2,233 research outputs found
Rayleigh-B\'{e}nard convection in a homeotropically aligned nematic liquid crystal
We report experimental results for convection near onset in a thin layer of a
homeotropically aligned nematic liquid crystal heated from below as a function
of the temperature difference and the applied vertical magnetic
field and compare them with theoretical calculations. The experiments cover
the field range 8 \alt h \equiv H/ H_{F} \alt 80 ( is the
Fr\'eedericksz field). For less than a codimension-two field the bifurcation is subcritical and oscillatory, with travelling- and
standing-wave transients. Beyond the bifurcation is stationary and
subcritical until a tricritical field is reached, beyond which it
is supercritical. The bifurcation sequence as a function of found in the
experiment confirms the qualitative aspects of the theoretical predictions.
However, the value of is about 10% higher than the predicted value and
the results for are systematically below the theory by about 2% at small
and by as much as 7% near . At , is continuous within
the experimental resolution whereas the theory indicates a 7% discontinuity.
The theoretical tricritical field is somewhat below the
experimental one. The fully developed flow above for is
chaotic. For the subcritical stationary bifurcation also
leads to a chaotic state. The chaotic states persist upon reducing the Rayleigh
number below , i.e. the bifurcation is hysteretic. Above the tricritical
field , we find a bifurcation to a time independent pattern which within
our resolution is non-hysteretic.Comment: 15 pages incl. 23 eps figure
Singularity in the boundary resistance between superfluid He and a solid surface
We report new measurements in four cells of the thermal boundary resistance
between copper and He below but near the superfluid-transition
temperature . For fits of to the data yielded ,
whereas a fit to theoretical values based on the renormalization-group theory
yielded . Alternatively, a good fit of the theory to the data could
be obtained if the {\it amplitude} of the prediction was reduced by a factor
close to two. The results raise the question whether the boundary conditions
used in the theory should be modified.Comment: 4 pages, 4 figures, revte
Power-Law Behavior of Power Spectra in Low Prandtl Number Rayleigh-Benard Convection
The origin of the power-law decay measured in the power spectra of low
Prandtl number Rayleigh-Benard convection near the onset of chaos is addressed
using long time numerical simulations of the three-dimensional Boussinesq
equations in cylindrical domains. The power-law is found to arise from
quasi-discontinuous changes in the slope of the time series of the heat
transport associated with the nucleation of dislocation pairs and roll
pinch-off events. For larger frequencies, the power spectra decay exponentially
as expected for time continuous deterministic dynamics.Comment: (10 pages, 6 figures
Spiral Defect Chaos in Large Aspect Ratio Rayleigh-Benard Convection
We report experiments on convection patterns in a cylindrical cell with a
large aspect ratio. The fluid had a Prandtl number of approximately 1. We
observed a chaotic pattern consisting of many rotating spirals and other
defects in the parameter range where theory predicts that steady straight rolls
should be stable. The correlation length of the pattern decreased rapidly with
increasing control parameter so that the size of a correlated area became much
smaller than the area of the cell. This suggests that the chaotic behavior is
intrinsic to large aspect ratio geometries.Comment: Preprint of experimental paper submitted to Phys. Rev. Lett. May 12
1993. Text is preceeded by many TeX macros. Figures 1 and 2 are rather lon
Pattern Formation and Dynamics in Rayleigh-B\'{e}nard Convection: Numerical Simulations of Experimentally Realistic Geometries
Rayleigh-B\'{e}nard convection is studied and quantitative comparisons are
made, where possible, between theory and experiment by performing numerical
simulations of the Boussinesq equations for a variety of experimentally
realistic situations. Rectangular and cylindrical geometries of varying aspect
ratios for experimental boundary conditions, including fins and spatial ramps
in plate separation, are examined with particular attention paid to the role of
the mean flow. A small cylindrical convection layer bounded laterally either by
a rigid wall, fin, or a ramp is investigated and our results suggest that the
mean flow plays an important role in the observed wavenumber. Analytical
