332 research outputs found
On the crescentic shape of barchan dune
Aeolian sand dunes originate from wind flow and sand bed interactions.
According to wind properties and sand availability, they can adopt different
shapes, ranging from huge motion-less star dunes to small and mobile barchan
dunes. The latter are crescentic and emerge under a unidirectional wind, with a
low sand supply. Here, a 3d model for barchan based on existing 2d model is
proposed. After describing the intrinsic issues of 3d modeling, we show that
the deflection of reptating particules due to the shape of the dune leads to a
lateral sand flux deflection, which takes the mathematical form of a non-linear
diffusive process. This simple and physically meaningful coupling method is
used to understand the shape of barchan dunes.Comment: 8 pages, 9 figures, submitted to Eur. Phys. J. B v2 : major changes
in grammar and in presentatio
Barchan dune corridors: field characterization and investigation of control parameters
The structure of the barchan field located between Tarfaya and Laayoune
(Atlantic Sahara, Morocco) is quantitatively investigated and compared to that
in La Pampa de la Joya (Arequipa, Peru). On the basis of field measurements, we
show how the volume, the velocity and the output sand flux of a dune can be
computed from the value of its body and horn widths. The dune size distribution
is obtained from the analysis of aerial photographs. It shows that these fields
are in a statistically homogeneous state along the wind direction and present a
`corridor' structure in the transverse direction, in which the dunes have a
rather well selected size. Investigating the possible external parameters
controlling these corridors, we demonstrate that none among topography,
granulometry, wind and sand flux is relevant. We finally discuss the dynamical
processes at work in these fields (collisions and wind fluctuations), and
investigate the way they could regulate the size of the dunes. Furthermore we
show that the overall sand flux transported by a dune field is smaller than the
maximum transport that could be reached in the absence of dunes, i.e. in
saltation over the solid ground.Comment: revised version for JGR-ES, 36 pages, 21 figure
Polygons on a Rotating Fluid Surface
We report a novel and spectacular instability of a fluid surface in a
rotating system. In a flow driven by rotating the bottom plate of a partially
filled, stationary cylindrical container, the shape of the free surface can
spontaneously break the axial symmetry and assume the form of a polygon
rotating rigidly with a speed different from that of the plate. With water we
have observed polygons with up to 6 corners. It has been known for many years
that such flows are prone to symmetry breaking, but apparently the polygonal
surface shapes have never been observed. The creation of rotating internal
waves in a similar setup was observed for much lower rotation rates, where the
free surface remains essentially flat. We speculate that the instability is
caused by the strong azimuthal shear due to the stationary walls and that it is
triggered by minute wobbling of the rotating plate. The slight asymmetry
induces a tendency for mode-locking between the plate and the polygon, where
the polygon rotates by one corner for each complete rotation of the plate
3D Dune Skeleton Model as a Coupled Dynamical System of 2D Cross-Sections
To analyze theoretically the stability of the shape and the migration process
of transverse dunes and barchans, we propose a {\it skeleton model} of 3D dunes
described with coupled dynamics of 2D cross-sections. First, 2D cross-sections
of a 3D dune parallel to the wind direction are extracted as elements of a
skeleton of the 3D dune, hence, the dynamics of each and interaction between
them is considered. This model simply describes the essential dynamics of 3D
dunes as a system of coupled ordinary differential equations. Using the model
we study the stability of the shape of 3D transversal dunes and their
deformation to barchans depending on the amount of available sand in the dune
field, sand flow in parallel and perpendicular to wind direction.Comment: 6 pages, 6 figures, lette
Genetic drift at expanding frontiers promotes gene segregation
Competition between random genetic drift and natural selection plays a
central role in evolution: Whereas non-beneficial mutations often prevail in
small populations by chance, mutations that sweep through large populations
typically confer a selective advantage. Here, however, we observe chance
effects during range expansions that dramatically alter the gene pool even in
large microbial populations. Initially well-mixed populations of two
fluorescently labeled strains of Escherichia coli develop well-defined,
sector-like regions with fractal boundaries in expanding colonies. The
formation of these regions is driven by random fluctuations that originate in a
thin band of pioneers at the expanding frontier. A comparison of bacterial and
yeast colonies (Saccharomyces cerevisiae) suggests that this large-scale
genetic sectoring is a generic phenomenon that may provide a detectable
footprint of past range expansions.Comment: Please visit http://www.pnas.org/content/104/50/19926.abstract for
