771 research outputs found
Investigating organic phase change behavior with thermal photography
The United Kingdom has a target of reducing fossil fuel emissions to 80% of 1990s emissions levels by 2050 and the National Grid is highly likely to see a vast portion of displaced currently fossil fuelled heating provision. There is therefore a significant need for research into alternatives to grid re-enforcement and additional power stations. Thermal energy storage systems present one such alternative via the decoupling of energy production and consumption in both industrial and domestic heating demand. Reversible phase change in materials such as paraffin waxes, inorganic salts and metal alloys represent such a technology. For these, the ability to empirically assess a given phase change is invaluable for determining materials of interest, system characteristics and optimisation of systems design in addition to quantitatively validate any given numerical system models.
In this work the development and evolution of the solid to liquid phase transition in two organic phase change materials: beeswax and paraffin wax is investigated. These are characterised via visible and thermal photography of an ongoing melt/solidification in a 5cm dimensioned cube constructed of several differing wall materials. Isothermal heat is supplied via the bottom surface by a laboratory hot plate.
During the melting and re-solidification process, the thermal conduction through non-infra-red transparent wall materials has been unable to indicate the solid-liquid interface or melt/solidification times. Direct infra-red transmission through the infra-red transparent materials and visible light analysis through the transparent walls has been able to provide a solid-liquid interface track and melting time to completion but is unable to confer any useful information during the solidification process due to premature solidification on the wall exposed to imaging
A Pinned Polymer Model of Posture Control
A phenomenological model of human posture control is posited. The dynamics
are modelled as an elastically pinned polymer under the influence of noise. The
model accurately reproduces the two-point correlation functions of experimental
posture data and makes predictions for the response function of the postural
control system. The physiological and clinical significance of the model is
discussed.Comment: uuencoded post script file, 17 pages with 3 figure
Closed string tachyons, flips and conifolds
Following the analysis of tachyons and orbifold flips described in
hep-th/0412337, we study nonsupersymmetric analogs of the supersymmetric
conifold singularity and show using their toric geometry description that they
are nonsupersymmetric orbifolds of the latter. Using linear sigma models, we
see that these are unstable to localized closed string tachyon condensation and
exhibit flip transitions between their two small resolutions (involving
2-cycles), in the process mediating mild dynamical topology change. Our
analysis shows that the structure of these nonsupersymmetric conifolds as
quotients of the supersymmetric conifold obstructs the 3-cycle deformation of
such singularities, suggesting that these nonsupersymmetric conifolds decay by
evolving towards their stable small resolutions.Comment: Latex, 22 pgs, 2 figs. v4: matches JHEP version, 29 pgs, 3 figures,
more elaborate Introduction, various clarifications adde
A numerical study of the development of bulk scale-free structures upon growth of self-affine aggregates
During the last decade, self-affine geometrical properties of many growing
aggregates, originated in a wide variety of processes, have been well
characterized. However, little progress has been achieved in the search of a
unified description of the underlying dynamics. Extensive numerical evidence
has been given showing that the bulk of aggregates formed upon ballistic
aggregation and random deposition with surface relaxation processes can be
broken down into a set of infinite scale invariant structures called "trees".
These two types of aggregates have been selected because it has been
established that they belong to different universality classes: those of
Kardar-Parisi-Zhang and Edward-Wilkinson, respectively. Exponents describing
the spatial and temporal scale invariance of the trees can be related to the
classical exponents describing the self-affine nature of the growing interface.
Furthermore, those exponents allows us to distinguish either the compact or
non-compact nature of the growing trees. Therefore, the measurement of the
statistic of the process of growing trees may become a useful experimental
technique for the evaluation of the self-affine properties of some aggregates.Comment: 19 pages, 5 figures, accepted for publication in Phys.Rev.
