4,087 research outputs found
Slow flows of an relativistic perfect fluid in a static gravitational field
Relativistic hydrodynamics of an isentropic fluid in a gravitational field is
considered as the particular example from the family of Lagrangian
hydrodynamic-type systems which possess an infinite set of integrals of motion
due to the symmetry of Lagrangian with respect to relabeling of fluid particle
labels. Flows with fixed topology of the vorticity are investigated in
quasi-static regime, when deviations of the space-time metric and the density
of fluid from the corresponding equilibrium configuration are negligibly small.
On the base of the variational principle for frozen-in vortex lines dynamics,
the equation of motion for a thin relativistic vortex filament is derived in
the local induction approximation.Comment: 4 pages, revtex, no figur
Energetics, skeletal dynamics and long-term predictions in Kolmogorov-Lorenz systems
We study a particular return map for a class of low dimensional chaotic
models called Kolmogorov Lorenz systems, which received an elegant general
Hamiltonian description and includes also the famous Lorenz63 case, from the
viewpoint of energy and Casimir balance. In particular it is considered in
detail a subclass of these models, precisely those obtained from the Lorenz63
by a small perturbation on the standard parameters, which includes for example
the forced Lorenz case in Ref.[6]. The paper is divided into two parts. In the
first part the extremes of the mentioned state functions are considered, which
define an invariant manifold, used to construct an appropriate Poincare surface
for our return map. From the experimental observation of the simple orbital
motion around the two unstable fixed points, together with the circumstance
that these orbits are classified by their energy or Casimir maximum, we
construct a conceptually simple skeletal dynamics valid within our sub class,
reproducing quite well the Lorenz map for Casimir. This energetic approach
sheds some light on the physical mechanism underlying regime transitions. The
second part of the paper is devoted to the investigation of a new type of
maximum energy based long term predictions, by which the knowledge of a
particular maximum energy shell amounts to the knowledge of the future
(qualitative) behaviour of the system. It is shown that, in this respect, a
local analysis of predictability is not appropriate for a complete
characterization of this behaviour. A perspective on the possible extensions of
this type of predictability analysis to more realistic cases in (geo)fluid
dynamics is discussed at the end of the paper.Comment: 21 pages, 14 figure
Vortex line representation for flows of ideal and viscous fluids
It is shown that the Euler hydrodynamics for vortical flows of an ideal fluid
coincides with the equations of motion of a charged {\it compressible} fluid
moving due to a self-consistent electromagnetic field. Transition to the
Lagrangian description in a new hydrodynamics is equivalent for the original
Euler equations to the mixed Lagrangian-Eulerian description - the vortex line
representation (VLR). Due to compressibility of a "new" fluid the collapse of
vortex lines can happen as the result of breaking (or overturning) of vortex
lines. It is found that the Navier-Stokes equation in the vortex line
representation can be reduced to the equation of the diffusive type for the
Cauchy invariant with the diffusion tensor given by the metric of the VLR
High-pressure behaviour of GeO2: a simulation study
In this work we study the high pressure behaviour of liquid and glassy GeO2
by means of molecular dynamics simulations. The interaction potential, which
includes dipole polarization effects, was parameterized from first-principles
calculations. Our simulations reproduce the most recent experimental data to a
high degree of precision. The proportion of the various GeOn polyhedra is
determined as a function of the pressure: a smooth transition from tetrahedral
to octahedral network is observed. Finally, the study of high-pressure, liquid
germania confirms that this material presents an anomalous behaviour of the
diffusivity as observed in analog systems such as silica and water. The
importance of penta-coordinated germanium ions for such behaviour is stressed.Comment: 16 pages, 4 figures, accepted as a Fast Track Communication on
Journal of Physics: Condensed Matte
The {\gamma} Dor stars as revealed by Kepler : A key to reveal deep-layer rotation in A and F stars
The {\gamma} Dor pulsating stars present high-order gravity modes, which make
them important targets in the intermediate-and low-mass main-sequence region of
the Hertzsprung-Russell diagram. Whilst we have only access to rotation in the
envelope of the Sun, the g modes of {\gamma} Dor stars can in principle deliver
us constraints on the inner layers. With the puzzling discovery of unexpectedly
low rotation rates in the core of red giants, the {\gamma} Dor stars appear now
as unique targets to explore internal angular momentum transport in the
progenitors of red giants. Yet, the {\gamma} Dor pulsations remain hard to
detect from the ground for their periods are close to 1 day. While the CoRoT
space mission first revealed intriguing frequency spectra, the almost
uninterrupted 4-year photometry from the Kepler mission eventually shed a new
light on them. It revealed regularities in the spectra, expected to bear
signature of physical processes, including rotation, in the shear layers close
to the convective core. We present here the first results of our effort to
derive exploitable seismic diagnosis for mid- to fast rotators among {\gamma}
Dor stars. We confirm their potential to explore the rotation history of this
early phase of stellar evolution.Comment: 4 pages, 1 figure, proceedings of the 22nd Los Alamos Stellar
Pulsation Conference, "Wide-field variability surveys: a 21st-century
perspective" held in San Pedro de Atacama, Chile, Nov. 28-Dec. 2, 201
Extending the Grazing Season with Mixtures of Spring-Planted Spring and Winter Cereals
