65 research outputs found
Zone clearance in an infinite TASEP with a step initial condition
The TASEP is a paradigmatic model of out-of-equilibrium statistical physics,
for which many quantities have been computed, either exactly or by approximate
methods. In this work we study two new kinds of observables that have some
relevance in biological or traffic models. They represent the probability for a
given clearance zone of the lattice to be empty (for the first time) at a given
time, starting from a step density profile. Exact expressions are obtained for
single-time quantities, while more involved history-dependent observables are
studied by Monte Carlo simulation, and partially predicted by a
phenomenological approach
Environmental control of microtubule-based bidirectional cargo-transport
Inside cells, various cargos are transported by teams of molecular motors.
Intriguingly, the motors involved generally have opposite pulling directions,
and the resulting cargo dynamics is a biased stochastic motion. It is an open
question how the cell can control this bias. Here we develop a model which
takes explicitly into account the elastic coupling of the cargo with each
motor. We show that bias can be simply controlled or even reversed in a
counterintuitive manner via a change in the external force exerted on the cargo
or a variation of the ATP binding rate to motors. Furthermore, the
superdiffusive behavior found at short time scales indicates the emergence of
motor cooperation induced by cargo-mediated coupling
Two dimensional outflows for cellular automata with shuffle updates
In this paper, we explore the two-dimensional behavior of cellular automata
with shuffle updates. As a test case, we consider the evacuation of a square
room by pedestrians modeled by a cellular automaton model with a static floor
field. Shuffle updates are characterized by a variable associated to each
particle and called phase, that can be interpreted as the phase in the step
cycle in the frame of pedestrian flows. Here we also introduce a dynamics for
these phases, in order to modify the properties of the model. We investigate in
particular the crossover between low- and high-density regimes that occurs when
the density of pedestrians increases, the dependency of the outflow in the
strength of the floor field, and the shape of the queue in front of the exit.
Eventually we discuss the relevance of these results for pedestrians.Comment: 20 pages, 5 figures. v2: 16 pages, 5 figures; changed the title,
abstract and structure of the paper. v3: minor change
Fluctuation effects in bidirectional cargo transport
We discuss a theoretical model for bidirectional cargo transport in
biological cells, which is driven by teams of molecular motors and subject to
thermal fluctuations. The model describes explicitly the directed motion of the
molecular motors on the filament. The motor-cargo coupling is implemented via
linear springs. By means of extensive Monte Carlo simulations we show that the
model describes the experimentally observed regimes of anomalous diffusion,
i.e. subdiffusive behavior at short times followed by superdiffusion at
intermediate times. The model results indicate that subdiffuse regime is
induced by thermal fluctuations while the superdiffusive motion is generated by
correlations of the motors' activity. We also tested the efficiency of
bidirectional cargo transport in crowded areas by measuring its ability to pass
barriers with increased viscosity. Our results show a remarkable gain of
efficiency for high viscosities.Comment: 10 pages, 6 figure
Des embouteillages Ă toutes les Ă©chelles
National audienceNous avons tous fait l'expérience des embouteillages, que ce soit au volant de notre voiture, ou aux heures de pointe sur les quais du métro. Expérience souvent agaçante ou tragique selon l'urgence du rendezvous qui va suivre. On peut se demander pourquoi nos réseaux de transport n'ont toujours pas convergé vers une configuration qui nous assurerait un écoulement fluide en toutes circonstances. Est-il au moins possible de comprendre pourquoi les embouteillages sont notre lot quotidien ? Il s'agit d'un phénomène qui se retrouve à toutes les échelles, et nous verrons dans cet article qu'il existe dans chacune de nos cellules des marcheurs moléculaires qui eux aussi, en cas de maladies neurodégénératives, peuvent se retrouver coincés dans des embouteillages qui compromettent la vie de nos neurones. Réciproquement, nous pouvons nous émerveiller de ce que la cellule saine sache réguler un transport autrement plus complexe que celui auquel nous sommes confrontés en tant que piétons ou conducteurs
A bottleneck model for bidirectional transport controlled by fluctuations
We introduce a new model to study the oscillations of opposite flows sharing
a common bottleneck and moving on two Totally Asymmetric Simple Exclusion
Process (TASEP) lanes. We provide a theoretical analysis of the phase diagram,
valid when the flow in the bottleneck is dominated by local stationary states.
