3,756 research outputs found
MHD turbulence and heating of the open field-line solar corona
This paper discusses the possibility that heating of the solar corona in open field-line regions emanating from coronal holes is due to a nonlinear cascade, driven by low-frequency or quasi-static magnetohydrodynamic fluctuations. Reflection from coronal inhomogeneities plays an important role in sustaining the cascade. Physical and observational constraints are discussed. Kinetic processes that convert cascaded energy into heat must occur in regions of turbulent small-scale reconnection, and may be similar in some respects to ion heating due to intense electron beams observed in the aurora
Determination of Gd concentration profile in UO2-Gd2O3 fuel pellets
A transversal mapping of the Gd concentration was measured in UO2-Gd2O3
nuclear fuel pellets by electron paramagnetic resonance spectroscopy (EPR). The
quantification was made from the comparison with a Gd2O3 reference sample. The
nominal concentration in the pellets is UO2: 7.5 % Gd2O3. A concentration
gradient was found, which indicates that the Gd2O3 amount diminishes towards
the edges of the pellets. The concentration varies from (9.3 +/- 0.5)% in the
center to (5.8 +/- 0.3)% in one of the edges. The method was found to be
particularly suitable for the precise mapping of the distribution of Gd3+ ions
in the UO2 matrix.Comment: 10 pages, 5 figures, 2 tables. Submitted to Journal of Nuclear
Material
Apparent suppression of turbulent magnetic dynamo action by a dc magnetic field
Numerical studies of the effect of a dc magnetic field on dynamo action
(development of magnetic fields with large spatial scales), due to
helically-driven magnetohydrodynamic turbulence, are reported. The apparent
effect of the dc magnetic field is to suppress the dynamo action, above a
relatively low threshold. However, the possibility that the suppression results
from an improper combination of rectangular triply spatially-periodic boundary
conditions and a uniform dc magnetic field is addressed: heretofore a common
and convenient computational convention in turbulence investigations. Physical
reasons for the observed suppression are suggested. Other geometries and
boundary conditions are offered for which the dynamo action is expected not to
be suppressed by the presence of a dc magnetic field component.Comment: To appear in Physics of Plasma
A reduced magnetohydrodynamic model of coronal heating in open magnetic regions driven by reflected low-frequency waves
A reduced magnetohydrodynamic (RMHD) description is employed to examine a suggestion made by W. H. Matthaeus and colleagues in 1999 that coronal heating might be sustained by a cascade of low-frequency MHD turbulence. Here RMHD simulations show that the low-frequency cascade to high transverse wavenumbers can be driven by an externally maintained flux of low-frequency propagating Alfvén waves, in combination with reflection caused by an inhomogeneous background medium. The simulations show that the suggestions made previously on the basis of a phenomenology are indeed realizable. In addition, the effect is seen to sensitively depend on the presence of reflection, as the background turbulence level needed to maintain the cascade can be sustained only when reflection is imposed. The steady level of turbulence is insensitive to the initial seed turbulence level (provided it is nonzero). Consequences of this model for realistic models of coronal heating in open field-line regions are discussed
Facing the small aortic root in aortic valve replacement: Enlarge or not enlarge?
In patients with severe aortic stenosis, aortic valve replacement (AVR) should aim to implant a prosthesis of adequate size to effectively eliminate left ventricular obstruction and avoid the risk of patient–prosthesis mismatch (PPM). PPM has been demonstrated to be associated with increased mortality, decreased exercise tolerance, and reduced left ventricular mass regression after AVR for aortic stenosis
Magnetic helicity and cosmological magnetic field
The magnetic helicity has paramount significance in nonlinear saturation of
galactic dynamo. We argue that the magnetic helicity conservation is violated
at the lepton stage in the evolution of early Universe. As a result, a
cosmological magnetic field which can be a seed for the galactic dynamo obtains
from the beginning a substantial magnetic helicity which has to be taken into
account in the magnetic helicity balance at the later stage of galactic dynamo.Comment: 11 pages, no figures; v3: new references and new paragraphs added,
discussion extended, some mistypings correcte
Milestone Operations in Heart Valve and Aortic Replacement: Anniversaries Worth Remembering
Seventy years ago, in 1952, Charles A. Hufnagel implanted a caged-ball prosthesis into the descending thoracic aorta, to treat a patient with aortic valve insufficiency. In 1962, 60 years ago, the first aortic homograft was implanted in a subcoronary position by Donald N. Ross and Brian G. Barratt-Boyes. Forty years ago, in 1982, the first anticalcification treatment was introduced in commercially manufactured porcine bioprostheses. All such important or even milestone events should be remembered, since they witness efforts made by those who have significantly influenced the clinical history of aortic and valvular diseases
Finite driving rate and anisotropy effects in landslide modeling
In order to characterize landslide frequency-size distributions and
individuate hazard scenarios and their possible precursors, we investigate a
cellular automaton where the effects of a finite driving rate and the
anisotropy are taken into account. The model is able to reproduce observed
features of landslide events, such as power-law distributions, as
experimentally reported. We analyze the key role of the driving rate and show
that, as it is increased, a crossover from power-law to non power-law behaviors
occurs. Finally, a systematic investigation of the model on varying its
anisotropy factors is performed and the full diagram of its dynamical behaviors
is presented.Comment: 8 pages, 9 figure
Energy spectrum of turbulent fluctuations in boundary driven reduced magnetohydrodynamics
The nonlinear dynamics of a bundle of magnetic flux ropes driven by
stationary fluid motions at their endpoints is studied, by performing numerical
simulations of the magnetohydrodynamic (MHD) equations. The development of MHD
turbulence is shown, where the system reaches a state that is characterized by
the ratio between the Alfven time (the time for incompressible MHD waves to
travel along the field lines) and the convective time scale of the driving
motions. This ratio of time scales determines the energy spectra and the
relaxation toward different regimes ranging from weak to strong turbulence. A
connection is made with phenomenological theories for the energy spectra in MHD
turbulence.Comment: Published in Physics of Plasma
- …