13 research outputs found
Nonsymmetric Interactions Trigger Collective Swings in Globally Ordered Systems
Many systems in nature, from ferromagnets to flocks of birds, exhibit ordering phenomena on the large scale. In condensed matter systems, order is statistically robust for large enough dimensions, with relative fluctuations due to noise vanishing with system size. Several biological systems, however, are less stable and spontaneously change their global state on relatively short time scales. Here we show that there are two crucial ingredients in these systems that enhance the effect of noise, leading to collective changes of state on finite time scales and off-equilibrium behavior: the nonsymmetric nature of interactions between individuals, and the presence of local heterogeneities in the topology of the network. Our results might explain what is observed in several living systems and are consistent with recent experimental data on bird flocks and other animal groups
Silent Flocks
Experiments find coherent information transfer through biological groups on
length and time scales distinctly below those on which asymptotically correct
hydrodynamic theories apply. We present here a new continuum theory of
collective motion coupling the velocity and density fields of Toner and Tu to
the inertial spin field recently introduced to describe information propagation
in natural flocks of birds. The long-wavelength limit of the new equations
reproduces Toner-Tu theory, while at shorter wavelengths (or, equivalently,
smaller damping), spin fluctuations dominate over density fluctuations and
second sound propagation of the kind observed in real flocks emerges. We study
the dispersion relation of the new theory and find that when the speed of
second sound is large, a gap sharply separates first from second sound modes.
This gap implies the existence of `silent' flocks, namely medium-sized systems
across which neither first nor second sound can propagate
Flocking and turning: a new model for self-organized collective motion
Birds in a flock move in a correlated way, resulting in large polarization of
velocities. A good understanding of this collective behavior exists for linear
motion of the flock. Yet observing actual birds, the center of mass of the
group often turns giving rise to more complicated dynamics, still keeping
strong polarization of the flock. Here we propose novel dynamical equations for
the collective motion of polarized animal groups that account for correlated
turning including solely social forces. We exploit rotational symmetries and
conservation laws of the problem to formulate a theory in terms of generalized
coordinates of motion for the velocity directions akin to a Hamiltonian
formulation for rotations. We explicitly derive the correspondence between this
formulation and the dynamics of the individual velocities, thus obtaining a new
model of collective motion. In the appropriate overdamped limit we recover the
well-known Vicsek model, which dissipates rotational information and does not
allow for polarized turns. Although the new model has its most vivid success in
describing turning groups, its dynamics is intrinsically different from
previous ones in a wide dynamical regime, while reducing to the hydrodynamic
description of Toner and Tu at very large length-scales. The derived framework
is therefore general and it may describe the collective motion of any strongly
polarized active matter system.Comment: Accepted for the Special Issue of the Journal of Statistical Physics:
Collective Behavior in Biological Systems, 17 pages, 4 figures, 3 video
Time-delayed Follow-the-Leader model for pedestrians walking in line
International audienceWe use the results of a pedestrian tracking experiment to identify a follow-the-leader model for pedestrians walking-in-line. We demonstrate the existence of a time-delay between a subject's response and the predecessor's corresponding behavior. This time-delay induces an instability which can be damped out by a suitable relaxation. By comparisons with the experimental data, we show that the model reproduces well the emergence of large-scale structures such as congestions waves. The resulting model can be used either for modeling pedestrian queuing behavior or can be incorporated into bi-dimensional models of pedestrian traffic. Acknowledgements: This work has been supported by the french 'Agence Nationale pour la Recherche (ANR)' in the frame of the contract "Pedigree" (ANR-08-SYSC-015-01). JH acknowledges support of the ANR and the Institut de Mathématiques de Toulouse, where he conducted this research. AJ acknowledges support of the ANR and of the Laboratoire de physique t A c orique in Orsay where she conducted this research. PD is on leave from CNRS, Institut de Mat A c matiques de Toulouse, France
GReTA - a novel Global and Recursive Tracking Algorithm in three dimensions
Tracking multiple moving targets allows quantitative measure of the dynamic
behavior in systems as diverse as animal groups in biology, turbulence in fluid
dynamics and crowd and traffic control. In three dimensions, tracking several
targets becomes increasingly hard since optical occlusions are very likely,
i.e. two featureless targets frequently overlap for several frames. Occlusions
are particularly frequent in biological groups such as bird flocks, fish
schools, and insect swarms, a fact that has severely limited collective animal
behavior field studies in the past. This paper presents a 3D tracking method
that is robust in the case of severe occlusions. To ensure robustness, we adopt
a global optimization approach that works on all objects and frames at once. To
achieve practicality and scalability, we employ a divide and conquer
formulation, thanks to which the computational complexity of the problem is
reduced by orders of magnitude. We tested our algorithm with synthetic data,
with experimental data of bird flocks and insect swarms and with public
benchmark datasets, and show that our system yields high quality trajectories
for hundreds of moving targets with severe overlap. The results obtained on
very heterogeneous data show the potential applicability of our method to the
most diverse experimental situations.Comment: 13 pages, 6 figures, 3 tables. Version 3 was slightly shortened, and
new comprative results on the public datasets (thermal infrared videos of
flying bats) by Z. Wu and coworkers (2014) were included. in A. Attanasi et
al., "GReTA - A Novel Global and Recursive Tracking Algorithm in Three
Dimensions", IEEE Trans. Pattern Anal. Mach. Intell., vol.37 (2015
Un modèle de suivi réaliste pour la simulation de foules
International audienc
Experimental study of the following dynamics of pedestrians
International audienceWe report some experimental study of the behavior of pedestrians when they follow each other. In the frame of the PEDIGREE project, trajectories of pedestrians walking along a one-dimensional path were tracked through a high-precision motion capture. Data analysis allowed to obtain the fundamental diagram at different scales. Two unexpected transitions in the way pedestrians follow each other have been evidenced. The interest of the experiment is to capture at the same time microscopic and macroscopic characteristics of the flow. Indeed, macroscopic structures such as stop-and-go waves can also be studied from the data. Eventually, a data-based following model has been proposed. Its calibration/validation can be performed both at the microscopic or macroscopic level. It is possible to extend the model to quasi-one-dimensional flows for the modeling of pedestrian flows in corridors