19,969 research outputs found
Data management study, volume 5. Appendix K - Contractor data package data management /DM/ Final report
Contractor data management system for Voyager projec
Weakly Nonlinear Analysis of Electroconvection in a Suspended Fluid Film
It has been experimentally observed that weakly conducting suspended films of
smectic liquid crystals undergo electroconvection when subjected to a large
enough potential difference. The resulting counter-rotating vortices form a
very simple convection pattern and exhibit a variety of interesting nonlinear
effects. The linear stability problem for this system has recently been solved.
The convection mechanism, which involves charge separation at the free surfaces
of the film, is applicable to any sufficiently two-dimensional fluid. In this
paper, we derive an amplitude equation which describes the weakly nonlinear
regime, by starting from the basic electrohydrodynamic equations. This regime
has been the subject of several recent experimental studies. The lowest order
amplitude equation we derive is of the Ginzburg-Landau form, and describes a
forward bifurcation as is observed experimentally. The coefficients of the
amplitude equation are calculated and compared with the values independently
deduced from the linear stability calculation.Comment: 26 pages, 2 included eps figures, submitted to Phys Rev E. For more
information, see http://mobydick.physics.utoronto.c
Electroconvection in a Suspended Fluid Film: A Linear Stability Analysis
A suspended fluid film with two free surfaces convects when a sufficiently
large voltage is applied across it. We present a linear stability analysis for
this system. The forces driving convection are due to the interaction of the
applied electric field with space charge which develops near the free surfaces.
Our analysis is similar to that for the two-dimensional B\'enard problem, but
with important differences due to coupling between the charge distribution and
the field. We find the neutral stability boundary of a dimensionless control
parameter as a function of the dimensionless wave number .
, which is proportional to the square of the applied voltage, is
analogous to the Rayleigh number. The critical values and
are found from the minimum of the stability boundary, and its
curvature at the minimum gives the correlation length . The
characteristic time scale , which depends on a second dimensionless
parameter , analogous to the Prandtl number, is determined from the
linear growth rate near onset. and are coefficients in the
Ginzburg-Landau amplitude equation which describes the flow pattern near onset
in this system. We compare our results to recent experiments.Comment: 36 pages, 7 included eps figures, submitted to Phys Rev E. For more
info, see http://mobydick.physics.utoronto.ca
van Vleck determinants: traversable wormhole spacetimes
Calculating the van Vleck determinant in traversable wormhole spacetimes is
an important ingredient in understanding the physical basis behind Hawking's
chronology protection conjecture. This paper presents extensive computations of
this object --- at least in the short--throat flat--space approximation. An
important technical trick is to use an extension of the usual junction
condition formalism to probe the full Riemann tensor associated with a thin
shell of matter. Implications with regard to Hawking's chronology protection
conjecture are discussed. Indeed, any attempt to transform a single isolated
wormhole into a time machine results in large vacuum polarization effects
sufficient to disrupt the internal structure of the wormhole before the onset
of Planck scale physics, and before the onset of time travel. On the other
hand, it is possible to set up a putative time machine built out of two or more
wormholes, each of which taken in isolation is not itself a time machine. Such
``Roman configurations'' are much more subtle to analyse. For some particularly
bizarre configurations (not traversable by humans) the vacuum polarization
effects can be arranged to be arbitrarily small at the onset of Planck scale
physics. This indicates that the disruption scale has been pushed down into the
Planck slop. Ultimately, for these configurations, questions regarding the
truth or falsity of Hawking's chronology protection can only be addressed by
entering the uncharted wastelands of full fledged quantum gravity.Comment: 42 pages, ReV_TeX 3.
Doctors and Artists
In "The Doctor's Dilemma," Bernard Shaw suggests that there is more antagonism than attraction between the doctor-scientist and the painter. The average doctor may, however, make some claim to be an artist. In his professional work,art plays some role, even if it is restricted to that much maligned professional accessory the bedside manner. Doctors in their hobbies sometimes display a wider taste in art-for instance as painters or collectors.It is, however, my desire in this article to deal less with the links between doctors and art than with those between certain doctors and their artists. From the introduction of printing these links have been very close. The medical text-book has always required skilled illustration to make intelligible a letterpress which has not always been artistic. Many works of great medical importance owe their lasting fame as much to the perspicacity and skill of the illustrator as to any other inherent merit
Active Galaxies and Cluster Gas
Two lines of evidence indicate that active galaxies, principally radio
galaxies, have heated the diffuse hot gas in clusters. The first is the general
need for additional heating to explain the steepness of the X-ray
luminosity--temperature relation in clusters, the second is to solve the
cooling flow problem in cluster cores. The inner core of many clusters is
radiating energy as X-rays on a timescale much shorter than its likely age.
