286 research outputs found
Flow of magnetic repelling grains in a two-dimensional silo
During a typical silo discharge, the material flow rate is determined by the
contact forces between the grains. Here, we report an original study concerning
the discharge of a two-dimensional silo filled with repelling magnetic grains.
This non-contact interaction leads to a different dynamics from the one
observed with conventional granular materials. We found that, although the flow
rate dependence on the aperture size follows roughly the power-law with an
exponent found in non-repulsive systems, the density and velocity
profiles during the discharge are totally different. New phenomena must be
taken into account. Despite the absence of contacts, clogging and intermittence
were also observed for apertures smaller than a critical size determined by the
effective radius of the repulsive grains.Comment: 6 pages, 8 figure
Forecasting characteristic earthquakes in a minimalist model
International audienceUsing error diagrams, we quantify the forecasting of characteristic-earthquake occurrence in a recently introduced minimalist model. Initially we connect the earthquake alarm at a fixed time after the ocurrence of a characteristic event. The evaluation of this strategy leads to a one-dimensional numerical exploration of the loss function. This first strategy is then refined by considering a classification of the seismic cycles of the model according to the presence, or not, of some factors related to the seismicity observed in the cycle. These factors, statistically speaking, enlarge or shorten the length of the cycles. The independent evaluation of the impact of these factors in the forecast process leads to two-dimensional numerical explorations. Finally, and as a third gradual step in the process of refinement, we combine these factors leading to a three-dimensional exploration. The final improvement in the loss function is about 8.5%
Using synchronization to improve earthquake forecasting in a cellular automaton model
A new forecasting strategy for stochastic systems is introduced. It is
inspired by the concept of anticipated synchronization between pairs of chaotic
oscillators, recently developed in the area of Dynamical Systems, and by the
earthquake forecasting algorithms in which different pattern recognition
functions are used for identifying seismic premonitory phenomena. In the new
strategy, copies (clones) of the original system (the master) are defined, and
they are driven using rules that tend to synchronize them with the master
dynamics. The observation of definite patterns in the state of the clones is
the signal for connecting an alarm in the original system that efficiently
marks the impending occurrence of a catastrophic event. The power of this
method is quantitatively illustrated by forecasting the occurrence of
characteristic earthquakes in the so-called Minimalist Model.Comment: 4 pages, 3 figure
Synchronization of Kuramoto Oscillators in Scale-Free Networks
In this work, we study the synchronization of coupled phase oscillators on
the underlying topology of scale-free networks. In particular, we assume that
each network's component is an oscillator and that each interacts with the
others following the Kuramoto model. We then study the onset of global phase
synchronization and fully characterize the system's dynamics. We also found
that the resynchronization time of a perturbed node decays as a power law of
its connectivity, providing a simple analytical explanation to this interesting
behavior.Comment: 7 pages and 4 eps figures, the text has been slightly modified and
new references have been included. Final version to appear in Europhysics
Letter
Multiple and Precessing Collimated Outflows in the Planetary Nebula IC 4634
With its remarkable double-S shape, IC 4634 is an archetype of
point-symmetric planetary nebulae (PN). In this paper, we present a detailed
study of this PN using archival HST WFPC2 and ground-based narrow-band images
to investigate its morphology, and long-slit spectroscopic observations to
determine its kinematics and to derive its physical conditions and excitation.
