144 research outputs found
Cluster evolution in steady-state two-phase flow in porous media
We report numerical studies of the cluster development of two-phase flow in a
steady-state environment of porous media. This is done by including biperiodic
boundary conditions in a two-dimensional flow simulator. Initial transients of
wetting and non-wetting phases that evolve before steady-state has occurred,
undergo a cross-over where every initial patterns are broken up. For flow
dominated by capillary effects with capillary numbers in order of , we
find that around a critical saturation of non-wetting fluid the non-wetting
clusters of size have a power-law distribution with
the exponent for large clusters. This is a lower value
than the result for ordinary percolation. We also present scaling relation and
time evolution of the structure and global pressure.Comment: 12 pages, 11 figures. Minor corrections. Accepted for publication in
Phys. Rev.
Ions Accelerated by Sounder-Plasma Interaction as Observed by Mars Express
The ion sensor of the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) experiment detects accelerated ions during pulses of radio emissions from the powerful topside sounder: the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard Mars Express. Accelerated ions (O+2, O+, and lighter ions) are observed in an energy range up to 800Â eV when MARSIS transmits at a frequency close to the plasma frequency. Individual observations consist of almost monoenergetic ion beams aligned with the MARSIS antenna or lying in the plane perpendicular to the antenna. The observed ion beams are often accompanied by a small decrease in the electron flux observed by the electron sensor of Analyzer of Space Plasmas and Energetic Atoms 3. Observations indicate that the voltage applied to the antenna causes charging of the spacecraft to several hundreds of volts by the electrons of the ambient plasma. Positively charged ions are accelerated when the spacecraft discharges
Dynamical brittle fractures of nanocrystalline silicon using large-scale electronic structure calculations
A hybrid scheme between large-scale electronic structure calculations is
developed and applied to nanocrystalline silicon with more than 10 atoms.
Dynamical fracture processes are simulated under external loads in the [001]
direction. We shows that the fracture propagates anisotropically on the (001)
plane and reconstructed surfaces appear with asymmetric dimers. Step structures
are formed in larger systems, which is understood as the beginning of a
crossover between nanoscale and macroscale samples.Comment: 10 pages, 4 figure
Te covered Si(001): a variable surface reconstruction
At a given temperature, clean and adatom covered silicon surfaces usually
exhibit well-defined reconstruction patterns. Our finite temperature ab-initio
molecular dynamics calculations show that the tellurium covered Si(001) surface
is an exception. Soft longitudinal modes of surface phonons due to the strongly
anharmonic potential of the bridged tellurium atoms prevent the reconstruction
structure from attaining any permanent, two dimensional periodic geometry. This
explains why experiments attempting to find a definite model for the
reconstruction have reached conflicting conclusions.Comment: 4 pages, 3 gif figure
Transient Foreshock Structures Upstream of Mars: Implications of the Small Martian Bow Shock
We characterize the nature of magnetic structures in the foreshock region of
Mars associated with discontinuities in the solar wind. The structures form at
the upstream edge of moving foreshocks caused by slow rotations in the
interplanetary magnetic field (IMF). The solar wind plasma density and the IMF
strength noticeably decrease inside the structures' core, and a compressional
shock layer is present at their sunward side, making them consistent with
foreshock bubbles (FBs). Ion populations responsible for these structures
include backstreaming ions that only appear within the moving foreshock, and
accelerated reflected ions from the quasi-perpendicular bow shock. Both ion
populations accumulate near the upstream edge of the moving foreshock which
facilitates FB formation. Reflected ions with hybrid trajectories that straddle
between the quasi-perpendicular and quasi-parallel bow shocks during slow IMF
rotations contribute to formation of foreshock transients.Comment: Submitted to Geophysical Research Letter
Study of a Nonlocal Density scheme for electronic--structure calculations
An exchange-correlation energy functional beyond the local density
approximation, based on the exchange-correlation kernel of the homogeneous
electron gas and originally introduced by Kohn and Sham, is considered for
electronic structure calculations of semiconductors and atoms. Calculations are
carried out for diamond, silicon, silicon carbide and gallium arsenide. The
lattice constants and gaps show a small improvement with respect to the LDA
results.
