensional (3D) calculations of the gravitational-wave emission in the collapse of uniformly rotating stars to black holes. The initial models are polytropes which are dynamically unstable and near the mass-shedding limit. The waveforms have been extracted using a gauge-invariant approach and reflect the properties of both the initial stellar models and of newly produced black holes, being in good qualitative agreement with those computed in previous 2D simulations. The wave amplitudes, however, are about 1 order of magnitude smaller, giving, for a source at 10 kpc, a signal-to-noise ratio S/N~0.25 for LIGO-VIRGO and S/N<~4 for LIGO II
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