209 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
Análisis del proceso de descarga de un silo con un obstáculo cerca del orificio
Jamming is an important problem in numerous industrial processes, and in other situation such as traffic and evacuation. Some reports show that an obstacle placed before the exit may prevent jamming a pedestrian flow. However, this is a general hypothesis and there are still related questions that have not been fully addressed, mainly the dynamics of the system or the optimal position of the obstacle.
The present work aims at shedding some more light on these phenomena. We present an experimental work where we analyze systematically and under well controlled conditions, the macroscopic and microscopic processes involved during the discharge of a silo by gravity with an obstacle placed before an orifice. We fixed at the size of the orifice and change the position of the insert. In order to do that, we have designed a 2D silo with transparent walls which allowed visualization of the particles.
The first conclusion of this work is the existence of an optimal position of the obstacle where the jamming probability is drastically reduced. If the obstacle is far away from the orifice, it does not have any effect. When the obstacle is close to the orifice, the avalanche size is higher and the probability that a particle clogs the outlet decreases. We find that, if the insert position is properly selected, the probability that the granular flow gets jammed can be decreased by a factor of 100. This dramatic effect occurs without any remarkable modification of the flow rate or the packing fraction above the outlet. However, for low positions of the insert we saw that some particles in the region of arch formation can be displaced upwards. This phenomenon is less evident when the insert is at high positions. This effect could be related with the reduction of the clogging probability. So, we propose that the mechanism by which the insert prevents clogging is a reduction of the pressure exerted to the particles in the region of arch formation
Master curves for the stress tensor invariants in stationary states of static granular beds: implications for the thermodynamic phase space
We prepare static granular beds under gravity in different stationary states by tapping the system with pulsed excitations of controlled amplitude and duration. The macroscopic state-defined by the ensemble of static configurations explored by the system tap after tap-for a given tap intensity and duration is studied in terms of volume, V, and force moment tensor, Σ. In a previous paper [Pugnaloni et al., Phys. Rev. E 82, 050301(R) (2010)], we reported evidence supporting that such macroscopic states cannot be fully described by using only V or Σ, apart from the number of particles N. In this work, we present an analysis of the fluctuations of these variables that indicates that V and Σ may be sufficient to define the macroscopic states. Moreover, we show that only one of the invariants of Σ is necessary, since each component of Σ falls onto a master curve when plotted as a function of Tr(Σ). This implies that these granular assemblies have a common shape for the stress tensor, even though it does not correspond to the hydrostatic type. Although most results are obtained by molecular dynamics simulations, we present supporting experimental results.Facultad de Ciencias ExactasInstituto de Física de Líquidos y Sistemas Biológico
Master curves for the stress tensor invariants in stationary states of static granular beds. Implications for the thermodynamic phase space
We prepare static granular beds under gravity in different stationary states
by tapping the system with pulsed excitations of controlled amplitude and
duration. The macroscopic state---defined by the ensemble of static
configurations explored by the system tap after tap---for a given tap intensity
and duration is studied in terms of volume, V, and force moment tensor, \Sigma.
In a previous paper [Pugnaloni et al., Phys. Rev. E 82, 050301(R) (2010)], we
reported evidence supporting that such macroscopic states cannot be fully
described by using only V or \Sigma, apart from the number of particles N. In
this work, we present an analysis of the fluctuations of these variables that
indicates that V and \Sigma may be sufficient to define the macroscopic states.
Moreover, we show that only one of the invariants of \Sigma is necessary, since
each component of \Sigma falls onto a master curve when plotted as a function
of Tr(\Sigma). This implies that these granular assemblies have a common shape
for the stress tensor, even though it does not correspond to the hydrostatic
type. Although most results are obtained by molecular dynamics simulations, we
present supporting experimental results.Comment: 13 pages, 14 figure
- …