406 research outputs found
Invited review: Clogging of granular materials in bottlenecks
During the past decades, notable improvements have been achieved in the
understanding of static and dynamic properties of granular materials, giving
rise to appealing new concepts like jamming, force chains, non-local rheology
or the inertial number. The `saltcellar' can be seen as a canonical example of
the characteristic features displayed by granular materials: an apparently
smooth flow is interrupted by the formation of a mesoscopic structure (arch)
above the outlet that causes a quick dissipation of all the kinetic energy
within the system. In this manuscript, I will give an overview of this field
paying special attention to the features of statistical distributions appearing
in the clogging and unclogging processes. These distributions are essential to
understand the problem and allow subsequent study of topics such as the
influence of particle shape, the structure of the clogging arches and the
possible existence of a critical outlet size above which the outpouring will
never stop. I shall finally offer some hints about general ideas that can be
explored in the next few years.Comment: 13 pages, 7 figure
Role of vibrations in the jamming and unjamming of grains discharging from a silo
We present experimental results of the jamming of non-cohesive particles
discharged from a flat bottomed silo subjected to vertical vibration. When the
exit orifice is only a few grain diameter wide, the flow can be arrested due to
the formation of blocking arches. Hence, an external excitation is needed to
resume the flow. The use of a continuous gentle vibration is a usual technique
to ease the flow in such situations. Even though jamming is less frequent, it
is still an issue in vibrated silos. There are, in principle, two possible
mechanisms through which vibrations may facilitate the flow: (i) a decrease in
the probability of the formation of blocking arches, and (ii) the breakage of
blocking arches once they have been formed. By measuring the time intervals
inside an avalanche during which no particles flow through the outlet, we are
able to estimate the probability of breaking a blocking arch by vibrations. The
result agrees with the prediction of a bivariate probabilistic model in which
the formation of blocking arches is equally probable in vibrated and
non-vibrated silos. This indicates that the second aforementioned mechanism is
the main responsible for improving the flowability in gently vibrated silos
Multifractal intermittency in granular flow through bottlenecks
We experimentally analyze the intermittent nature of granular silo flow when the discharge is controlled by an extracting belt at the bottom. We discover the existence of four different scenarios. For low extraction rates, the system is characterized by an on-off intermittency. When the extraction rate is increased the structure functions of the grains velocity increments, calculated for different lag times, reveal the emergence of multifractal intermittency. Finally, for very high extraction rates that approach the purely gravitational discharge, we observe that the dynamics become dependent on the outlet size. For large orifices the behavior is monofractal, whereas for small ones, the fluctuations of the velocity increments deviate from Gaussianity even for very large time lags
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
Breaking arches with vibrations: the role of defects
We present experimental results about the stability of arches against
external vibrations. Two dimensional strings of mutually stabilizing grains are
geometrically analyzed and subsequently submitted to a periodic forcing at
fixed frequency and increasing amplitude. The main factor that determines the
granular arch resistance against vibrations is the maximum angle among those
formed between any particle of the arch and its two neighbors: the higher the
maximum angle is, the easier to break the arch. Based in an analysis of the
forces, a simple explanation is given for this dependence. From this,
interesting information can be extracted about the expected magnitudes of
normal forces and friction coefficients of the particles conforming the arches
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