However, the corresponding corrections to the total energy of the isolated
atoms are not large enough to yield a substantial improvement for the cohesive
energy of solids, which remains hence overestimated as in the LDA.Comment: 4 postscript figure
Krylov Subspace Method for Molecular Dynamics Simulation based on Large-Scale Electronic Structure Theory
For large scale electronic structure calculation, the Krylov subspace method
is introduced to calculate the one-body density matrix instead of the
eigenstates of given Hamiltonian. This method provides an efficient way to
extract the essential character of the Hamiltonian within a limited number of
basis set. Its validation is confirmed by the convergence property of the
density matrix within the subspace. The following quantities are calculated;
energy, force, density of states, and energy spectrum. Molecular dynamics
simulation of Si(001) surface reconstruction is examined as an example, and the
results reproduce the mechanism of asymmetric surface dimer.Comment: 7 pages, 3 figures; corrected typos; to be published in Journal of
the Phys. Soc. of Japa
Phase Separation of Crystal Surfaces: A Lattice Gas Approach
We consider both equilibrium and kinetic aspects of the phase separation
(``thermal faceting") of thermodynamically unstable crystal surfaces into a
hill--valley structure. The model we study is an Ising lattice gas for a simple
cubic crystal with nearest--neighbor attractive interactions and weak
next--nearest--neighbor repulsive interactions. It is likely applicable to
alkali halides with the sodium chloride structure. Emphasis is placed on the
fact that the equilibrium crystal shape can be interpreted as a phase diagram
and that the details of its structure tell us into which surface orientations
an unstable surface will decompose. We find that, depending on the temperature
and growth conditions, a number of interesting behaviors are expected. For a
crystal in equilibrium with its vapor, these include a low temperature regime
with logarithmically--slow separation into three symmetrically--equivalent
facets, and a higher temperature regime where separation proceeds as a power
law in time into an entire one--parameter family of surface orientations. For a
crystal slightly out of equilibrium with its vapor (slow crystal growth or
etching), power--law growth should be the rule at late enough times. However,
in the low temperature regime, the rate of separation rapidly decreases as the
chemical potential difference between crystal and vapor phases goes to zero.Comment: 16 pages (RevTex 3.0); 12 postscript figures available on request
([email protected]). Submitted to Physical Review E. SFU-JDSDJB-94-0
Methylphenidate for attention-deficit/hyperactivity disorder in children and adolescents: Cochrane systematic review with meta-analyses and trial sequential analyses of randomised clinical trials
Study question: Is methylphenidate beneficial or harmful for the treatment of attention-deficit/hyperactivity disorder (ADHD) in children and adolescents? / Methods:
Electronic databases were searched up to February 2015 for parallel and crossover randomised clinical trials comparing methylphenidate with placebo or no intervention in children and adolescents with ADHD. Meta-analyses and trial sequential analyses (TSA) were conducted. Quality was assessed using GRADE. Teachers, parents, and observers rated ADHD symptoms and general behaviour. / Study answer and limitations: The analyses included 38 parallel group trials (n=5111, median treatment duration 49 days) and 147 crossover trials (n=7134, 14 days). The average age across all studies was 9.7 years. The analysis suggested a beneficial effect of methylphenidate on teacher rated symptoms in 19 parallel group trials (standardised mean difference (SMD) −0.77, n=1698), corresponding to a mean difference of −9.6 points on the ADHD rating scale. There was no evidence that methylphenidate was associated with an increase in serious adverse events (risk ratio 0.98, nine trials, n=1532; TSA adjusted intervention effect RR 0.91). Methylphenidate was associated with an increased risk of non-serious adverse events (1.29, 21 trials, n=3132; TSA adjusted RR 1.29). Teacher rated general behaviour seemed to improve with methylphenidate (SMD −0.87, five trials, n=668) A change of 7 points on the child health questionnaire (CHQ) has been deemed a minimal clinically relevant difference. The change reported in a meta-analysis of three trials corresponds to a mean difference of 8.0 points on the CHQ (range 0-100 points), which suggests that methylphenidate may improve parent reported quality of life (SMD 0.61, three trials, n=514). 96.8% of trials were considered high risk of bias trials according to the Cochrane guidelines. All outcomes were assessed very low quality according to GRADE. / What this study adds: The results suggest that among children and adolescents with a diagnosis of ADHD, methylphenidate may improve teacher reported symptoms of ADHD and general behaviour and parent reported quality of life. However, given the risk of bias in the included studies, and the very low quality of outcomes, the magnitude of the effects is uncertain. Methylphenidate is associated with an increased risk of non-serious but not serious adverse events. / Funding, competing interests, data sharing: Region Zealand Research Foundation and Copenhagen Trial Unit. Competing interests are given in the full paper on bmj.com. Full data are available in the version of this review published in The Cochrane Library
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