The long-time dynamics of two hydrodynamically-coupled swimming cells
Swimming micro-organisms such as bacteria or spermatozoa are typically found
in dense suspensions, and exhibit collective modes of locomotion qualitatively
different from that displayed by isolated cells. In the dilute limit where
fluid-mediated interactions can be treated rigorously, the long-time
hydrodynamics of a collection of cells result from interactions with many other
cells, and as such typically eludes an analytical approach. Here we consider
the only case where such problem can be treated rigorously analytically, namely
when the cells have spatially confined trajectories, such as the spermatozoa of
some marine invertebrates. We consider two spherical cells swimming, when
isolated, with arbitrary circular trajectories, and derive the long-time
kinematics of their relative locomotion. We show that in the dilute limit where
the cells are much further away than their size, and the size of their circular
motion, a separation of time scale occurs between a fast (intrinsic) swimming
time, and a slow time where hydrodynamic interactions lead to change in the
relative position and orientation of the swimmers. We perform a multiple-scale
analysis and derive the effective dynamical system - of dimension two -
describing the long-time behavior of the pair of cells. We show that the system
displays one type of equilibrium, and two types of rotational equilibrium, all
of which are found to be unstable. A detailed mathematical analysis of the
dynamical systems further allows us to show that only two cell-cell behaviors
are possible in the limit of , either the cells are attracted to
each other (possibly monotonically), or they are repelled (possibly
monotonically as well), which we confirm with numerical computations
Stereoscopic Analysis of the 19 May 2007 Erupting Filament
A filament eruption, accompanied by a B9.5 flare, coronal dimming and an EUV
wave, was observed by the Solar TERrestrial Relations Observatory (STEREO) on
19 May 2007, beginning at about 13:00 UT. Here, we use observations from the
SECCHI/EUVI telescopes and other solar observations to analyze the behavior and
geometry of the filament before and during the eruption. At this time, STEREO A
and B were separated by about 8.5 degrees, sufficient to determine the
three-dimensional structure of the filament using stereoscopy. The filament
could be followed in SECCHI/EUVI 304 A stereoscopic data from about 12 hours
before to about 2 hours after the eruption, allowing us to determine the 3D
trajectory of the erupting filament. From the 3D reconstructions of the
filament and the chromospheric ribbons in the early stage of the eruption,
simultaneous heating of both the rising filamentary material and the
chromosphere directly below is observed, consistent with an eruption resulting
from magnetic reconnection below the filament. Comparisons of the filament
during eruption in 304 A and Halpha show that when it becomes emissive in He
II, it tends to disappear in Halpha, indicating that the disappearance probably
results from heating or motion, not loss, of filamentary material.Comment: Accepted for publication in Solar Physic
Glassy Vortex State in a Two-Dimensional Disordered XY-Model
The two-dimensional XY-model with random phase-shifts on bonds is studied.
The analysis is based on a renormalization group for the replicated system. The
model is shown to have an ordered phase with quasi long-range order. This
ordered phase consists of a glass-like region at lower temperatures and of a
non-glassy region at higher temperatures. The transition from the disordered
phase into the ordered phase is not reentrant and is of a new universality
class at zero temperature. In contrast to previous approaches the disorder
strength is found to be renormalized to larger values. Several correlation
functions are calculated for the ordered phase. They allow to identify not only
the transition into the glassy phase but also an additional crossover line,
where the disconnected vortex correlation changes its behavior on large scales
non-analytically. The renormalization group approach yields the glassy features
without a breaking of replica symmetry.Comment: latex 12 pages with 3 figures, using epsf.sty and multicol.st
On Dark Matter Annihilation in the Local Group
Under the hypothesis of a Dark Matter composed by supersymmetric particles
like neutralinos, we investigate the possibility that their annihilation in the
haloes of nearby galaxies could produce detectable fluxes of -photons.
Expected fluxes depend on several, poorly known quantities such as the density
profiles of Dark Matter haloes, the existence and prominence of central density
cusps and the presence of a population of sub-haloes. We find that, for all
reasonable choices of Dark Matter halo models, the intensity of the
-ray flux from some of the nearest extragalactic objects, like M31, is
comparable or higher than the diffuse Galactic foreground. We show that next
generation ground-based experiments could have the sensitivity to reveal such
fluxes which could help us unveiling the nature of Dark Matter particles.Comment: 11 pages, 10 figures. Accepted for publication in Phys. Rev. D.;
added a new paragraph on the detectability of Galactic sub-halos in our
Galaxy; added a discussion on their model dependence. The relation of our
results with the "CDM crisis" issue has also been adde
Large scale numerical investigation of excited states in poly(phenylene)
A density matrix renormalisation group scheme is developed, allowing for the
first time essentially exact numerical solutions for the important excited
states of a realistic semi-empirical model for oligo-phenylenes. By monitoring
the evolution of the energies with chain length and comparing them to the
experimental absorption peaks of oligomers and thin films, we assign the four
characteristic absorption peaks of phenyl-based polymers. We also determine the
position and nature of the nonlinear optical states in this model.Comment: RevTeX, 10 pages, 4 eps figures included using eps
- …