Journals

Magazine:
GRANULAR MATTER
ISSN:
1434-5021

Year:
2022
Vol:
24
N°:
2
Pp:
42

In the present work, we investigate experimentally and numerically the motion of solid macroscopic spheres (Brownian and colloidal effects are negligible) when settling from rest in a quiescent fluid toward a solid wall under confined and unconfined configurations. Particle trajectories for spheres of two types of materials are measured using a high-speed digital camera. For unconfined configurations, our experimental findings are in excellent agreement with well-established analytical frameworks, used to describe the forces acting on the sphere. Besides, the experimental values of the terminal velocity obtained for different confinements are also in very good agreement with previous theoretical formulations. Similar conditions are simulated using a resolved CFD-DEM approach. After adjusting the parameters of the numerical model, we analyze the particle dynamics under several confinement conditions. The simulations results are contrasted with the experimental findings, obtaining a good agreement. We analyze several systems varying the radius of the bead and show the excellent agreement of our results with previous analytical approaches. However, the results indicate that confined particles have a distinct dynamics response when approaching the wall. Consequently, their motion cannot be described by the analytical framework introduced for the infinite system. Indeed, the confinement strongly affects the spatial scale where the particle is affected by the bottom wall and, accordingly, the dimensionless results cannot be collapsed in a single master curve, using the particle size as a characteristic length. Alternatively, we rationalize our findings using a kinematic approximation to highlight the relevant scale of the problem. Our outcomes suggest it is possible to determine a new spatial scale to describe the collisional process, depending on the specific confining conditions.

Authors:
Blanco-Rodríguez, R.; Cruz, Raúl; Pérez-Ángel, G.; et al.
Magazine:
GRANULAR MATTER
ISSN:
1434-5021

Year:
2021
Vol:
23
N°:
4
Pp:
86

We present a critical comparative analysis between numerical and experimental results of quasi-two-dimensional silo and hopper flows. In our approach, the Discrete Element Method was employed to describe a single-layer mono-disperse sphere confined by two parallel walls with an orifice at the bottom. As a first step, we examined the discharge process, varying the size of the outlet and the hopper angle. Next, we set the simulation parameters fitting the experimental flow rate values obtained experimentally. Remarkably, the numerical model captured the slight non-monotonic dependence of the flow rate with the hopper angle, which was detected experimentally. Additionally, we analyzed the vertical velocity and solid fractions profiles at the outlet numerically and experimentally. Although numerical results also agreed with the experimental observations, a slight deviation appeared systematically between both approaches. Finally, we explored the impact of the system's confinement on this process, examining the consequences of particle-particle and particle-wall friction on the system macroscopic response. We mainly found that the degree of confinement and particle-wall friction have a relevant impact on the outflow dynamics. Our analysis demonstrated that the naive 2D approximation of this 3D flow process fails to describe it accurately.

Magazine:
JOURNAL OF FLUID MECHANICS
ISSN:
0022-1120

Year:
2021
Vol:
925
Pgs:
A24

The dynamics of granular average within a silo in which the grain velocities are controlled by a conveyor belt has been experimentally investigated. To this end, the building of coarse-grained field maps of different magnitudes has allowed a deep analysis of the flow properties as a function of two parameters: the orifice size and the belt velocity. First, the internal dynamics of the particles within the silo has been fully characterized by the solid fraction, the velocity of the particles and the kinetic stress. Then, the analysis of the vertical profiles of the same magnitude (plus the acceleration) has allowed connection of the internal dynamics with the flow rate. In particular, we show that the gamma parameter - which accounts for the integration of the normalized acceleration along the vertical direction - can successfully discriminate the kind of flow established within the silo (from the quasistatic regime to the free discharge) depending on the outlet size and belt velocity.

Magazine:
GRANULAR MATTER
ISSN:
1434-7636

Year:
2021
Vol:
23
N°:
2
Pp:
34

In this work, we reported experimental and numerical results of granular flows in silos and hoppers. We used a very flexible experimental setup, allowing us to explore the entire domain of the hopper angles. In addition, the granular flow was also studied numerically using Computational Fluid Dynamics. First, the numerical protocol was validated, comparing the output with experimental data of mass flow rate. In general, we obtained a good quantitative agreement between numerical and experimental results using a single set of the model parameters. Remarkably, the numerical results reproduced very well the weak non-monotonic behavior of the mass flow rate dependence on the hopper angle obtained experimentally. Stepping forward, we examined the scaling properties of the simulated velocity v(r) and density (r) profiles at the outlet region. Finally, we also analysed the velocity and volume fraction field inside the silo. The outcomes suggested that fast dynamics at orifice perturbs the system distinctly, depending on the hopper angle. Interestingly, small and large angles showed a larger zone of influence in comparison with intermediate angles.

