8,312 research outputs found
Jamming transition of a granular pile below the angle of repose
We study experimentally the relaxation towards mechanical equilibrium of a
granular pile which has just experienced an avalanche and discuss it in the
more general context of the granular jamming transition. Two coexisting
dynamics are observed in the surface layer: a short time exponential decay
consisting in rapid and independent moves of grains and intermittent bursts
consisting in spatially correlated moves lasting for longer time. The
competition of both dynamics results in long-lived intermittent transients, the
total duration of which can late more than a thousand of seconds. We measure a
two-time relaxation function, and relate it via a simple statistical model to a
more usual two-time correlation function which exhibits strong similarities
with auto-correlation functions found in aging systems. Localized perturbation
experiments also allow us to test the pile surface layer receptivity.Comment: 9 pages, 10 figure
Correlated bursts and the role of memory range
Inhomogeneous temporal processes in natural and social phenomena have been
described by bursts that are rapidly occurring events within short time periods
alternating with long periods of low activity. In addition to the analysis of
heavy-tailed inter-event time distributions, higher-order correlations between
inter-event times, called correlated bursts, have been studied only recently.
As the possible mechanisms underlying such correlated bursts are far from being
fully understood, we devise a simple model for correlated bursts by using a
self-exciting point process with variable memory range. Here the probability
that a new event occurs is determined by a memory function that is the sum of
decaying memories of the past events. In order to incorporate the noise and/or
limited memory capacity of systems, we apply two memory loss mechanisms, namely
either fixed number or variable number of memories. By using theoretical
analysis and numerical simulations we find that excessive amount of memory
effect may lead to a Poissonian process, which implies that for memory effect
there exists an intermediate range that will generate correlated bursts of
magnitude comparable to empirical findings. Hence our results provide deeper
understanding of how long-range memory affects correlated bursts.Comment: 9 pages, 7 figure
The Contagion Effects of Repeated Activation in Social Networks
Demonstrations, protests, riots, and shifts in public opinion respond to the
coordinating potential of communication networks. Digital technologies have
turned interpersonal networks into massive, pervasive structures that
constantly pulsate with information. Here, we propose a model that aims to
analyze the contagion dynamics that emerge in networks when repeated activation
is allowed, that is, when actors can engage recurrently in a collective effort.
We analyze how the structure of communication networks impacts on the ability
to coordinate actors, and we identify the conditions under which large-scale
coordination is more likely to emerge.Comment: Submitted for publicatio
Astrobiological Complexity with Probabilistic Cellular Automata
Search for extraterrestrial life and intelligence constitutes one of the
major endeavors in science, but has yet been quantitatively modeled only rarely
and in a cursory and superficial fashion. We argue that probabilistic cellular
automata (PCA) represent the best quantitative framework for modeling
astrobiological history of the Milky Way and its Galactic Habitable Zone. The
relevant astrobiological parameters are to be modeled as the elements of the
input probability matrix for the PCA kernel. With the underlying simplicity of
the cellular automata constructs, this approach enables a quick analysis of
large and ambiguous input parameters' space. We perform a simple clustering
analysis of typical astrobiological histories and discuss the relevant boundary
conditions of practical importance for planning and guiding actual empirical
astrobiological and SETI projects. In addition to showing how the present
framework is adaptable to more complex situations and updated observational
databases from current and near-future space missions, we demonstrate how
numerical results could offer a cautious rationale for continuation of
practical SETI searches.Comment: 37 pages, 11 figures, 2 tables; added journal reference belo
Current challenges for preseismic electromagnetic emissions: shedding light from micro-scale plastic flow, granular packings, phase transitions and self-affinity notion of fracture process
Are there credible electromagnetic (EM) EQ precursors? This a question
debated in the scientific community and there may be legitimate reasons for the
critical views. The negative view concerning the existence of EM precursors is
enhanced by features that accompany their observation which are considered as
paradox ones, namely, these signals: (i) are not observed at the time of EQs
occurrence and during the aftershock period, (ii) are not accompanied by large
precursory strain changes, (iii) are not accompanied by simultaneous geodetic
or seismological precursors and (v) their traceability is considered
problematic. In this work, the detected candidate EM precursors are studied
through a shift in thinking towards the basic science findings relative to
granular packings, micron-scale plastic flow, interface depinning, fracture
size effects, concepts drawn from phase transitions, self-affine notion of
fracture and faulting process, universal features of fracture surfaces, recent
high quality laboratory studies, theoretical models and numerical simulations.
Strict criteria are established for the definition of an emerged EM anomaly as
a preseismic one, while, precursory EM features, which have been considered as
paradoxes, are explained. A three-stage model for EQ generation by means of
preseismic fracture-induced EM emissions is proposed. The claim that the
observed EM precursors may permit a real-time and step-by-step monitoring of
the EQ generation is tested
Micro-plasticity and intermittent dislocation activity in a simplified micro structural model
Here we present a model to study the micro-plastic regime of a stress-strain
curve. In this model an explicit dislocation population represents the mobile
dislocation content and an internal shear-stress field represents a mean-field
description of the immobile dislocation content. The mobile dislocations are
constrained to a simple dipolar mat geometry and modelled via a dislocation
dynamics algorithm, whilst the shear-stress field is chosen to be a sinusoidal
function of distance along the mat direction. The latter, defined by a periodic
length and a shear-stress amplitude, represents a pre-existing micro-structure.
These model parameters, along with the mobile dislocation density, are found to
admit a diversity of micro-plastic behaviour involving intermittent plasticity
in the form of a scale-free avalanche phenomenon, with an exponent for the
strain burst magnitude distribution similar to those seen in experiment and
more complex dislocation dynamics simulations.Comment: 30 pages, 12 figures, to appear in "Modelling and Simulation in
Materials Science and Engineering
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