172 research outputs found
Redefining the role of obstacles in pedestrian evacuation
The placement of obstacles in front of doors is believed to be an effective strategy to increase the flow of pedestrians, hence improving the evacuation process. Since it was first suggested, this counterintuitive feature is considered a hallmark of pedestrian flows through bottlenecks. Indeed, despite the little experimental evidence, the placement of an obstacle has been hailed as the panacea for solving evacuation problems. In this work, we challenge this idea and experimentally demonstrate that the pedestrians flow rate is not necessarily altered by the presence of an obstacle. This result - which is at odds with recent demonstrations on its suitability for the cases of granular media, sheep and mice - differs from the outcomes of most of existing numerical models, and warns about the risks of carelessly extrapolating animal behaviour to humans. Our experimental findings also reveal an unnoticed phenomenon in relation with the crowd movement in front of the exit: in competitive evacuations, an obstacle attenuates the development of collective transversal rushes, which are hazardous as they might cause falls.Fil: Garcimartín, A.. Universidad de Navarra; EspañaFil: Maza, D.. Universidad de Navarra; EspañaFil: Pastor, J. M.. Focke Meler Gluing Solutions S.A.; EspañaFil: Parisi, Daniel Ricardo. Instituto Tecnológico de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Martín Gómez, C.. Universidad de Navarra; EspañaFil: Zuriguel, I.. Universidad de Navarra; Españ
The shape of jamming arches in two-dimensional deposits of granular materials
We present experimental results on the shape of arches that block the outlet
of a two dimensional silo. For a range of outlet sizes, we measure some
properties of the arches such as the number of particles involved, the span,
the aspect ratio, and the angles between mutually stabilizing particles. These
measurements shed light on the role of frictional tangential forces in arching.
In addition, we find that arches tend to adopt an aspect ratio (the quotient
between height and half the span) close to one, suggesting an isotropic load.
The comparison of the experimental results with data from numerical models of
the arches formed in the bulk of a granular column reveals the similarities of
both, as well as some limitations in the few existing models.Comment: 8 pages; submitted to Physical Review
Jamming during the discharge of granular matter from a silo
In this work we present an experimental study of the jamming that stops the
free flow of grains from a silo discharging by gravity. When the outlet size is
not much bigger than the beads, granular material jams the outlet of the
container due to the formation of an arch. Statistical data from the number of
grains fallen between consecutive jams are presented. The information that they
provide can help to understand the jamming phenomenon. As the ratio between the
size of the orifice and the size of the beads is increased, the probability
that an arch blocks the outlet decreases. We show here that there is a power
law divergence of the mean avalanche size for a finite critical radius. Beyond
this critical radius no jamming can occur and the flow is never stopped. The
dependence of the arch formation on the shape and the material of the grains
has been explored. It has been found that the material properties of the grains
do not affect the arch formation probability. On the contrary, the shape of the
grains deeply influences it. A simple model to interpret the results is also
discussed.Comment: Submitted to Phys. Rev.
Time resolved particle dynamics in granular convection
We present an experimental study of the movement of individual particles in a
layer of vertically shaken granular material. High-speed imaging allows us to
investigate the motion of beads within one vibration period. This motion
consists mainly of vertical jumps, and a global ordered drift. The analysis of
the system movement as a whole reveals that the observed bifurcation in the
flight time is not adequately described by the Inelastic Bouncing Ball Model.
Near the bifurcation point, friction plays and important role, and the branches
of the bifurcation do not diverge as the control parameter is increased. We
quantify the friction of the beads against the walls, showing that this
interaction is the underlying mechanism responsible for the dynamics of the
flow observed near the lateral wall
Detection of zooplankton predator-prey interactions in Alboran Sea by combining acoustic backscatter data and different sampling systems
Multifrequency acoustic technology was used to detect the summer epipelagic scattering layer (SESL) associated with a heterogenic zooplankton community in Alboran Sea. Simultaneously, identification hauls by means of two different plankton net (bongo 40 equipped with 250 and 333 μm mesh and bongo 90 equipped with 500 and 2000 μm mesh) were performed to obtain taxonomical and quantitative characteristics of the zooplankton community. Collected data revealed a strong correlation between
the abundance of small crustacean (prey) captured by the 250 μm mesh with the abundance of chaetognaths, siphonophores and fish larvae (predators) captured by 500 μm mesh
Failure time and microcrack nucleation
The failure time of samples of heterogeneous materials (wood, fiberglass) is
studied as a function of the applied stress. It is shown that in these
materials the failure time is predicted with a good accuracy by a model of
microcrack nucleation proposed by Pomeau. It is also shown that the crack
growth process presents critical features when the failure time is approached.Comment: 13 pages, 4 figures, submitted to Europhysics Letter
THE ROLE OF CAPITAL FLOWS IN EXTERNAL CONSTRAINED GROWTH: THE EXPERIENCES OF SPAIN AND PORTUGAL IN THE EUROZONE
Abstract Broadly speaking, the balance-of-payments constraint hypothesis has been empirically supported. Yet, it shows some shortcomings highlighted in the literature. In our opinion, two of them must be analysed. First, temporary disequilibria and capital flows must be incorporated into the balance-ofpayments constrained growth models. Second, the role of relative prices must be made explicit, since it can be relevant even in an external constraint framework. This study is aimed at developing a model that incorporates both possibilities: temporary external disequilibria and the impact of relative prices. This model is subsequently used to analyse the evolution of the Spanish and Portuguese economies in last decades, and, in particular, the different paths shown by both countries since their accession to the Eurozone. JEL: E12, F41, F4
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