226 research outputs found
Stability boundaries of roll and square convection in binary fluid mixtures with positive separation ratio
Rayleigh-B\'{e}nard convection in horizontal layers of binary fluid mixtures
heated from below with realistic horizontal boundary conditions is studied
theoretically using multi-mode Galerkin expansions. For positive separation
ratios the main difference between the mixtures and pure fluids lies in the
existence of stable three dimensional patterns near onset in a wide range of
the parameter space. We evaluated the stationary solutions of roll, crossroll,
and square convection and we determined the location of the stability
boundaries for many parameter combinations thereby obtaining the Busse balloon
for roll and square patterns.Comment: 19 pages + 15 figures, accepted by Journal of Fluid Mechanic
Roll convection of binary fluid mixtures in porous media
We investigate theoretically the nonlinear state of ideal straight rolls in
the Rayleigh-B\'enard system of a fluid layer heated from below with a porous
medium using a Galerkin method. Applying the Oberbeck-Boussinesq approximation,
binary mixtures with positive separation ratio are studied and compared to
one-component fluids. Our results for the structural properties of roll
convection resemble qualitatively the situation in the Rayleigh--B\'enard
system without porous medium except for the fact that the streamlines of binary
mixtures are deformed in the so-called Soret regime. The deformation of the
streamlines is explained by means of the Darcy equation which is used to
describe the transport of momentum. In addition to the properties of the rolls,
their stability against arbitrary infinitesimal perturbations is investigated.
We compute stability balloons for the pure fluid case as well as for a wide
parameter range of Lewis numbers and separation ratios which are typical for
binary gas and fluid mixtures. The stability regions of rolls are found to be
restricted by a crossroll, a zigzag and a new type of oscillatory instability
mechanism, which can be related to the crossroll mechanism
Faraday instability in a two-component Bose Einstein condensate
Motivated by recent experiments on Faraday waves in Bose Einstein condensates
(BEC) we investigate the dynamics of two component cigar shaped BEC subject to
periodic modulation of the strength of the transverse confinement. It is shown
that two coupled Mathieu equations govern the dynamics of the system. We found
that the two component BEC in a phase mixed state is relatively more unstable
towards pattern formation than the phase segregated state.Comment: 6 pages, 4 figure
Quantum mechanical ab-initio simulation of the electron screening effect in metal deuteride crystals
In antecedent experiments the electron screening energies of the d+d
reactions in metallic environments have been determined to be enhanced by an
order of magnitude in comparison to the case of gaseous deuterium targets. The
analytical models describing averaged material properties have not been able to
explain the experimental results so far. Therefore, a first effort has been
undertaken to simulate the dynamics of reacting deuterons in a metallic lattice
by means of an ab-initio Hartree-Fock calculation of the total electrostatic
force between the lattice and the successively approaching deuterons via path
integration. The calculations have been performed for Li and Ta, clearly
showing a migration of electrons from host metallic to the deuterium atoms.
However, in order to avoid more of the necessary simplifications in the model
the utilization of a massive parallel supercomputer would be required.Comment: 11 pages, 12 figures, svjour class. To be published in Eur. Phys. J.
Subharmonic bifurcation cascade of pattern oscillations caused by winding number increasing entrainment
Convection structures in binary fluid mixtures are investigated for positive
Soret coupling in the driving regime where solutal and thermal contributions to
the buoyancy forces compete. Bifurcation properties of stable and unstable
stationary square, roll, and crossroll (CR) structures and the oscillatory
competition between rolls and squares are determined numerically as a function
of fluid parameters. A novel type of subharmonic bifurcation cascade (SC) where
the oscillation period grows in integer steps as is found
and elucidated to be an entrainment process.Comment: 7 pages, 4 figure
Anomalous enhancements of low-energy fusion rates in plasmas: the role of ion momentum distributions and inhomogeneous screening
Non-resonant fusion cross-sections significantly higher than corresponding
theoretical predictions are observed in low-energy experiments with deuterated
matrix target. Models based on thermal effects, electron screening, or
quantum-effect dispersion relations have been proposed to explain these
anomalous results: none of them appears to satisfactory reproduce the
experiments. Velocity distributions are fundamental for the reaction rates and
deviations from the Maxwellian limit could play a central role in explaining
the enhancement. We examine two effects: an increase of the tail of the target
Deuteron momentum distribution due to the Galitskii-Yakimets quantum
uncertainty effect, which broadens the energy-momentum relation; and spatial
fluctuations of the Debye-H\"{u}ckel radius leading to an effective increase of
electron screening. Either effect leads to larger reaction rates especially
large at energies below a few keV, reducing the discrepancy between
observations and theoretical expectations.Comment: 6 pages, 3 figure
Enhancement of the Deuteron-Fusion Reactions in Metals and its Experimental Implications
Recent measurements of the reaction d(d,p)t in metallic environments at very
low energies performed by different experimental groups point to an enhanced
electron screening effect. However, the resulting screening energies differ
strongly for divers host metals and different experiments. Here, we present new
experimental results and investigations of interfering processes in the
irradiated targets. These measurements inside metals set special challenges and
pitfalls which make them and the data analysis particularly error-prone. There
are multi-parameter collateral effects which are crucial for the correct
interpretation of the observed experimental yields. They mainly originate from
target surface contaminations due to residual gases in the vacuum as well as
from inhomogeneities and instabilities in the deuteron density distribution in
the targets. In order to address these problems an improved differential
analysis method beyond the standard procedures has been implemented. Profound
scrutiny of the other experiments demonstrates that the observed unusual
changes in the reaction yields are mainly due to deuteron density dynamics
simulating the alleged screening energy values. The experimental results are
compared with different theoretical models of the electron screening in metals.
The Debye-H\"{u}ckel model that has been previously proposed to explain the
influence of the electron screening on both nuclear reactions and radioactive
decays could be clearly excluded.Comment: 22 pages, 12 figures, REVTeX4, 2-column format. Submitted to Phys.
Rev. C; accepte
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