1,054 research outputs found
Efficiency of Magnetized Thin Accretion Disks in the Kerr Metric
The efficiency of thin disk accretion onto black holes depends on the inner
boundary condition, specifically the torque applied to the disk at the last
stable orbit. This is usually assumed to vanish. I estimate the torque on a
magnetized disk using a steady magnetohydrodynamic inflow model originally
developed by Takahashi et al., 1990. I find that the efficiency epsilon can
depart significantly from the classical thin disk value. In some cases epsilon
> 1, i.e. energy is extracted from the black hole.Comment: 11 pages, 3 figures, aastex, submitted to ApJ Letter
Dissipation in Compressible MHD Turbulence
We report results of a three dimensional, high resolution (up to 512^3)
numerical investigation of supersonic compressible magnetohydrodynamic
turbulence. We consider both forced and decaying turbulence. The model
parameters are appropriate to conditions found in Galactic molecular clouds. We
find that the dissipation time of turbulence is of order the flow crossing time
or smaller, even in the presence of strong magnetic fields. About half the
dissipation occurs in shocks. Weak magnetic fields are amplified and tangled by
the turbulence, while strong fields remain well ordered.Comment: 5 pages, 3 Postscript figures, LaTeX, accepted by Ap.J.Let
Formal Design of Asynchronous Fault Detection and Identification Components using Temporal Epistemic Logic
Autonomous critical systems, such as satellites and space rovers, must be
able to detect the occurrence of faults in order to ensure correct operation.
This task is carried out by Fault Detection and Identification (FDI)
components, that are embedded in those systems and are in charge of detecting
faults in an automated and timely manner by reading data from sensors and
triggering predefined alarms. The design of effective FDI components is an
extremely hard problem, also due to the lack of a complete theoretical
foundation, and of precise specification and validation techniques. In this
paper, we present the first formal approach to the design of FDI components for
discrete event systems, both in a synchronous and asynchronous setting. We
propose a logical language for the specification of FDI requirements that
accounts for a wide class of practical cases, and includes novel aspects such
as maximality and trace-diagnosability. The language is equipped with a clear
semantics based on temporal epistemic logic, and is proved to enjoy suitable
properties. We discuss how to validate the requirements and how to verify that
a given FDI component satisfies them. We propose an algorithm for the synthesis
of correct-by-construction FDI components, and report on the applicability of
the design approach on an industrial case-study coming from aerospace.Comment: 33 pages, 20 figure
HARM: A Numerical Scheme for General Relativistic Magnetohydrodynamics
We describe a conservative, shock-capturing scheme for evolving the equations
of general relativistic magnetohydrodynamics. The fluxes are calculated using
the Harten, Lax, and van Leer scheme. A variant of constrained transport,
proposed earlier by T\'oth, is used to maintain a divergence free magnetic
field. Only the covariant form of the metric in a coordinate basis is required
to specify the geometry. We describe code performance on a full suite of test
problems in both special and general relativity. On smooth flows we show that
it converges at second order. We conclude by showing some results from the
evolution of a magnetized torus near a rotating black hole.Comment: 38 pages, 18 figures, submitted to Ap
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