Magazine:
PHYSICAL REVIEW E
ISSN:
2470-0045

Year:
2020
Vol:
102
N°:
1
Pp:
010902(R)

We report experimental evidence of clogging due to the spontaneous development of hanging arches when a granular sample composed of spherical particles flows down a narrow vertical pipe. These arches, akin to the ones responsible for silo clogging, can only be possible due to the role of frictional forces; otherwise they will be unstable. We find that, contrary to the silo case, the probability of clogging in vertical narrow tubes does not decrease monotonically with the ratio of the pipe-to-particle diameters. This behaviour is related to the clogging prevention caused by the spontaneous ordering of particles apparent in certain aspect ratios. More importantly, by means of numerical simulations, we discover that the interparticle normal force distributions broaden in systems with higher probability of clogging. This feature, which has been proposed before as a distinctive feature of jamming in sheared granular samples, suggests that clogging and jamming are connected in pipe flow.

Magazine:
SAFETY SCIENCE
ISSN:
0925-7535

Year:
2020
Vol:
121
Ppgs:
394 - 402

We report experimental measurements obtained during the evacuation of 180 soldiers through a narrow door. Several conditions are analyzed in the evacuation drills, such as the degree of competitiveness (from rush to shove) and the influence of an obstacle placed before the exit. From the data, we compute the flow rate through the door and the velocity and density fields, as well as a map of the local evacuation time. We also present novel results on the pressure that the individuals exert on the wall adjacent to the door. Our study challenges the idea that an obstacle could be beneficial for pedestrian evacuations because of a hypothetical alleviation of pressure at the door. At the same time, we discover a correlation among the largest pressure peaks and the development of clogging.

Magazine:
POWDER TECHNOLOGY
ISSN:
0032-5910

Year:
2020
Vol:
360
Ppgs:
104 - 111

By means of an experimental analysis we study the granular flow in a two-dimensional silo discharged through a conveyor belt placed below the outlet. The results exhibit a saturation of the flow rate, W, with the belt velocity, v(b). Moreover, we find a dependence of the flow rate and grains velocity on the outlet size D which differs from the purely gravitational regime. To explain it, we propose an analysis based on mass conservation arguments that agrees with the experimental data when v(b) is sufficiently low. For large values of this variable, it seems to be a smooth transition between a free discharge regime (for small D) and a belt extraction regime (for large D) where the proposed model is also valid. Our analysis provides a useful connection between the flow rate and the exit geometry, a feature that may be very useful from a practical point of view. (C) 2019 Elsevier B.V. All rights reserved.

Magazine:
SCIENTIFIC REPORTS
ISSN:
2045-2322

Year:
2020
Vol:
10
N°:
1
Pp:
15947

Although some experimental evidence showed that an obstacle placed in front of a door allows making people's evacuations faster, the efficacy of such a solution has been debated for over 15 years. Researchers are split between those who found the obstacle beneficial and those who could not find a significant difference without it. One of the reasons for the several conclusions lies in the variety of the experiments performed so far, both in terms of competitiveness among participants, geometrical configuration and number of participants. In this work, two unique datasets relative to evacuations with/without obstacle and comprising low and high competitiveness are analyzed using state-of-the-art definitions for crowd dynamics. In particular, the so-called congestion level is employed to measure the smoothness of collective motion. Results for extreme conditions show that, on the overall, the obstacle does not reduce density and congestion level and it could rather slightly increase it. From this perspective, the obstacle was found simply shifting the dangerous spots from the area in front of the exit to the regions between the obstacle and the wall. On the other side, it was however confirmed, that the obstacle can stabilize longitudinal crowd waves, thus reducing the risk of trampling, which could be as important (in terms of safety) as improving the evacuation time.

Magazine:
POWDER TECHNOLOGY
ISSN:
0032-5910

Year:
2020
Vol:
366
Pgs:
488 - 496

Hoppers are one of the most popular devices implemented to allow precise flow control mechanism when dispensing granulate materials from silos and other containers. Despite its ubiquity in many industrial processes, the effect that hopper geometry has on the flow rate is still poorly understood. In this work, we study the influence of the hopper angle on the main two variables that determine the flow rate: the solid-fraction and the yelorities of the particles. To this end, we use a quasi-two-dimensional system which allows a precise characterization of the profiles of these variables at the orifice. Using these experimental results, we compute the flow-density vector and obtain the resulting expression for the volumetric flow rate. Finally, we compare this expression with an equation introduced back in 1961 by RL Brown. (C) 2020 Elsevier B.V. All rights reserved.

Magazine:
PHYSICAL REVIEW E
ISSN:
2470-0045

Year:
2019
Vol:
99
N°:
3
Pp:
1 - 6

We present experimental results of the effect of the hopper angle on the clogging of grains discharged from a two-dimensional silo under gravity action. We observe that the probability of clogging can be reduced by three orders of magnitude by increasing the hopper angle. In addition, we find that for very large hopper angles, the avalanche size (s) grows with the outlet size (D) stepwise, in contrast to the case of a flat-bottom silo for which s grows smoothly with D. This surprising effect is originated from the static equilibrium requirement imposed by the hopper geometry to the arch that arrests the flow. The hopper angle sets the bounds of the possible angles of the vectors connecting consecutive beads in the arch. As a consequence, only a small and specific portion of
the arches that jam a flat-bottom silo can survive in hoppers.

