10,716 research outputs found
Information-theoretic measurements of coupling between structure and dynamics in glass-formers
We analyse the connections between structure and dynamics in two model
glass-formers, using the mutual information between an initial configuration
and the ensuing dynamics to compare the predictive value of different
structural observables. We consider the predictive power of normal modes,
locally favoured structures, and coarse-grained measurements of local energy
and density. The mutual information allows the influence of the liquid
structure on the dynamics to be analysed quantitatively as a function of time,
showing that normal modes give the most useful predictions on short time scales
while local energy and density are most strongly predictive at long times.Comment: 10 pages, 7 fig
A language comparison for scientific computing on MIMD architectures
Choleski's method for solving banded symmetric, positive definite systems is implemented on a multiprocessor computer using three FORTRAN based parallel programming languages, the Force, PISCES and Concurrent FORTRAN. The capabilities of the language for expressing parallelism and their user friendliness are discussed, including readability of the code, debugging assistance offered, and expressiveness of the languages. The performance of the different implementations is compared. It is argued that PISCES, using the Force for medium-grained parallelism, is the appropriate choice for programming Choleski's method on the multiprocessor computer, Flex/32
Cryogenic Insulation System
This invention relates to reusable, low density, high temperature cryogenic foam insulation systems and the process for their manufacture. A pacing technology for liquid hydrogen fueled, high speed aircraft is the development of a fully reusable, flight weight cryogenic insulation system for propellant tank structures. In the invention cryogenic foam insulation is adhesively bonded to the outer wall of the fuel tank structure. The cryogenic insulation consists of square sheets fabricated from an array of abutting square blocks. Each block consists of a sheet of glass cloth adhesively bonded between two layers of polymethacrylimide foam. Each block is wrapped in a vapor impermeable membrane, such as Kapton(R) aluminum Kapton(R), to provide a vapor barrier. Very beneficial results can be obtained by employing the present invention in conjunction with fibrous insulation and an outer aeroshell, a hot fuselage structure with an internal thermal protection system
Gas physical conditions and kinematics of the giant outflow Ou4
Ou4 is a recently discovered bipolar outflow with a projected size of more
than one degree in the plane of the sky. It is apparently centred on the young
stellar cluster -whose most massive representative is the triple system HR8119-
inside the HII region Sh 2-129. The driving source, the nature, and the
distance of Ou4 are not known. Deep narrow-band imagery of the whole nebula at
arcsec resolution was obtained to study its morphology. Long-slit spectroscopy
of the tips of the bipolar lobes was secured to determine the gas ionization
mechanism, physical conditions, and line-of-sight velocities. An estimate of
the proper motions at the tip of the south lobe using archival images is
attempted. The existing multi-wavelength data for Sh 2-129 and HR 8119 are also
comprehensively reviewed. The morphology of Ou4, its emission-line spatial
distribution, line flux ratios, and the kinematic modelling adopting a
bow-shock parabolic geometry, illustrate the expansion of a shock-excited fast
collimated outflow. The radial velocities and reddening are consistent with
those of Sh 2-129 and HR 8119. The improved determination of the distance to
HR8119 (composed of two B0 V and one B0.5 V stars) and Sh 2-129 is 712 pc. We
identify in WISE images a 5 arcmin-radius (1 pc at the distance above) bubble
of emission at 22 micron emitted by hot (107 K) dust, located inside the
central part of Ou4 and corresponding to several [O III] features of Ou4. The
apparent position and the properties studied in this work are consistent with
the hypothesis that Ou4 is located inside the Sh 2-129 HII region, suggesting
that it was launched some 90 000 yrs ago by HR8119. The outflow total kinetic
energy is estimated to be ~4e47~ergs. However, the alternate possibility that
Ou4 is a bipolar planetary nebula, or the result of an eruptive event on a
massive AGB or post-AGB star not yet identified, cannot be ruled out.Comment: Accepted for publication in Astronomy and Astrophysics. Also
available at http://hal.archives-ouvertes.fr/hal-0102228
The effect of boundary adaptivity on hexagonal ordering and bistability in circularly confined quasi hard discs
The behaviour of materials under spatial confinement is sensitively dependent
on the nature of the confining boundaries. In two dimensions, confinement
within a hard circular boundary inhibits the hexagonal ordering observed in
bulk systems at high density. Using colloidal experiments and Monte Carlo
simulations, we investigate two model systems of quasi hard discs under
circularly symmetric confinement. The first system employs an adaptive circular
boundary, defined experimentally using holographic optical tweezers. We show
that deformation of this boundary allows, and indeed is required for, hexagonal
ordering in the confined system. The second system employs a circularly
symmetric optical potential to confine particles without a physical boundary.
We show that, in the absence of a curved wall, near perfect hexagonal ordering
is possible. We propose that the degree to which hexagonal ordering is
suppressed by a curved boundary is determined by the `strictness' of that wall.Comment: 10 pages, 8 figure
Effects of vertical confinement on gelation and sedimentation of colloids
We consider the sedimentation of a colloidal gel under confinement in the
direction of gravity. The confinement allows us to compare directly experiments
and computer simulations, for the same system size in the vertical direction.
The confinement also leads to qualitatively different behaviour compared to
bulk systems: in large systems gelation suppresses sedimentation, but for small
systems sedimentation is enhanced relative to non-gelling suspensions, although
the rate of sedimentation is reduced when the strength of the attraction
between the colloids is strong. We map interaction parameters between a model
experimental system (observed in real space) and computer simulations.
Remarkably, we find that when simulating the system using Brownian dynamics in
which hydrodynamic interactions between the particles are neglected, we find
that sedimentation occurs on the same timescale as the experiments, however the
thickness of the "arms" of the gel is rather larger in the experiments,
compared with the simulations. An analysis of local structure in the
simulations showed similar behaviour to gelation in the absence of gravity
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