22,957 research outputs found
Corner and finger formation in Hele--Shaw flow with kinetic undercooling regularisation
We examine the effect of a kinetic undercooling condition on the evolution of
a free boundary in Hele--Shaw flow, in both bubble and channel geometries. We
present analytical and numerical evidence that the bubble boundary is unstable
and may develop one or more corners in finite time, for both expansion and
contraction cases. This loss of regularity is interesting because it occurs
regardless of whether the less viscous fluid is displacing the more viscous
fluid, or vice versa. We show that small contracting bubbles are described to
leading order by a well-studied geometric flow rule. Exact solutions to this
asymptotic problem continue past the corner formation until the bubble
contracts to a point as a slit in the limit. Lastly, we consider the evolving
boundary with kinetic undercooling in a Saffman--Taylor channel geometry. The
boundary may either form corners in finite time, or evolve to a single long
finger travelling at constant speed, depending on the strength of kinetic
undercooling. We demonstrate these two different behaviours numerically. For
the travelling finger, we present results of a numerical solution method
similar to that used to demonstrate the selection of discrete fingers by
surface tension. With kinetic undercooling, a continuum of corner-free
travelling fingers exists for any finger width above a critical value, which
goes to zero as the kinetic undercooling vanishes. We have not been able to
compute the discrete family of analytic solutions, predicted by previous
asymptotic analysis, because the numerical scheme cannot distinguish between
solutions characterised by analytic fingers and those which are corner-free but
non-analytic
A three-dimensional Galactic extinction model
A large-scale three-dimensional model of Galactic extinction is presented
based on the Galactic dust distribution model of Drimmel and Spergel (2001).
The extinction A_V to any point within the Galactic disk can be quickly deduced
using a set of three-dimensional cartesian grids. Extinctions from the model
are compared to empirical extinction measures, including lines-of-sight in and
near the Galactic plane using optical and NIR extinction measures; in
particular we show how extinction can be derived from NIR color-magnitude
diagrams in the Galactic plane to a distance of 8 kiloparsec.Comment: 12 pages, to be published in A&
A metamaterial frequency-selective super-absorber that has absorbing cross section significantly bigger than the geometric cross section
Using the idea of transformation optics, we propose a metamaterial device
that serves as a frequency-selective super-absorber, which consists of an
absorbing core material coated with a shell of isotropic double negative
metamaterial. For a fixed volume, the absorption cross section of the
super-absorber can be made arbitrarily large at one frequency. The double
negative shell serves to amplify the evanescent tail of the high order incident
cylindrical waves, which induces strong scattering and absorption. Our
conclusion is supported by both analytical Mie theory and numerical finite
element simulation. Interesting applications of such a device are discussed.Comment: 16 pages, 5 figure
Paleontology: Outrunning Time
In this paper, I discuss several temporal aspects of paleontology from a
philosophical perspective. I begin by presenting the general problem of âtamingâ
deep time to make it comprehensible at a human scale, starting with the traditional
geologic time scale: an event-based, relative time scale consisting of a hierarchy of
chronological units. Not only does the relative timescale provide a basis for
reconstructing many of the general features of the history of life, but it is also
consonant with the cognitive processes humans use to think about time. Absolute
dating of rocks, fossils, and evolutionary events (such as branching events on the
tree of life) can be accomplished through the use of radiometric dating, chronological
signals extractable from fossil growth patterns, and the âmolecular clock.â
Sometimes these different methods of absolute dating, which start from largely
independent assumptions and evidentiary bases, converge in their temporal estimates,
resulting in a consilience of inductions. At other times they fail to agree,
either because fossils and molecules are giving temporal information about different
aspects of nature and should not be expected to agree, or because of flawed
assumptions that give rise to an inaccurate estimate. I argue that in general, despite
the fact that it can be difficult to integrate disparate kinds of evidence, the principle
of total evidence should be applied to the dating of evolutionary events. As a
historical science, paleontology studies past events we cannot observe directly.
This raises questions of epistemic access, meaning that due to the fragmentary
nature of the fossil record we may find ourselves without access to the relevant
traces to adjudicate between rival hypotheses about the past. The problems and
prospects of epistemic access are explored through a case study of the reconstruction
of the colors of dinosaurs. The paper closes with a reflection on the Darwin-
Lyell metaphor of the fossil record as a highly fragmentary history book, and a call
for a reconsideration of the book metaphor in favor of a systems view of the
geologic and fossil records
Influence of hand position on the near-effect in 3D attention
Voluntary reorienting of attention in real depth situations is characterized by an attentional bias to locations near the viewer once attention is deployed to a spatially cued object in depth. Previously this effect (initially referred to as the ânear-effectâ) was attributed to access of a 3D viewer-centred spatial representation for guiding attention in 3D space. The aim of this study was to investigate whether the near-bias could have been associated with the position of the response-hand, always near the viewer in previous studies investigating endogenous attentional shifts in real depth. In Experiment 1, the response-hand was placed at either the near or far target depth in a depth cueing task. Placing the response-hand at the far target depth abolished the near-effect, but failed to bias spatial attention to the far location. Experiment 2 showed that the response-hand effect was not modulated by the presence of an additional passive hand, whereas Experiment 3 confirmed that attentional prioritization of the passive hand was not masked by the influence of the responding hand on spatial attention in Experiment 2. The pattern of results is most consistent with the idea that response preparation can modulate spatial attention within a 3D viewer-centred spatial representation
Biological evolution through mutation, selection, and drift: An introductory review
Motivated by present activities in (statistical) physics directed towards
biological evolution, we review the interplay of three evolutionary forces:
mutation, selection, and genetic drift. The review addresses itself to
physicists and intends to bridge the gap between the biological and the
physical literature. We first clarify the terminology and recapitulate the
basic models of population genetics, which describe the evolution of the
composition of a population under the joint action of the various evolutionary
forces. Building on these foundations, we specify the ingredients explicitly,
namely, the various mutation models and fitness landscapes. We then review
recent developments concerning models of mutational degradation. These predict
upper limits for the mutation rate above which mutation can no longer be
controlled by selection, the most important phenomena being error thresholds,
Muller's ratchet, and mutational meltdowns. Error thresholds are deterministic
phenomena, whereas Muller's ratchet requires the stochastic component brought
about by finite population size. Mutational meltdowns additionally rely on an
explicit model of population dynamics, and describe the extinction of
populations. Special emphasis is put on the mutual relationship between these
phenomena. Finally, a few connections with the process of molecular evolution
are established.Comment: 62 pages, 6 figures, many reference
Endangered by Sprawl: How Runaway Development Threatens America's Wildlife
Estimates the growth of land consumption in metropolitan areas over the next 25 years, investigates locally implemented strategies to protect natural lands from overdevelopment, and offers "smart growth" as an option for reducing suburban sprawl
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