Magazine:
CHAOS SOLITONS AND FRACTALS
ISSN:
0960-0779

Year:
2019
Vol:
119
Pgs:
237 - 242

We present experimental data of the motion of a cylindrical slider interacting only by friction with a polished horizontal tray. The tray is harmonically shacked in the horizontal direction. Below a certain threshold of the driver acceleration, the slider permanently sticks to its substrate due to the static friction. Above that threshold, the observed slider dynamics is periodic (synchronous with the driver oscillation frequency) but not wholly harmonic: for driver accelerations little beyond the threshold, the slider velocity signal is quasi-triangular. A Markovian model shows that, with increasing driver acceleration, the slider motion increasingly tends to be harmonic again, though with a prominent phase difference respect to the driver.

Magazine:
GRANULAR MATTER
ISSN:
1434-5021

Year:
2019
Vol:
21
N°:
3
Pp:
47

In this paper we report experimental and numerical results on the velocity fluctuations of grains inside silos. Although simple models exist for the stationary and continuous approximation of the flow, the variability at the microscopic level (both ensemble averages and the fluctuations of individual particles around the average) reveal non-Gaussian statistics that resist a straightforward treatment. We also show that decreasing the orifice size causes an increase in the relative amplitude of the velocity fluctuations, meaning that the intermittency grows bigger.

Authors:
Aumaître, S.; Behringer, R. P.; Cazaubiel, A.; et al.

Magazine:
REVIEW OF SCIENTIFIC INSTRUMENTS
ISSN:
0034-6748

Year:
2018
Vol:
89
N°:
075103
Pgs:
1 - 10

A new experimental facility has been designed and constructed to study driven granular average in a low-gravity environment. This versatile instrument, fully automated, with a modular design based on several interchangeable experimental cells, allows us to investigate research topics ranging from dilute to dense regimes of granular average such as granular gas, segregation, convection, sound propagation, jamming, and rheology¿all without the disturbance by gravitational stresses active on Earth. Here, we present the main parameters, protocols, and performance characteristics of the instrument. The current scientific objectives are then briefly described and, as a proof of concept, some first selected results obtained in low gravity during parabolic flight campaigns are presented.

Magazine:
PHYSICAL REVIEW LETTERS
ISSN:
0031-9007

Year:
2018
Vol:
121
N°:
13
Pp:
138001

Based on the implementation of a novel silo discharge procedure, we are able to control the grains velocities regardless of the outlet size. This allows isolating the geometrical and kinematic contributions to the clogging process. We find that, for a given outlet size, reducing the grains velocities to extremely low values leads to a clogging probability increment of almost two orders of magnitude, hence revealing the importance of particle kinematics in the silo clogging process. Then, we explore the contribution of both variables, outlet size and grains velocity, and we find that our results agree with an already known exponential expression that relates clogging probability with outlet size. We propose a modification of such expression revealing that only two parameters are necessary to fit all the data: one is related with the geometry of the problem, and the other with the grains kinematics.

Magazine:
NEW JOURNAL OF PHYSICS
ISSN:
1367-2630

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 counter-intuitive 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 average, 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.

Magazine:
PHYSICAL REVIEW LETTERS
ISSN:
0031-9007

Year:
2017
Vol:
119
N°:
22
Pp:
228002

We experimentally analyze the compaction dynamics of an ensemble of cubic particles submitted to a novel type of excitation. Instead of the standard tapping procedure used in granular materials we apply alternative twists to the cylindrical container. Under this agitation, the development of shear forces among the different layers of cubes leads to particle alignment. As a result, the packing fraction grows monotonically with the number of twists. If the intensity of the excitations is sufficiently large, an ordered final state is reached where the volume fraction is the densest possible compatible with the boundary condition. This ordered final state resembles the tetratic or cubatic phases observed in colloids.

Magazine:
COMPUTATIONAL PARTICLE MECHANICS
ISSN:
2196-4378

Year:
2017
Vol:
4
N°:
4
Pp:
419 - 427

Very recently, we have examined experimentally and numerically the micro-mechanical details of monodisperse particle flows through an orifice placed at the bottom of a silo (Rubio-Largo et al. in Phys Rev Lett 114:238002, 2015). Our findings disentangled the paradoxical ideas associated to the free-fall arch concept, which has historically served to justify the dependence of the flow rate on the outlet size. In this work, we generalize those findings by examining large-scale polydisperse particle flows in silos. In the range of studied apertures, both velocity and density profiles at the aperture are self-similar, and the obtained scaling functions confirm that the relevant scale of the problem is the size of the aperture. Moreover, we find that the contact stress monotonically decreases when the particles approach the exit and vanish at the outlet. The behavior of this magnitude is practically independent of the size of the orifice. However, the total and partial kinetic stress profiles suggest that the outlet size controls the propagation of the velocity fluctuations inside the silo. Examining this magnitude, we conclusively argue that indeed there is a well-defined transition region where the particle flow changes its nature. The general trend of the partial kinetic pressure profiles and the location of the transition region results the same for all particle types. We find that the partial kinetic stress is larger for bigger particles. However, the small particles carry a higher fraction of kinetic stress respect to their concentration, which suggest that the small particles have larger velocity fluctuations than the large ones and showing lower strength of correlation with the global flow. Our outcomes explain why the free-fall arch picture has served to describe the polydisperse flow rate in the discharge of silos.

