3,594 research outputs found
A Note on the Period Enforcer Algorithm for Self-Suspending Tasks
The period enforcer algorithm for self-suspending real-time tasks is a technique for suppressing the "back-to-back" scheduling penalty associated with deferred execution. Originally proposed in 1991, the algorithm has attracted renewed interest in recent years. This note revisits the algorithm in the light of recent developments in the analysis of self-suspending tasks, carefully re-examines and explains its underlying assumptions and limitations, and points out three observations that have not been made in the literature to date: (i) period enforcement is not strictly superior (compared to the base case without enforcement) as it can cause deadline misses in self-suspending task sets that are schedulable without enforcement; (ii) to match the assumptions underlying the analysis of the period enforcer, a schedulability analysis of self-suspending tasks subject to period enforcement requires a task set transformation for which no solution is known in the general case, and which is subject to exponential time complexity (with current techniques) in the limited case of a single self-suspending task; and (iii) the period enforcer algorithm is incompatible with all existing analyses of suspension-based locking protocols, and can in fact cause ever-increasing suspension times until a deadline is missed
Turbulent transport in hydromagnetic flows
The predictive power of mean-field theory is emphasized by comparing theory
with simulations under controlled conditions. The recently developed test-field
method is used to extract turbulent transport coefficients both in kinematic as
well as nonlinear and quasi-kinematic cases. A striking example of the
quasi-kinematic method is provided by magnetic buoyancy-driven flows that
produce an alpha effect and turbulent diffusion.Comment: 17 pages, 6 figures, topical issue of Physica Scripta on turbulent
mixing and beyon
Inertial range scaling in numerical turbulence with hyperviscosity
Numerical turbulence with hyperviscosity is studied and compared with direct
simulations using ordinary viscosity and data from wind tunnel experiments. It
is shown that the inertial range scaling is similar in all three cases.
Furthermore, the bottleneck effect is approximately equally broad (about one
order of magnitude) in these cases and only its height is increased in the
hyperviscous case--presumably as a consequence of the steeper decent of the
spectrum in the hyperviscous subrange. The mean normalized dissipation rate is
found to be in agreement with both wind tunnel experiments and direct
simulations. The structure function exponents agree with the She-Leveque model.
Decaying turbulence with hyperviscosity still gives the usual t^{-1.25} decay
law for the kinetic energy, and also the bottleneck effect is still present and
about equally strong.Comment: Final version (7 pages
On the Detection of Magnetic Helicity
Magnetic fields in various astrophysical settings may be helical and, in the
cosmological context, may provide a measure of primordial CP violation during
baryogenesis. Yet it is difficult, even in principle, to devise a scheme by
which magnetic helicity may be detected, except in some very special systems.
We propose that charged cosmic rays originating from known sources may be
useful for this purpose. We show that the correlator of the arrival momenta of
the cosmic rays is sensitive to the helicity of an intervening magnetic field.
If the sources themselves are not known, the method may still be useful
provided we have some knowledge of their spatial distribution.Comment: 5 pages, 1 figure, discussions and references added, submited to
Phys. Rev.
AGOR status report
The operations of the superconducting cyclotron AGOR over the past years will be reviewed. Reliability issues encountered after nearly 25 years of operation and mitigation measures to warrant reliable operation for the coming decade will be discussed. The research performed with AGOR has significantly shifted from fundamental physics to radiation biology and medical radiation physics, both in collaboration with the Groningen Proton Therapy Center, and radiation hardness studies. The radiation biology research will be substantially expanded in the coming years with a new beam line for image guided preclinical research. For this research new dose delivery modalities including scanning, spatial fractionation and very high dose rates are developed. In addition, a new program has been started on the production of exotic nuclei, for which a new superconducting solenoid fragment separator will be developed. For the radiation hardness testing a cocktail beam at 30 MeV/amu with several ion species up to Xe has been developed and is now routinely delivered for experiments. A cocktail at 15 MeV/amu up to Bi is under development
A model of driven and decaying magnetic turbulence in a cylinder
Using mean-field theory, we compute the evolution of the magnetic field in a
cylinder with outer perfectly conducting boundaries, an imposed axial magnetic
and electric field. The thus injected magnetic helicity in the system can be
redistributed by magnetic helicity fluxes down the gradient of the local
current helicity of the small-scale magnetic field. A weak reversal of the
axial magnetic field is found to be a consequence of the magnetic helicity flux
in the system. Such fluxes are known to alleviate so-called catastrophic
quenching of the {\alpha}-effect in astrophysical applications. Application to
the reversed field pinch in plasma confinement devices is discussed.Comment: 7 pages, 4 figures, submitted to Phys. Rev.
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