The data reveal new structural components, including a distant string of knots
distributed along an arc-like feature 40"-60" from the center of the nebula, a
skin of enhanced [O III]/H-alpha ratio enveloping the inner shell and the
double-S feature, and a triple-shell structure. The spatio-kinematical study
also finds an equatorial component of the main nebula that is kinematically
independent from the bright inner S-shaped arc. We have investigated in detail
the bow shock-like features in IC 4634 and found that their morphological,
kinematical and emission properties are consistent with the interaction of a
collimated outflow with surrounding material. Indeed, the morphology and
kinematics of some of these features can be interpreted using a 3D numerical
simulation of a collimated outflow precessing at a moderate, time-dependent
velocity. Apparently, IC 4634 has experienced several episodes of
point-symmetric ejections oriented at different directions with the outer
S-shaped feature being related to an earlier point-symmetric ejection and the
outermost arc-like string of knots being the relic of an even much earlier
point-symmetric ejection. There is tantalizing evidence that the action of
these collimated outflows has also taken part in the shaping of the innermost
shell and inner S-shaped arc of IC 4634.Comment: 16 pages, 11 figures, accepted for publication in The Astrophysical
Journa
Parámetros reproductivos y distribución geográfica potencial de las áreas de anidación de Grus canadensis nesiotes (Aves, Gruidae) en Cuba: implicaciones para su conservación
Grus canadensis nesiotes (grulla cubana) es una subespecie endémica de Cuba que se encuentra en peligro de extinción. A pesar de estar directamente relacionada con los humedales, no existen estudios que contribuyan a su gestión y conservación. Por ello, se registraron parámetros reproductivos de la subespecie durante ocho temporadas reproductivas entre 2005 y 2015 en un humedal de Cuba; asimismo, se modeló y caracterizó la distribución geográfica potencial de las áreas de anidación, y se analizó su representación dentro de las áreas protegidas (AP). Para elaborar el modelo, se utilizaron el algoritmo de máxima entropía y variables de hábitat (100 m de tamaño de píxel). Para caracterizar la distribución potencial, se calculó la superficie ocupada por cada uso de suelo y tipo de vegetación dentro de dicha área de distribución. Se empleó el mismo procedimiento para calcular la superficie de la distribución que está protegida. Se localizaron 151 nidos en herbazales de ciénaga. Se trataba de plataformas simples sobre suelo húmedo o agua; los más grandes se observaron en 2006. El 70% de los nidos tuvieron dos huevos (1,7 huevos/nido) y el 63,5% fueron exitosos con 1,6 polluelos/nido exitoso. El hábitat potencial de anidación es estrecho (242 km2) y se localiza en el centro del humedal. De la distribución prevista, la superficie con alta probabilidad de presencia es del 13,8%. El 60% del herbazal de ciénaga de la zona del estudio estaba comprendido dentro de la distribución potencial, mientras que la proporción de cultivos (1,2%) y pastizales (2,1%) era baja. Las AP gestionadas solo protegen el 39,1% de la distribución potencial de los sitios de anidación y el 12% de las zonas con alta probabilidad. Se proponen tres sitios prioritarios para estudiar la anidación de la subespecie y hacer un seguimiento de la misma. Las medidas de conservación de la subespecie deberían considerar la distribución geográfica potencial de los sitios de anidación dentro y fuera de las AP.Grus canadensis nesiotes (grulla cubana) es una subespecie endémica de Cuba que se encuentra en peligro de extinción. A pesar de estar directamente relacionada con los humedales, no existen estudios que contribuyan a su gestión y conservación. Por ello, se registraron parámetros reproductivos de la subespecie durante ocho temporadas reproductivas entre 2005 y 2015 en un humedal de Cuba; asimismo, se modeló y caracterizó la distribución geográfica potencial de las áreas de anidación, y se analizó su representación dentro de las áreas protegidas (AP). Para elaborar el modelo, se utilizaron el algoritmo de máxima entropía y variables de hábitat (100 m de tamaño de píxel). Para caracterizar la distribución potencial, se calculó la superficie ocupada por cada uso de suelo y tipo de vegetación dentro de dicha área de distribución. Se empleó el mismo procedimiento para calcular la superficie de la distribución que está protegida. Se localizaron 151 nidos en herbazales de ciénaga. Se trataba de plataformas simples sobre suelo húmedo o agua; los más grandes se observaron en 2006. El 70% de los nidos