published articl
The song of the dunes as a self-synchronized instrument
Since Marco Polo (1) it has been known that some sand dunes have the peculiar
ability of emitting a loud sound with a well defined frequency, sometimes for
several minutes. The origin of this sustained sound has remained mysterious,
partly because of its rarity in nature (2). It has been recognized that the
sound is not due to the air flow around the dunes but to the motion of an
avalanche (3), and not to an acoustic excitation of the grains but to their
relative motion (4-7). By comparing several singing dunes and two controlled
experiments, one in the laboratory and one in the field, we here demonstrate
that the frequency of the sound is the frequency of the relative motion of the
sand grains. The sound is produced because some moving grains synchronize their
motions. The existence of a velocity threshold in both experiments further
shows that this synchronization comes from an acoustic resonance within the
flowing layer: if the layer is large enough it creates a resonance cavity in
which grains self-synchronize.Comment: minor changes, essentially more references
Collision dynamics of two barchan dunes simulated by a simple model
The collision processes of two crescentic dunes called barchans are
systematically studied using a simple computer simulation model. The simulated
processes, coalescence, ejection and reorganization, qualitatively correspond
to those observed in a water tank experiment. Moreover we found the realized
types of collision depend both on the mass ratio and on the lateral distance
between barchans under initial conditions. A simple set of differential
equations to describe the collision of one-dimensional (1D) dunes is
introduced.Comment: 4 pages, 5 figures : To be published in Journal of the Physical
Society of Japa
Corridors of barchan dunes: stability and size selection
Barchans are crescentic dunes propagating on a solid ground. They form dune
fields in the shape of elongated corridors in which the size and spacing
between dunes are rather well selected. We show that even very realistic models
for solitary dunes do not reproduce these corridors. Instead, two instabilities
take place. First, barchans receive a sand flux at their back proportional to
their width while the sand escapes only from their horns. Large dunes
proportionally capture more than they loose sand, while the situation is
reversed for small ones: therefore, solitary dunes cannot remain in a steady
state. Second, the propagation speed of dunes decreases with the size of the
dune: this leads -- through the collision process -- to a coarsening of barchan
fields. We show that these phenomena are not specific to the model, but result
from general and robust mechanisms. The length scales needed for these
instabilities to develop are derived and discussed. They turn out to be much
smaller than the dune field length. As a conclusion, there should exist further
- yet unknown - mechanisms regulating and selecting the size of dunes.Comment: 13 pages, 13 figures. New version resubmitted to Phys. Rev. E.
Pictures of better quality available on reques
The fidelity of dynamic signaling by noisy biomolecular networks
This is the final version of the article. Available from Public Library of Science via the DOI in this record.Cells live in changing, dynamic environments. To understand cellular decision-making, we must therefore understand how fluctuating inputs are processed by noisy biomolecular networks. Here we present a general methodology for analyzing the fidelity with which different statistics of a fluctuating input are represented, or encoded, in the output of a signaling system over time. We identify two orthogonal sources of error that corrupt perfect representation of the signal: dynamical error, which occurs when the network responds on average to other features of the input trajectory as well as to the signal of interest, and mechanistic error, which occurs because biochemical reactions comprising the signaling mechanism are stochastic. Trade-offs between these two errors can determine the system's fidelity. By developing mathematical approaches to derive dynamics conditional on input trajectories we can show, for example, that increased biochemical noise (mechanistic error) can improve fidelity and that both negative and positive feedback degrade fidelity, for standard models of genetic autoregulation. For a group of cells, the fidelity of the collective output exceeds that of an individual cell and negative feedback then typically becomes beneficial. We can also predict the dynamic signal for which a given system has highest fidelity and, conversely, how to modify the network design to maximize fidelity for a given dynamic signal. Our approach is general, has applications to both systems and synthetic biology, and will help underpin studies of cellular behavior in natural, dynamic environments.We acknowledge support from a Medical Research Council and Engineering and Physical Sciences Council funded Fellowship in Biomedical Informatics (CGB) and a Scottish Universities Life Sciences Alliance chair in Systems Biology (PSS). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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