In particular, we predict and find an inhomogeneous high density phase, with a
striped spatio-temporal structure. At the same time, our results also show that
some other features of the model cannot be explained by the stationarity
hypothesis and require consideration of the transients in the bottleneck at
each reversal of the flow. In particular, we show that for short bottlenecks,
the capacity of the system is at least as high as for uni-directional flow, in
spite of having to empty the bottleneck at each reversal - a feature that can
be explained only by efficient transients. Looking at more sensitive quantities
like the distribution of flipping times, we show that, in most regimes, the
bottleneck is driven by rare fluctuations and descriptions beyond the
stationary state are required.Comment: 6 pages, 7 figures, accepted for publication in EPL (2012
Properties of pedestrians walking in line without density constraint
This article deals with the study of pedestrian behaviour in one-dimensional
traffic situations. We asked participants to walk either in a straight line
with a fast or slow leader, or to form a circle, without ever forcing the
conditions of density. While the observed density results from individual
decisions in the line case, both density and velocity have to be collectively
chosen in the case of circle formation. In the latter case, interestingly, one
finds that the resulting velocity is very stable among realizations, as if
collective decision was playing the role of an average. In the line experiment,
though participants could choose comfortable headways, they rather stick to
short headways requiring a faster adaption - a fact that could come from a
``social pressure from behind''. For flows close to the jamming transition, the
same operating point is chosen as in previous experiments where it was not
velocity but density that was imposed. All these results show that the walking
values preferred by humans in following tasks depend on more factors than
previously considered.Comment: Main paper (11 pages, 13 figures) + Suppl. Mat. (8 pages, 9 figures
Properties of pedestrians walking in line: Stepping behavior
In human crowds, interactions among individuals give rise to a variety of
self-organized collective motions that help the group to effectively solve the
problem of coordination. However, it is still not known exactly how humans
adjust their behavior locally, nor what are the direct consequences on the
emergent organization. One of the underlying mechanisms of adjusting individual
motions is the stepping dynamics. In this paper, we present first quantitative
analysis on the stepping behavior in a one-dimensional pedestrian flow studied
under controlled laboratory conditions. We find that the step length is
proportional to the velocity of the pedestrian, and is directly related to the
space available in front of him, while the variations of the step duration are
much smaller. This is in contrast with locomotion studies performed on isolated
pedestrians and shows that the local density has a direct influence on the
stepping characteristics. Furthermore, we study the phenomena of
synchronization -walking in lockstep- and show its dependence on flow
densities. We show that the synchronization of steps is particularly important
at high densities, which has direct impact on the studies of optimizing
pedestrians flow in congested situations. However, small synchronization and
antisynchronization effects are found also at very low densities, for which no
steric constraints exist between successive pedestrians, showing the natural
tendency to synchronize according to perceived visual signals.Comment: 8 pages, 5 figure
Properties of pedestrians walking in line - Fundamental diagrams
We present experimental results obtained for a one-dimensional flow using
high precision motion capture. The full pedestrians' trajectories are obtained.
In this paper, we focus on the fundamental diagram, and on the relation between
the instantaneous velocity and spatial headway (distance to the predecessor).
While the latter was found to be linear in previous experiments, we show that
it is rather a piecewise linear behavior which is found if larger density
ranges are covered. Indeed, our data clearly exhibits three distinct regimes in
the behavior of pedestrians that follow each other. The transitions between
these regimes occur at spatial headways of about 1.1 and 3 m, respectively.
This finding could be useful for future modeling.Comment: 9 figures, 3 table
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