Magazine:
PHYSICAL REVIEW FLUIDS
ISSN:
2469-990X

Year:
2017
Vol:
2
N°:
8
Pp:
084304

We demonstrate experimentally that clogging in a silo correlates with some features of the particle velocities in the outlet proximities. This finding, that links the formation of clogs with a kinematic property of the system, is obtained by looking at the effect that the position of the lateral walls of the silo has on the flow and clogging behavior. Surprisingly, the avalanche size depends nonmonotonically on the distance of the outlet from the lateral walls. Apart from evidencing the relevance of a parameter that has been traditionally overlooked in bottleneck flow, this nonmonotonicity supposes a benchmark with which to explore the correlation of clogging probability with different variables within the system. Among these, we find that the velocity of the particles above the outlet and their fluctuations seem to be behind the nonmonotonicity in the avalanche size versus wall distance curve.

Magazine:
PHYSICAL REVIEW E
ISSN:
2470-0045

Year:
2017
Vol:
95
N°:
5
Pp:
052904

We experimentally analyze the effect that particle size has on the mass flow rate of a quasi two-dimensional silo discharged by gravity. In a previous work, Janda et al. [Phys. Rev. Lett. 108, 248001 (2012)PRLTAO0031-900710.1103/PhysRevLett.108.248001] introduced a new expression for the mass flow rate based on a detailed experimental analysis of the flow for 1-mm diameter beads. Here, we aim to extend these results by using particles of larger sizes and a variable that was not explicitly included in the proposed expression. We show that the velocity and density profiles at the outlet are self-similar and scale with the outlet size with the same functionalities as in the case of 1-mm particles. Nevertheless, some discrepancies are evidenced in the values of the fitting parameters. In particular, we observe that larger particles lead to higher velocities and lower packing fractions at the orifice. Intriguingly, both magnitudes seem to compensate giving rise to very similar flow rates. In order to shed light on the origin of this behavior we have computed fields of a solid fraction, velocity, and a kinetic-stress like variable in the region above the orifice.

Magazine:
PHYSICAL REVIEW E
ISSN:
2470-0045

Year:
2017
Vol:
96
N°:
2
Pp:
022904

We present numerical and experimental results on the mass flow rate during the discharge of three-dimensional silos filled with a bidisperse mixture of grains of different sizes. We analyzed the influence of the ratio between coarse and fine particles on the profile of volume fraction and velocity across the orifice. By using numerical simulations, we have shown that the velocity profile has the same shape as that in the monodisperse case and is insensitive to the composition of the mixture. On the contrary, the volume fraction profile is strongly affected by the composition of the mixture. Assuming that an effective particle size can be introduced to characterize the mixture, we have shown that previous expression for the mass flow rate of monodisperse particles can be used for binary mixtures. A comparison with Beverloo's correlation is also presented.

Authors:
Gago, P. A.; Maza, D; Pugnaloni, L. A.
Magazine:
PAPERS IN PHYSICS
ISSN:
1852-4249

Year:
2016
Vol:
8
Pgs:
080001

Static granular packs have been studied in the last three decades in the frame of a modified equilibrium statistical mechanics that assumes ergodicity as a basic postulate. The canonical example on which this framework is tested consists in the series of static configurations visited by a granular column subjected to taps. By analyzing the response of a realistic model of grains, we demonstrate that volume and stress variables visit different regions of the phase space at low tap intensities in different realizations of the experiment. We show that the tap intensity beyond which sampling by tapping becomes ergodic coincides with the forcing necessary to break all particle-particle contacts during each tap. These results imply that the well-known "reversible" branch of tapped granular columns is only valid at relatively high tap intensities.

Magazine:
GRANULAR MATTER
ISSN:
1434-5021

Year:
2015
Vol:
17
N°:
5
Pgs:
545 - 551

We report experimental results for pipe flow of granular materials discharged through vertical narrow tubes by means of a conveyor belt placed at the bottom. When the diameter of the tube is not much larger than the particle size, the system clogs due to the development of hanging arches that are able to support the weight of the grains above them. We find that the time it takes to develop a stable clog decays exponentially, which is compatible with a clogging probability that remains constant during the discharge. From this, and making an analogy with the discharge of silos, we introduce the avalanche size, measured in terms of the number of discharged tubes before the system clogs. The mean avalanche size is found to increase as the tube diameter is enlarged, the velocity of the conveyor belt grows, and the tube tilt deviates from the vertical. 2015, Springer-Verlag Berlin Heidelberg.

