7 research outputs found
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 - 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
Bridging length and time scales in sheared demixing systems: from the Cahn-Hilliard to the Doi-Ohta model
We develop a systematic coarse-graining procedure which establishes the
connection between models of mixtures of immiscible fluids at different length
and time scales. We start from the Cahn-Hilliard model of spinodal
decomposition in a binary fluid mixture under flow from which we derive the
coarse-grained description. The crucial step in this procedure is to identify
the relevant coarse-grained variables and find the appropriate mapping which
expresses them in terms of the more microscopic variables. In order to capture
the physics of the Doi-Ohta level, we introduce the interfacial width as an
additional variable at that level. In this way, we account for the stretching
of the interface under flow and derive analytically the convective behavior of
the relevant coarse-grained variables, which in the long wavelength limit
recovers the familiar phenomenological Doi-Ohta model. In addition, we obtain
the expression for the interfacial tension in terms of the Cahn-Hilliard
parameters as a direct result of the developed coarse-graining procedure.
Finally, by analyzing the numerical results obtained from the simulations on
the Cahn-Hilliard level, we discuss that dissipative processes at the Doi-Ohta
level are of the same origin as in the Cahn-Hilliard model. The way to estimate
the interface relaxation times of the Doi-Ohta model from the underlying
morphology dynamics simulated at the Cahn-Hilliard level is established.Comment: 29 pages, 2 figures, accepted for publication in Phys. Rev.
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
Superfluid transport of information in turning flocks of starlings
Collective decision-making in biological systems requires all individuals in
the group to go through a behavioural change of state. During this transition,
the efficiency of information transport is a key factor to prevent cohesion
loss and preserve robustness. The precise mechanism by which natural groups
achieve such efficiency, though, is currently not fully understood. Here, we
present an experimental study of starling flocks performing collective turns in
the field. We find that the information to change direction propagates across
the flock linearly in time with negligible attenuation, hence keeping group
decoherence to a minimum. This result contrasts with current theories of
collective motion, which predict a slower and dissipative transport of
directional information. We propose a novel theory whose cornerstone is the
existence of a conserved spin current generated by the gauge symmetry of the
system. The theory turns out to be mathematically identical to that of
superfluid transport in liquid helium and it explains the dissipationless
propagating mode observed in turning flocks. Superfluidity also provides a
quantitative expression for the speed of propagation of the information,
according to which transport must be swifter the stronger the group's
orientational order. This prediction is verified by the data. We argue that the
link between strong order and efficient decision-making required by
superfluidity may be the adaptive drive for the high degree of behavioural
polarization observed in many living groups. The mathematical equivalence
between superfluid liquids and turning flocks is a compelling demonstration of
the far-reaching consequences of symmetry and conservation laws across
different natural systems
Properties of pedestrians walking in line: Stepping behavior
7 pages, 5 figuresInternational audienceIn 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 how exactly are the humans adjusting 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 weaker. Furthermore, we study the phenomena of synchronization --walking in lock-steps-- and show its dependence on the 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, some small synchronization and antisynchronization effects are found even at very low densities, for which no steric constraints exist between successive pedestrians, showing the natural tendency to synchronize according to the perceived visual signals