The objective of the study was to extend the grazing season into the fall using crop-combinations of spring-planted spring and winter cereals. Treatments established at Lacombe, Alberta, Canada were spring oat (Avena sativa L.) and barley (Hordeum vulgare L.) monocrops (SMC), spring-planted winter wheat (Tritcum aestivum L.) and winter triticale (X Triticosecale Wittmack) monocrops (WMC), spring and winter cereal binary mixtures seeded together in the spring (MX) and the winter cereal seeded after the first clipping of the spring cereal (double crop-DC). Clippings were carried out at 4 to 6 wk intervals after the initial cut (Boot and Late Milk Stage). MX produced more total yield than other systems when cut initially at the Late Milk stage ( 92% of SMC at initial cut and 65% of WMC for regrowth). MX was superior to DC and SMC for regrowth yield, but not WMC. Treatments containing winter triticale were superior to those containing winter wheat for fall regrowth. Cropping systems like MX have the potential to economically extend the grazing season in the parkland of the Canadian prairies
A Parallel Tree code for large Nbody simulation: dynamic load balance and data distribution on CRAY T3D system
N-body algorithms for long-range unscreened interactions like gravity belong
to a class of highly irregular problems whose optimal solution is a challenging
task for present-day massively parallel computers. In this paper we describe a
strategy for optimal memory and work distribution which we have applied to our
parallel implementation of the Barnes & Hut (1986) recursive tree scheme on a
Cray T3D using the CRAFT programming environment. We have performed a series of
tests to find an " optimal data distribution " in the T3D memory, and to
identify a strategy for the " Dynamic Load Balance " in order to obtain good
performances when running large simulations (more than 10 million particles).
The results of tests show that the step duration depends on two main factors:
the data locality and the T3D network contention. Increasing data locality we
are able to minimize the step duration if the closest bodies (direct
interaction) tend to be located in the same PE local memory (contiguous block
subdivison, high granularity), whereas the tree properties have a fine grain
distribution. In a very large simulation, due to network contention, an
unbalanced load arises. To remedy this we have devised an automatic work
redistribution mechanism which provided a good Dynamic Load Balance at the
price of an insignificant overhead.Comment: 16 pages with 11 figures included, (Latex, elsart.style). Accepted by
Computer Physics Communication
Spin-Glass Attractor on Tridimensional Hierarchical Lattices in the Presence of an External Magnetic Field
A nearest-neighbor-interaction Ising spin glass, in the presence of an
external magnetic field, is studied on different hierarchical lattices that
approach the cubic lattice. The magnetic field is considered as uniform, or
random (following either a bimodal or a Gaussian probability distribution). In
all cases, a spin-glass attractor is found, in the plane magnetic field versus
temperature, associated with a low-temperature phase. The physical consequences
of this attractor are discussed, in view of the present scenario of the
spin-glass problem.Comment: Accepted for publication in Physical Review
Development and validation of the child post-traumatic cognitions inventory (CPTCI)
Background: Negative trauma-related cognitions have been found to be a significant factor in the maintenance of post-traumatic stress disorder (PTSD) in adults. Initial studies of such appraisals in trauma-exposed children and adolescents suggest that this is an important line of research in youth, yet empirically validated measures for use with younger populations are lacking. A measure of negative trauma-related cognitions for use with children and adolescents, the Child Post-Traumatic Cognitions Inventory (CPTCI), is presented. The measure was devised as an age-appropriate version of the adult Post-Traumatic Cognitions Inventory (Foa et al., 1999). Methods: The CPTCI was developed and validated within a large (n = 570) sample, comprising community and trauma-exposed samples of children and adolescents aged 6-18 years. Results: Principal components analysis suggested a two-component structure. These components were labelled 'permanent and disturbing change' and 'fragile person in a scary world', and were each found to possess good internal consistency, test-retest reliability, convergent validity, and discriminative validity. The reliability and validity of these sub-scales was present regardless of whether the measure was completed in the acute phase or several months after a trauma. Scores on these sub-scales did not vary with age. Conclusions: The CPTCI is a reliable and valid measure that is not specific to the type of trauma exposure, and shows considerable promise as a research and clinical tool. The structure of this measure suggests that appraisals concerning the more abstract consequences of a trauma, as well as physical threat and vulnerability, are pertinent factors in trauma-exposed children and adolescents, even prepubescent children
Nambu representation of an extended Lorenz model with viscous heating
We consider the Nambu and Hamiltonian representations of Rayleigh-Benard
convection with a nonlinear thermal heating effect proportional to the Eckert
number (Ec). The model we use is an extension of the classical Lorenz-63 model
with 4 kinematic and 6 thermal degrees of freedom. The conservative parts of
the dynamical equations which include all nonlinearities satisfy Liouville's
theorem and permit a conserved Hamiltonian H for arbitrary Ec. For Ec=0 two
independent conserved Casimir functions exist, one of these is associated with
unavailable potential energy and is also present in the Lorenz-63 truncation.
This Casimir C is used to construct a Nambu representation of the conserved
part of the dynamical system. The thermal heating effect can be represented
either by a second canonical Hamiltonian or as a gradient (metric) system using
the time derivative of the Casimir. The results demonstrate the impact of
viscous heating in the total energy budget and in the Lorenz energy cycle for
kinetic and available potential energy.Comment: 15 pages, no figur
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