603 research outputs found
Competition, efficiency and collective behavior in the "El Farol" bar model
The El Farol bar model, proposed to study the dynamics of competition of
agents in a variety of contexts (W. B. Arthur, Amer. Econ. Assoc. Pap. and
Proc. 84, 406 (1994)) is studied. We characterize in detail the three regions
of the phase diagram (efficient, inefficient and better than random) of the
simplest version of the model (D. Challet and Y.-C. Zhang, Physica A, 246, 407
(1997)). The efficient region is shown to have a rich structure, which is
investigated in some detail. Changes in the payoff function enhance further the
tendency of the model towards a wasteful distribution of resources.Comment: 7 pages Latex, 7 Postscript figures; changed reference,
acknowledgments included. Accepted for publication in Europen Physics Journal
Cáculo del riesgo de ingición a partir de imágenes AVHRR (NOAA)
El riesgo de incendio forestal puede resumirse en dos factores principales: el riesgo de ignición y la
probabilidad de que el fuego se expanda y acabe produciendo un incendio forestal. El riesgo de ignición puede
ser debido a diferentes causas: factor humano y el estado de la vegetación. La probabilidad de expansión es
principalmente debido a: condiciones meteorológicas, situación geográfica, características de la vegetación y
facilidad de extinción.
El principal objetivo del presente trabajo es la obtención de un índice de riesgo de ignición debido al estado de
la vegetación. Para cumplir dicho objetivo se ha elaborado una serie temporal de 8 años de imágenes AVHRR
(NOAA). A partir de las imágenes diarias se ha calculado el índice de vegetación NDVI, promedios mensuales y
también promedios del mismo mes para los distintos años. A partir de la comparación del NDVI mensual del
año en curso con el promedio de la serie de temporal para el mes correspondiente se detectan las zonas con
diferencias importantes de estado de la vegetación. Las zonas con NDVI promedio más bajos para el año en
curso respecto a la serie temporal, son zonas con riesgo de ignición más elevado que el resto de zonas.The risk of forest fires can be summarized in two main factors: the risk of ignition and the probability of fire
spreading and causing a forest fires. The risk of ignition may be due to different causes: human factor and the
state of vegetation. The probability of expansion is mainly due to: weather, geography, vegetation
characteristics and ease of extinction.
The main aim of this study is to obtain an index of risk of ignition due to the state of vegetation. To meet this
objective a series of 8 years of images AVHRR (NOAA) has been developed. From daily images we have
calculated vegetation index NDVI, monthly averages and averages of the same month for different years. From
the comparison of monthly NDVI for current month with the average time series for the corresponding month,
in order to detect areas with significant differences in the state of the vegetation. Areas with lower average
NDVI for the current year with respect to the time series, are areas with higher fire risks than other areas
Energy radiation of moving cracks
The energy radiated by moving cracks in a discrete background is analyzed.
The energy flow through a given surface is expressed in terms of a generalized
Poynting vector. The velocity of the crack is determined by the radiation by
the crack tip. The radiation becomes more isotropic as the crack velocity
approaches the instability threshold.Comment: 7 pages, embedded figure
Mean Field Theory of Sandpile Avalanches: from the Intermittent to the Continuous Flow Regime
We model the dynamics of avalanches in granular assemblies in partly filled
rotating cylinders using a mean-field approach. We show that, upon varying the
cylinder angular velocity , the system undergoes a hysteresis cycle
between an intermittent and a continuous flow regimes. In the intermittent flow
regime, and approaching the transition, the avalanche duration exhibits
critical slowing down with a temporal power-law divergence. Upon adding a white
noise term, and close to the transition, the distribution of avalanche
durations is also a power-law. The hysteresis, as well as the statistics of
avalanche durations, are in good qualitative agreement with recent experiments
in partly filled rotating cylinders.Comment: 4 pages, RevTeX 3.0, postscript figures 1, 3 and 4 appended
On the driven Frenkel-Kontorova model: II. Chaotic sliding and nonequilibrium melting and freezing
The dynamical behavior of a weakly damped harmonic chain in a spatially
periodic potential (Frenkel-Kontorova model) under the subject of an external
force is investigated. We show that the chain can be in a spatio-temporally
chaotic state called fluid-sliding state. This is proven by calculating
correlation functions and Lyapunov spectra. An effective temperature is
attributed to the fluid-sliding state. Even though the velocity fluctuations
are Gaussian distributed, the fluid-sliding state is clearly not in equilibrium
because the equipartition theorem is violated. We also study the transition
between frozen states (stationary solutions) and=7F molten states
(fluid-sliding states). The transition is similar to a first-order phase
transition, and it shows hysteresis. The depinning-pinning transition
(freezing) is a nucleation process. The frozen state contains usually two
domains of different particle densities. The pinning-depinning transition
(melting) is caused by saddle-node bifurcations of the stationary states. It
depends on the history. Melting is accompanied by precursors, called
micro-slips, which reconfigurate the chain locally. Even though we investigate
the dynamics at zero temperature, the behavior of the Frenkel-Kontorova model
is qualitatively similar to the behavior of similar models at nonzero
temperature.Comment: Written in RevTeX, 13 figures in PostScript, appears in PR
Steady-State Cracks in Viscoelastic Lattice Models II
We present the analytic solution of the Mode III steady-state crack in a
square lattice with piecewise linear springs and Kelvin viscosity. We show how
the results simplify in the limit of large width. We relate our results to a
model where the continuum limit is taken only along the crack direction. We
present results for small velocity, and for large viscosity, and discuss the
structure of the critical bifurcation for small velocity. We compute the size
of the process zone wherein standard continuum elasticity theory breaks down.Comment: 17 pages, 3 figure
- …