Magazine:
PHYSICAL REVIEW LETTERS
ISSN:
0031-9007

Year:
2015
Vol:
114
N°:
23
Pp:
238002

link DADUN:

http: //hdl.handle.net/10171/38680

Several theoretical predictions of the mass flow rate of granular average discharged from a silo are based
on the spontaneous development of a free-fall arch region, the existence of which is still controversial. In
this Letter, we study experimentally and numerically the particle flow through an orifice placed at the
bottom of 2D and 3D silos. The implementation of a coarse-grained technique allows a thorough
description of all the kinetic and micromechanical properties of the particle flow in the outlet proximities.
Though the free-fall arch does not exist as traditionally understood¿a region above which particles have
negligible velocity and below which particles fall solely under gravity action¿we discover that the kinetic
pressure displays a well-defined transition in a position that scales with the outlet size. This universal
scaling explains why the free-fall arch picture has served as an approximation to describe the flow rate in
the discharge of silos.

Magazine:
PHYSICAL REVIEW E
ISSN:
2470-0045

Year:
2015
Vol:
92
N°:
6
Pp:
062817

The ¿faster-is-slower¿ (FIS) effect was first predicted by computer simulations of the egress of pedestrians through a narrow exit [D. Helbing, I. J. Farkas, and T. Vicsek, Nature (London) 407, 487 (2000)]. FIS refers to the finding that, under certain conditions, an excess of the individuals' vigor in the attempt to exit causes a decrease in the flow rate. In general, this effect is identified by the appearance of a minimum when plotting the total evacuation time of a crowd as a function of the pedestrian desired velocity. Here, we experimentally show that the FIS effect indeed occurs in three different systems of discrete particles flowing through a constriction: (a) humans evacuating a room, (b) a herd of sheep entering a barn, and (c) grains flowing out a 2D hopper over a vibrated incline. This finding suggests that FIS is a universal phenomenon for active matter passing through a narrowing.

Magazine:
COMPUTATIONAL PARTICLE MECHANICS
ISSN:
2196-4378

Year:
2015
Vol:
2
N°:
2
Pp:
127 - 138

We present a hybrid GPU¿CPU implementation of an accurate discrete element model for a system of ellipsoids. The ellipsoids have three translational degrees of freedom, their rotational motion being described through quaternions and the contact interaction between two ellipsoids is described by a force which accounts for the elastic and dissipative interactions. Further we combine the exact derivation of contact points between ellipsoids (Wang et al. in Computing 72(1¿2):235¿246, 2004) with the advantages of the GPU-NVIDIA parallelization strategy (Owens et al. in Comput Graph Forum 26:80¿113, 2007). This novelty makes the analytical algorithm computationally feasible when dealing with several thousands of particles. As a benchmark, we simulate a granular gas of frictionless ellipsoids identifying a classical homogeneous cooling state for ellipsoids. For low dissipative systems, the behavior of the granular temperature indicates that the cooling dynamics is governed by the elongation of the ellipsoids and the restitution coefficient. Our outcomes comply with the statistical mechanical laws and the results are in agreement with previous findings for hard ellipsoids (Bereolos et al. in J Chem Phys 99:6087, 1993; Villemot and Talbot in Granul Matter 14:91¿97, 2012). Additionally, new insight is provided namely suggesting that the mean field description of the cooling dynamics of elongated particles is conditioned by the particle shape and the degree of energy equipartition.

Authors:
Gago, P. A. ; Maza, D; Pugnaloni, L. A.
Magazine:
PHYSICAL REVIEW E
ISSN:
1539-3755

Year:
2015
Vol:
91
N°:
3
Pp:
032207

We investigate the steady-state packing fraction phi and force moment tensor Sigma of quasi-two-dimensional granular columns subjected to tapping. Systems of different height h and width L are considered. We find that phi and Sigma which describe the macroscopic state of the system, are insensitive to L for L > 50d (with d the grain diameter). However, results for granular columns of different heights cannot be conciliated. This suggests that comparison between results of different laboratories on this type of experiments can be done only for systems of same height. We show that a parameter epsilon = 1 + (A omega)(2)/(2gh), with A and omega the amplitude and frequency of the tap and g the acceleration of gravity, can be defined to characterize the tap intensity. This parameter is based on the effective flight of the granular bed, which takes into account the h dependency. When phi is plotted as a function of epsilon, the data collapses for systems of different h. However, this parameter alone is unable to determine the steady state to be reached since different Sigma can be observed for a given epsilon if different column heights are considered.

Magazine:
SCIENTIFIC REPORTS
ISSN:
2045-2322

Year:
2014
Vol:
4
Pgs:
7324

When a large set of discrete bodies passes through a bottleneck, the flow may become intermittent due to the development of clogs that obstruct the constriction. Clogging is observed, for instance, in colloidal suspensions, granular materials and crowd swarming, where consequences may be dramatic. Despite its ubiquity, a general framework embracing research in such a wide variety of scenarios is still lacking. We show that in systems of very different nature and scale -including sheep herds, pedestrian crowds, assemblies of grains, and colloids- the probability distribution of time lapses between the passages of consecutive bodies exhibits a power-law tail with an exponent that depends on the system condition. Consequently, we identify the transition to clogging in terms of the divergence of the average time lapse. Such a unified description allows us to put forward a qualitative clogging state diagram whose most conspicuous feature is the presence of a length scale qualitatively related to the presence of a finite size orifice. This approach helps to understand paradoxical phenomena, such as the faster-is-slower effect predicted for pedestrians evacuating a room and might become a starting point for researchers working in a wide variety of situations where clogging represents a hindrance.