results are developed quantifying the mean flow sources, generated by amplitude
gradients, and its effect on the pattern wavenumber for a large-aspect-ratio
cylinder with a ramped boundary. Numerical results are found to agree well with
these analytical predictions. We gain further insight into the role of mean
flow in pattern dynamics by employing a novel method of quenching the mean flow
numerically. Simulations of a spiral defect chaos state where the mean flow is
suddenly quenched is found to remove the time dependence, increase the
wavenumber and make the pattern more angular in nature.Comment: 9 pages, 10 figure
Long, Bellows-Free Vertical Helium Transfer Lines for the LHC Cryogenic System
The cryogenic system for the Large Hadron Collider (LHC) under construction at CERN will include four new vertical helium transfer lines connecting the new helium refrigerators to the underground areas. These four transfer lines will be installed between a refrigerator on the surface and an interconnection box located 80 m to 145 m underground. They consist of a vacuum jacket, a thermal screen and four internal helium pipes. Due to space and accessibility limitations, the lines have been specified without bellows or bends of any kind in the long vertical part; the thermal contractions must be compensated at the surface only. The displacement due to these contractions amounts to more than 35 cm in one case, and all four internal pipes, as well as the thermal screen, must be able to contract and expand independently. The lines will be built and installed by a consortium of Linde AG and Babcock Noell Nuclear GmbH. Their technical design choices are presented together with expected performance
Weak interactions and quasi-stable particle energy loss
We discuss the interplay between electromagnetic energy loss and weak
interactions in the context of quasistable particle particle propagation
through materials. As specific examples, we consider staus, where weak
interactions may play a role, and taus, where they don't.Comment: 4 pages, 4 figures, to appear in the proceedings of the Second
Workshop on TeV Particle Astrophysics (August 2006, Madison, WI
Finite-size effects lead to supercritical bifurcations in turbulent rotating Rayleigh-B\'enard convection
In turbulent thermal convection in cylindrical samples of aspect ratio \Gamma
= D/L (D is the diameter and L the height) the Nusselt number Nu is enhanced
when the sample is rotated about its vertical axis, because of the formation of
Ekman vortices that extract additional fluid out of thermal boundary layers at
the top and bottom. We show from experiments and direct numerical simulations
that the enhancement occurs only above a bifurcation point at a critical
inverse Rossby number 1/\Ro_c, with 1/\Ro_c \propto 1/\Gamma. We present a
Ginzburg-Landau like model that explains the existence of a bifurcation at
finite 1/\Ro_c as a finite-size effect. The model yields the proportionality
between 1/\Ro_c and and is consistent with several other measured
or computed system properties.Comment: 4 pages, 4 figure
From multiplicative noise to directed percolation in wetting transitions
A simple one-dimensional microscopic model of the depinning transition of an
interface from an attractive hard wall is introduced and investigated. Upon
varying a control parameter, the critical behaviour observed along the
transition line changes from a directed-percolation to a multiplicative-noise
type. Numerical simulations allow for a quantitative study of the multicritical
point separating the two regions, Mean-field arguments and the mapping on a yet
simpler model provide some further insight on the overall scenario.Comment: 4 pages, 3 figure
Quantized charge pumping through a quantum dot by surface acoustic waves
We present a realization of quantized charge pumping. A lateral quantum dot
is defined by metallic split gates in a GaAs/AlGaAs heterostructure. A surface
acoustic wave whose wavelength is twice the dot length is used to pump single
electrons through the dot at a frequency f=3GHz. The pumped current shows a
regular pattern of quantization at values I=nef over a range of gate voltage
and wave amplitude settings. The observed values of n, the number of electrons
transported per wave cycle, are determined by the number of electronic states
in the quantum dot brought into resonance with the fermi level of the electron
reservoirs during the pumping cycle.Comment: 8 page
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