tuvieron dos huevos (1,7 huevos/nido) y el 63,5% fueron exitosos con 1,6 polluelos/nido exitoso. El hábitat potencial de anidación es estrecho (242 km2) y se localiza en el centro del humedal. De la distribución prevista, la superficie con alta probabilidad de presencia es del 13,8%. El 60% del herbazal de ciénaga de la zona del estudio estaba comprendido dentro de la distribución potencial, mientras que la proporción de cultivos (1,2%) y pastizales (2,1%) era baja. Las AP gestionadas solo protegen el 39,1% de la distribución potencial de los sitios de anidación y el 12% de las zonas con alta probabilidad. Se proponen tres sitios prioritarios para estudiar la anidación de la subespecie y hacer un seguimiento de la misma. Las medidas de conservación de la subespecie deberían considerar la distribución geográfica potencial de los sitios de anidación dentro y fuera de las AP.Reproductive parameters and potential geographical distribution of nesting areas of Grus canadensis nesiotes (Aves, Gruidae) in Cuba: conservation implications Grus canadensis nesiotes (Cuban sandhill crane) is an endemic and endangered subspecies from Cuba. Protection of wetland habitats is essential for survival of this species, but studies that could contribute to its management and conservation are lacking. In this study we recorded the reproductive parameters of Grus canadensis nesiotes in eight breeding seasons between 2005 and 2015 in a wetland of Cuba. We modeled and characterized the potential geographical distribution of the nesting areas, analyzing its representation within protected areas. Maximum entropy algorithm and habitat variables were used for modeling (100 m of pixel size). To characterize the potential distribution we calculated each land–use–vegetation within the potential distribution. We used the same procedure to determine the extent of the protected area. A hundred and fifty–one nests were located in marsh grasslands. These nests were simple platforms built on wet soil/water; the largest nests were observed in 2006. Seventy percent of nests had two eggs (1.7 eggs/nest) and 63.5% were successful with 1.6 chicks per successful nest. The potential nesting habitat is a narrow stretch (242 km2) located in the center of the wetland. The area with high probability of presence makes up 13.8% of the predicted distribution. Sixty percent of marsh grassland of the study area was included in the potential distribution, while the proportion of crops (1.2%) and pastures (2.1%) was low. Managed protected areas cover only 39.1% of the potential distribution of the nesting sites and 12% of the high probability areas. We propose three priority sites to study and monitor nesting of the subspecies. Species conservation actions should consider the potential geographical distribution of nesting sites both inside and outside the protected areas
Epidemic Incidence in Correlated Complex Networks
We introduce a numerical method to solve epidemic models on the underlying
topology of complex networks. The approach exploits the mean-field like rate
equations describing the system and allows to work with very large system
sizes, where Monte Carlo simulations are useless due to memory needs. We then
study the SIR epidemiological model on assortative networks, providing
numerical evidence of the absence of epidemic thresholds. Besides, the time
profiles of the populations are analyzed. Finally, we stress that the present
method would allow to solve arbitrary epidemic-like models provided that they
can be described by mean-field rate equations.Comment: 5 pages, 4 figures. Final version published in PR
Bounds for the time to failure of hierarchical systems of fracture
For years limited Monte Carlo simulations have led to the suspicion that the
time to failure of hierarchically organized load-transfer models of fracture is
non-zero for sets of infinite size. This fact could have a profound
significance in engineering practice and also in geophysics. Here, we develop
an exact algebraic iterative method to compute the successive time intervals
for individual breaking in systems of height in terms of the information
calculated in the previous height . As a byproduct of this method,
rigorous lower and higher bounds for the time to failure of very large systems
are easily obtained. The asymptotic behavior of the resulting lower bound leads
to the evidence that the above mentioned suspicion is actually true.Comment: Final version. To appear in Phys. Rev. E, Feb 199
Experimental and numerical investigation of the compression and expansion of a granular bed of repelling magnetic disks
peer reviewe
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