Magazine:
PHYSICAL REVIEW E
ISSN:
1539-3755

Year:
2014
Vol:
89
N°:
4
Pp:
042205

We present numerical results of the effect that the driving force has on the clogging probability of inert particles passing through a bottleneck. When the driving force is increased by four orders of magnitude, the mean avalanche size remains almost unaltered (increases 1.6 times) while the flow rate and the avalanche duration display strong dependence on this magnitude. This indicates that in order to characterize the ability of a system to clog, the right variable to consider is the number of particles that pass through the outlet. The weak dependence of this magnitude on the driving force is explained in terms of the average kinetic energy of the flowing grains that has to be dissipated in order to get an arch stabilized.

Magazine:
GRANULAR MATTER
ISSN:
1434-5021

Year:
2014
Vol:
16
N°:
4
Pp:
411 - 420

We present experimental and numerical results for particle alignment and stress distribution in packings of faceted particles deposited in a small-scale two-dimensional silo. First, we experimentally characterize the deposits' morphology in terms of the particles' aspect ratio and feeding rate. Then we use the experimental results to validate our discrete element method (DEM) based on spheropolygons. After achieving excellent agreement, we use contact forces and fabric provided by the simulations to calculate the coarse-grained stress tensor. For low feeding rates, square particles display a strong tendency to align downwards, i.e., with a diagonal parallel to gravity. This morphology leads to stress transmission towards the walls, implying a quick development of pressure saturation, in agreement with the Janssen effect. When the feed rate is increased, both the disorder and the number of horizontal squares in the silo increase, hindering the Janssen effect. Conversely, for elongated particles the feed rate has a weak effect on the final deposit properties. Indeed, we always observe highly ordered structures of horizontal rods where the stress is transmitted mainly in the vertical direction.

Magazine:
PHYSICAL REVIEW E
ISSN:
1539-3755

Year:
2014
Vol:
89
Pgs:
052212

We use the first Betti number of a complex to analyze the morphological structure of granular samples in mechanical equilibrium. We investigate two-dimensional granular packings after a tapping process by means of both simulations and experiments. States with equal packing fraction obtained with different tapping intensities are distinguished after the introduction of a filtration parameter which determines the particles (nodes in the network) that are joined by an edge. This is accomplished by just using the position of the particles obtained experimentally and no other information about the possible contacts, or magnitude of forces.

Authors:
Arevalo, R.; Pugnaloni, L. A.; Maza, D; et al.
Magazine:
PHYSICAL REVIEW E
ISSN:
1539-3755

Year:
2013
Vol:
87
N°:
2
Pp:
022203

We analyze the contact network of simulated two-dimensional granular packings in different states of mechanical equilibrium obtained by tapping. We show that topological descriptors of the contact network allow one to distinguish steady states of the same mean density obtained with different tap intensities. These equal-density states were recently proven to be distinguishable through the mean force moment tensor. In contrast, geometrical descriptors, such as radial distribution functions, bond order parameters, and Voronoi cell distributions, can hardly discriminate among these states. We find that small-order loops of contacts-the polygons of the network-are especially sensitive probes for the contact structure. DOI: 10.1103/PhysRevE.87.022203

Magazine:
EDIFICACION MAGAZINE
ISSN:
0213-8948

Year:
2013
N°:
41 - 42
Pp:
102 - 107

Granular media ¿subject inert media composed of divided solids ¿ when passing through an opening that is only slightly larger than the particle size can clog.
than the particle size can clog. It has been studied how the probability of clogging is reduced by placing an obstacle in front of the outlet.
by placing an obstacle in front of the exit. The obtained result demonstrates that the probability of clogging is reduced
of jamming depending on where the obstacle is placed, being an optimal position where the distance between the obstacle and the exit is similar to the size of the exit.
the exit is similar to the size of the opening. These results can be interesting for the optimal design of an evacuation exit.
evacuation.

Magazine:
PHYSICAL REVIEW E
ISSN:
1539-3755

Year:
2013
Vol:
87
N°:
1
Pp:
012202

In a previous paper [Hidalgo et al., Phys. Rev. Lett. 103, 118001 (2009)] it was shown that square particles deposited in a silo tend to align with a diagonal parallel to the gravity, giving rise to a deposit with very particular properties. Here we explore, both experimentally and numerically, the effect on these properties of the filling mechanism. In particular, we modify the volume fraction of the initial configuration from which the grains are deposited. Starting from a very dilute case, increasing the volume fraction results in an enhancement of the disorder in the final deposit characterized by a decrease of the final packing fraction and a reduction of the number of particles oriented with their diagonal in the direction of gravity. However, for very high initial volume fractions, the final packing fraction increases again. This result implies that two deposits with the same final packing fraction can be obtained from very different initial conditions. The structural properties of such deposits are analyzed, revealing that, although the final volume fraction is the same, their micromechanical properties notably differ.

