291 research outputs found
Self-propelled non-linearly diffusing particles. Aggregation and continuum description
We introduce a model of self-propelled particles carrying out a Brownian
motion with a diffusion coefficient which depends on the local density of
particles within a certain finite radius. Numerical simulations show that in a
range of parameters the long-time spatial distribution of particles is
non-homogeneous, and clusters can be observed. A number density equation, which
explains the emergence of the aggregates and indicates the values of the
parameters for which they appear, is derived. Numerical results of this
continuum density equation are also shown.Comment: 5 pages, 5 figures. Major modifications. A new figure and some
references added. Final version accepted for publication in Phys. Rev.
Sustained plankton blooms under open chaotic flows
We consider a predator-prey model of planktonic population dynamics, of
excitable character, living in an open and chaotic fluid flow, i.e., a state of
fluid motion in which fluid trajectories are unbounded but a chaotic region
exists that is restricted to a localized area. Despite that excitability is a
transient phenomenon and that fluid trajectories are continuously leaving the
system, there is a regime of parameters where the excitation remains
permanently in the system, given rise to a persistent plankton bloom. This
regime is reached when the time scales associated to fluid stirring become
slower than the ones associated to biological growth.Comment: 14 pages, 3 figure
Efficiency of a stirred chemical reaction in a closed vessel
We perform a numerical study of the reaction efficiency in a closed vessel.
Starting with a little spot of product, we compute the time needed to complete
the reaction in the container following an advection-reaction-diffusion
process. Inside the vessel it is present a cellular velocity field that
transports the reactants. If the size of the container is not very large
compared with the typical length of the velocity field one has a plateau of the
reaction time as a function of the strength of the velocity field, . This
plateau appears both in the stationary and in the time-dependent flow. A
comparison of the results for the finite system with the infinite case (for
which the front speed, , gives a simple estimate of the reacting time)
shows the dramatic effect of the finite size.Comment: 4 pages, 4 figure
Spatial Patterns in Chemically and Biologically Reacting Flows
We present here a number of processes, inspired by concepts in Nonlinear
Dynamics such as chaotic advection and excitability, that can be useful to
understand generic behaviors in chemical or biological systems in fluid flows.
Emphasis is put on the description of observed plankton patchiness in the sea.
The linearly decaying tracer, and excitable kinetics in a chaotic flow are
mainly the models described. Finally, some warnings are given about the
difficulties in modeling discrete individuals (such as planktonic organisms) in
terms of continuous concentration fields.Comment: 41 pages, 10 figures; To appear in the Proceedings of the 2001 ISSAOS
School on 'Chaos in Geophysical Flows
Spatial patterns of competing random walkers
We review recent results obtained from simple individual-based models of
biological competition in which birth and death rates of an organism depend on
the presence of other competing organisms close to it. In addition the
individuals perform random walks of different types (Gaussian diffusion and
L\'{e}vy flights). We focus on how competition and random motions affect each
other, from which spatial instabilities and extinctions arise. Under suitable
conditions, competitive interactions lead to clustering of individuals and
periodic pattern formation. Random motion has a homogenizing effect and then
delays this clustering instability. When individuals from species differing in
their random walk characteristics are allowed to compete together, the ones
with a tendency to form narrower clusters get a competitive advantage over the
others. Mean-field deterministic equations are analyzed and compared with the
outcome of the individual-based simulations.Comment: 38 pages, including 6 figure
Species clustering in competitive Lotka-Volterra models
We study the properties of Lotka-Volterra competitive models in which the
intensity of the interaction among species depends on their position along an
abstract niche space through a competition kernel. We show analytically and
numerically that the properties of these models change dramatically when the
Fourier transform of this kernel is not positive definite, due to a pattern
forming instability. We estimate properties of the species distributions, such
as the steady number of species and their spacings, for different types of
kernels.Comment: 4 pages, 3 figure
Online games: a novel approach to explore how partial information influences human random searches
Many natural processes rely on optimizing the success ratio of a search
process. We use an experimental setup consisting of a simple online game in
which players have to find a target hidden on a board, to investigate the how
the rounds are influenced by the detection of cues. We focus on the search
duration and the statistics of the trajectories traced on the board. The
experimental data are explained by a family of random-walk-based models and
probabilistic analytical approximations. If no initial information is given to
the players, the search is optimized for cues that cover an intermediate
spatial scale. In addition, initial information about the extension of the cues
results, in general, in faster searches. Finally, strategies used by informed
players turn into non-stationary processes in which the length of each
displacement evolves to show a well-defined characteristic scale that is not
found in non-informed searches.Comment: 17 pages, 10 figure
Spatial patterns in mesic savannas: the local facilitation limit and the role of demographic stochasticity
We propose a model equation for the dynamics of tree density in mesic
savannas. It considers long-range competition among trees and the effect of
fire acting as a local facilitation mechanism. Despite short-range facilitation
is taken to the local-range limit, the standard full spectrum of spatial
structures obtained in general vegetation models is recovered. Long-range
competition is thus the key ingredient for the development of patterns. The
long time coexistence between trees and grass, and how fires affect the
survival of trees as well as the maintenance of the patterns is studied. The
influence of demographic noise is analyzed. The stochastic system, under the
parameter constraints typical of mesic savannas, shows irregular patterns
characteristics of realistic situations. The coexistence of trees and grass
still remains at reasonable noise intensities.Comment: 12 pages, 7 figure
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