Although the temperature in this region drops by a factor of about 3 from that
of the surrounding gas, little evidence is found for gas much cooler than that.
Some form of heating appears to be taking place, probably by energy transported
outward from the central accreting black hole or radio source. How that energy
heats the gas depends on poorly understood transport properties (conductivity
and viscosity) of the intracluster medium. Viscous heating is discussed as a
possibility. Such heating processes have consequences for the truncation of the
luminosity function of massive galaxies.Comment: 14 pages, 16 fig, Feb 2004 talk for Phil Trans Roy So
Autocatalytic plume pinch-off
A localized source of buoyancy flux in a non-reactive fluid medium creates a
plume. The flux can be provided by either heat, a compositional difference
between the fluid comprising the plume and its surroundings, or a combination
of both. For autocatalytic plumes produced by the iodate-arsenous acid
reaction, however, buoyancy is produced along the entire reacting interface
between the plume and its surroundings. Buoyancy production at the moving
interface drives fluid motion, which in turn generates flow that advects the
reaction front. As a consequence of this interplay between fluid flow and
chemical reaction, autocatalytic plumes exhibit a rich dynamics during their
ascent through the reactant medium. One of the more interesting dynamical
features is the production of an accelerating vortical plume head that in
certain cases pinches-off and detaches from the upwelling conduit. After
pinch-off, a new plume head forms in the conduit below, and this can lead to
multiple generations of plume heads for a single plume initiation. We
investigated the pinch-off process using both experimentation and simulation.
Experiments were performed using various concentrations of glycerol, in which
it was found that repeated pinch-off occurs exclusively in a specific
concentration range. Autocatalytic plume simulations revealed that pinch-off is
triggered by the appearance of accelerating flow in the plume conduit.Comment: 10 figures. Accepted for publication in Phys Rev E. See also
http://www.physics.utoronto.ca/nonlinear/papers_chemwave.htm
Wegner-Houghton equation and derivative expansion
We study the derivative expansion for the effective action in the framework
of the Exact Renormalization Group for a single component scalar theory. By
truncating the expansion to the first two terms, the potential and the
kinetic coefficient , our analysis suggests that a set of coupled
differential equations for these two functions can be established under certain
smoothness conditions for the background field and that sharp and smooth
cut-off give the same result. In addition we find that, differently from the
case of the potential, a further expansion is needed to obtain the differential
equation for , according to the relative weight between the kinetic and
the potential terms. As a result, two different approximations to the
equation are obtained. Finally a numerical analysis of the coupled equations
for and is performed at the non-gaussian fixed point in
dimensions to determine the anomalous dimension of the field.Comment: 15 pages, 3 figure
Kinetic pathways of multi-phase surfactant systems
The relaxation following a temperature quench of two-phase (lamellar and
sponge phase) and three-phase (lamellar, sponge and micellar phase) samples,
has been studied in an SDS/octanol/brine system. In the three-phase case we
have observed samples that are initially mainly sponge phase with lamellar and
micellar phase on the top and bottom respectively. Upon decreasing temperature
most of the volume of the sponge phase is replaced by lamellar phase. During
the equilibriation we have observed three regimes of behaviour within the
sponge phase: (i) disruption in the sponge texture, then (ii) after the sponge
phase homogenises there is a lamellar nucleation regime and finally (iii) a
bizarre plume connects the lamellar phase with the micellar phase. The
relaxation of the two-phase sample proceeds instead in two stages. First
lamellar drops nucleate in the sponge phase forming a onion `gel' structure.
Over time the lamellar structure compacts while equilibriating into a two phase
lamellar/sponge phase sample. We offer possible explanatioins for some of these
observations in the context of a general theory for phase kinetics in systems
with one fast and one slow variable.Comment: 1 textfile, 20 figures (jpg), to appear in PR
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