Authors:
Arevalo, R.; Pugnaloni, L. A.; Maza, D; et al.
Magazine:
PHILOSOPHICAL MAGAZINE
ISSN:
1478-6435

Year:
2013
Vol:
93
N°:
31 - 33
Ppgs:
4078 - 4089

We characterize the structure of simulated two-dimensional granular packings using concepts from complex networks theory. The packings are generated by a simulated tapping protocol, which allows us to obtain states in mechanical equilibrium in a wide range of densities. We show that our characterization method is able to discriminate non-equivalent states that have the same density. We do this by examining differences in the topological structure of the contact network of the packings. In particular, we find that the polygons of the network are specially sensitive probes for the contact structure. Additionally, we compare the network properties obtained in two different scenarios: the tapped and a compressed system.

Magazine:
PLOS ONE
ISSN:
1932-6203

Year:
2013
Vol:
8
N°:
8
Pp:
e67838

While ¿vibrational noise¿ induced by rotating components of machinery is a common problem constantly faced by engineers, the controlled conversion of translational into rotational motion or vice-versa is a desirable goal in many scenarios ranging from internal combustion engines to ultrasonic motors. In this work, we describe the underlying physics after isolating a single degree of freedom, focusing on devices that convert a vibration along the vertical axis into a rotation around this axis. A typical Vibrot (as we label these devices) consists of a rigid body with three or more cantilevered elastic legs attached to its bottom at an angle. We show that these legs are capable of transforming vibration into rotation by a ¿ratchet effect¿, which is caused by the anisotropic stick-slip-flight motion of the leg tips against the ground. Drawing an analogy with the Froude number used to classify the locomotion dynamics of legged animals, we discuss the walking regime of these robots. We are able to control the rotation frequency of the Vibrot by manipulating the shaking amplitude, frequency or waveform. Furthermore, we have been able to excite Vibrots with acoustic waves, which allows speculating about the possibility of reducing the size of the devices so they can perform tasks into the human body, excited by ultrasound waves from the outside.

Magazine:
PHYSICAL REVIEW E
ISSN:
1539-3755

Year:
2012
Vol:
86
N°:
3
Pp:
031306

In a recent paper [Zuriguel et al., Phys. Rev. Lett. 107, 278001 (2011)] it has been shown that the presence of an obstacle above the outlet can significantly reduce the clogging probability of granular matter pouring from a silo. The amount of this reduction strongly depends on the obstacle position. In this work, we present new measurements to analyze different outlet sizes, extending foregoing results and revealing that the effect of the obstacle is enhanced as the outlet size is increased. In addition, the effect of the obstacle position on the flow rate properties and in the geometrical features of arches is studied. These results reinforce previous evidence of the pressure reduction induced by the obstacle. In addition, it is shown how the mean avalanche size and the average flow rate are not necessarily linked. On the other hand, a close relationship is suggested between the mean avalanche size and the flow rate fluctuations.

Magazine:
PHYSICAL REVIEW LETTERS
ISSN:
0031-9007

Year:
2012
Vol:
108
N°:
24
Pp:
248001

"Beverloo's law" is considered as the standard expression to estimate the flow rate of particles through apertures. This relation was obtained by simple dimensional analysis and includes empirical parameters whose physical meaning is poorly justified. In this Letter, we study the density and velocity profiles in the flow of particles through an aperture. We find that, for the whole range of apertures studied, both profiles are self-similar. Hence, by means of the functionality obtained for them the mass flow rate is calculated. The comparison of this expression with the Beverloo's one reveals some differences which are crucial to understanding the mechanism that governs the flow of particles through orifices.

Magazine:
PHYSICAL REVIEW E
ISSN:
1539-3755

Year:
2011
Vol:
84
N°:
3
Pp:
1 - 8

We present experimental data corresponding to a two-dimensional dense granular flow, namely, the gravity-driven discharge of grains from a small opening in a silo. We study the local velocity field at the scale of single grains at different places with the help of particle-tracking techniques. From these data, the velocity profiles can be obtained and the validity of some long-standing approaches can be assessed. Moreover, the fluctuations of the velocities are taken into consideration to characterize the features of the advective motion (due to the gravity force) and the diffusive motion, which shows nontrivial behavior.

Magazine:
PAPERS IN PHYSICS
ISSN:
1852-4249

Year:
2011
Vol:
3
N°:
0
Pp:
030004

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 \rm{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.

Magazine:
PHYSICAL REVIEW LETTERS
ISSN:
0031-9007

Year:
2011
Vol:
107
N°:
27
Pp:
278001

Magazine:
EUROPEAN PHYSICAL JOURNAL E
ISSN:
1292-8941

Year:
2011
Vol:
34
N°:
12
Pp:
133

We present experimental and numerical results of the effect that a partial discharge has on the morphological and micro-mechanical properties of non-spherical, convex particles in a silo. The comparison of the particle orientation after filling the silo and its subsequent partial discharge reveals important shear-induced orientation, which affects stress propagation. For elongated particles, the flow induces an increase in the packing disorder which leads to a reduction of the vertical stress propagation developed during the deposit generated prior to the partial discharge. For square particles, the flow favors particle alignment with the lateral walls promoting a behavior opposite to the one of the elongated particles: vertical force transmission, parallel to gravity, is induced. Hence, for elongated particles the flow developed during the partial discharge of the silo leads to force saturation with depth whereas for squares the flow induces hindering of the force saturation observed during the silo filling.

Magazine:
JOURNAL OF STATISTICAL MECHANICS-THEORY AND EXPERIMENT
ISSN:
1742-5468

The cooling dynamics of a 2D granular gas of elongated particles is analysed. We perform simulations on the temporal evolution of soft particles, using a molecular dynamics algorithm. For weakly dissipative particles, we found a homogeneous cooling process where the overall translational kinetic energy decreases analogously to viscoelastic circular particles. In contrast, for strongly dissipative particles we observed an inhomogeneous cooling process where the diminishing of translational kinetic energy notably slows down. The rotational kinetic energy, however, always decays in agreement with Haff's prediction for the homogeneous cooling state of inelastic particles. We mainly found that the cooling kinetics of the system is controlled by the mechanisms that determine the local energy dissipation (collisions). However, we detected a strong influence of particle shape and inelasticity on the structure of the clusters which develop in the inhomogeneous cooling regimes. Our numerical outcomes suggest that strong dissipation and particle anisotropy induce the formation of ordered cluster structures that retards the relaxation to the final asymptotic regime.

Magazine:
JOURNAL OF STATISTICAL MECHANICS-THEORY AND EXPERIMENT
ISSN:
1742-5468

We report experimental and theoretical results of the effect that particle shape has on the packing properties of granular materials. We have systematically measured the particle angular distribution, the cluster size distribution and the stress profiles of ensembles of faceted elongated particles deposited in a two-dimensional box. Stress transmission through this granular system has been numerically simulated using a two-dimensional model of irregular particles. For grains of maximum symmetry (squares), the stress propagation localizes and forms chain-like forces analogous to those observed for granular materials composed of spheres. For thick layers of grains, a pressure saturation is observed for deposit depths beyond a characteristic length. This scenario correlates with packing morphology and can be understood in terms of stochastic models of aggregation and random multiplicative processes. As grains elongate and lose their symmetry, stress propagation is strongly affected. Lateral force transmission becomes less favoured than vertical transfer, and hence, an increase in the pressure develops with depth, hindering force saturation.

Magazine:
PHYSICAL REVIEW E
ISSN:
1539-3755

Year:
2010
Vol:
81
N°:
6
Pp:
062301

We present an experimental study of the displacement of a light intruder immersed in a vibrated granular bed. Using high speed video we resolve the motion, during one cycle of oscillation, of a cylindrical object inside a Plexiglas box partially filled with grains. We report experimental evidence that, in the absence of convection, at least two forces are behind the intruder's motion: an air drag force-due to the airflow through the granular bed-and a buoyancy force produced by an air-mediated granular fluid.

Magazine:
International Journal of Bifurcation and Chaos
ISSN:
0218-1274

Year:
2010
Vol:
20
N°:
3
Pp:
897 - 903

The existence of small order loops of contacts is presented as an intrinsic characteristic of force granular networks. Based on molecular dynamics simulations, it is proposed that the presence of these small order loops - and in particular third order loops of contacts - is important to understand the transition from fluid-like to solid-like behavior of granular packings. In addition, we show a close relationship between the development of third order loops and the small forces of the granular packing in the sense that almost all third order loops allocate a force component smaller than the average.

Magazine:
PHYSICAL REVIEW E
ISSN:
1539-3755

Year:
2010
Vol:
81
N°:
4
Pp:
41302

The jamming transition of an isotropically compressed granular packing is studied by means of molecular dynamics simulations. The system is shown to undergo a critical transition which is analyzed by looking at the topological structure of the force network. At the critical packing fraction there is a sudden growth of the number of polygons in the network. Above the critical packing fraction the number of triangles keeps growing while the number of the rest of polygons is weakly reduced. Then, we prove that in the jammed regime, there is a linear relationship between the number of triangles and the coordination number. Furthermore, the presence of these minimal structures is revealed to be connected with the evolution of some important topological properties, suggesting its importance to understand the physical properties of the packing and the onset of rigidity during the compression.

Authors:
Pugnaloni, Luis A.; Sánchez, Iván; Gago, Paula A.; et al.

Magazine:
PHYSICAL REVIEW E
ISSN:
1539-3755

Year:
2010
Vol:
82
N°:
5
Pp:
050301

We analyze, experimentally and numerically, the steady states, obtained by tapping, of a two-dimensional granular layer. Contrary to the usual assumption, we show that the reversible (steady state branch) of the density-acceleration curve is non-monotonous. Accordingly, steady states with the same mean volume can be reached by tapping the system with very different intensities. Simulations of dissipative frictional disks show that equal volume steady states have different values of the force moment tensor. Additionally, we find that steady states of equal stress can be obtained by changing the duration of the taps; however, these states present distinct mean volumes. These results confirm previous speculations that the volume and the force moment tensor are both needed to describe univocally equilibrium states in static granular assemblies.