Miembros del Grupo
Coordinador
Colaboradores
Iñaki
Echeverría Huarte
Diego
Gella Bitrián
Bruno Valdemar
Guerrero Borges
Diego
López Rodríguez
David
Mendez Esteban
Líneas de Investigación
 Silos.
 Métodos numéricos en medios discretos y continuos.
 Movimientos de personas, tráfico, embotellamientos y situaciones de pánico.
 Formación de atascos en estrechamientos.
 Dispersiones coloidales en estrechamientos.
 Dinámica de medios granulares y compactación.
Palabras Clave
 Silos
 Medios granulares
 Fluidos complejos
 DEM
 Compactación
 Atascos y embotellamientos
Publicaciones Científicas desde 2018

Autores: Fonceca Junior, José Ilberto (Autor de correspondencia); Cruz Hidalgo, Raúl; Maza Ozcoidi, DiegoRevista: GRANULAR MATTERISSN 14345021 Vol.24 N° 2 2022 págs. 42ResumenIn 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 highspeed digital camera. For unconfined configurations, our experimental findings are in excellent agreement with wellestablished 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 CFDDEM approach. After adjusting the parameters of the numerical model, we analyze the particle dynamic 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 can not 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.

Autores: Echeverría Huarte, Iñaki (Autor de correspondencia); Alexandre, N.; Cruz Hidalgo, Raúl; et al.Revista: SCIENTIFIC REPORTSISSN 20452322 Vol.12 N° 1 2022 págs. 2647ResumenThe emergence of coherent vortices has been observed in a wide variety of manybody systems such as animal flocks, bacteria, colloids, vibrated granular materials or human crowds. Here, we experimentally demonstrate that pedestrians roaming within an enclosure also form vortexlike patterns which, intriguingly, only rotate counterclockwise. By implementing simple numerical simulations, we evidence that the development of swirls in manyparticle systems can be described as a phase transition in which both the density of agents and their dissipative interactions with the boundaries play a determinant role. Also, for the specific case of pedestrians, we show that the preference of righthanded people (the majority in our experiments) to turn leftwards when facing a wall is the symmetry breaking mechanism needed to trigger the global counterclockwise rotation observed.

Autores: Hernández Delfín, Dariel (Autor de correspondencia); Weinhart, T.; Cruz Hidalgo, Raúl (Autor de correspondencia)Revista: SOFT MATTERISSN 1744683X Vol.18 N° 17 2022 págs. 3335  3341ResumenThis work is devoted to study numerically the selfdiffusion of spherocylindrical particles flowing down an inclined plane, using the discrete element method (DEM). This system is challenging due to particles being nonspherical and because they are subjected to a nonuniform shear rate. We performed simulations for several aspect ratios and inclination angles, tracking individual particle trajectories. Using the simulation data, we computed the diffusion coefficients D, and a coarsegraining methodology allowed accessing the shear rate spatial profiles (y)over dot (z). This data enabled us to identify the spatial regions where the diffusivity strongly correlates with the local shear rate. Introducing an effective particle size d(perpendicular to), we proposed a wellrationalized scaling law between D and (y)over dot. Our findings also identified specific locations where the diffusivity does not correlate with the shear rate. This observation corresponds to zones where has nonlinear spatial variation, and the velocity probability density distributions exhibit asymmetric shapes.

Autores: Puzyrev, D. (Autor de correspondencia); Fischer, D.; Harth, K.; et al.Revista: SCIENTIFIC REPORTSISSN 20452322 Vol.11 N° 1 2021 págs. 10621ResumenGranular multiparticle ensembles are of interest from fundamental statistical viewpoints as well as for the understanding of collective processes in industry and in nature. Extraction of physical data from optical observations of threedimensional (3D) granular ensembles poses considerable problems. Particlebased tracking is possible only at low volume fractions, not in clusters. We apply shadowbased and featuretracking methods to analyze the dynamics of granular gases in a container with vibrating side walls under microgravity. In order to validate the reliability of these optical analysis methods, we perform numerical simulations of ensembles similar to the experiment. The simulation output is graphically rendered to mimic the experimentally obtained images. We validate the output of the optical analysis methods on the basis of this ground truth information. This approach provides insight in two interconnected problems: the confirmation of the accuracy of the simulations and the test of the applicability of the visual analysis. The proposed approach can be used for further investigations of dynamical properties of such media, including the granular Leidenfrost effect, granular cooling, and gasclustering transitions.

Autores: Puzyrev, D. (Autor de correspondencia); Cruz Hidalgo, Raúl; Fischer, D.; et al.Revista: THE EUROPEAN PHYSICAL JOURNAL CONFERENCESISSN 21016275 Vol.249 2021 págs. 04004ResumenGranular gases are interesting multiparticle systems which, irrespective of the apparent simplicity of particle interactions, exhibit a rich scenario of so far only little understood features. We have numerically investigated a dense granular gas composed of frictional spherocylinders which are excited mechanically by lateral vibrating container walls. This study was stimulated by experiments in microgravity on parabolic flights. The formation of spatial inhomogeneities (clusters) was observed in a region near the corners of the container, about halfway from the excitation plates. The particles in the clusters show a tendency to align parallel to the container walls, seemingly increasing the stabilizing effect of friction. The simulation results provide hints that the phase difference of the vibrations of the two excitation walls might affect the cluster dynamics.

Autores: Parisi, D. R. (Autor de correspondencia); Sartorio, A. G.; Colonnello, J. R.; et al.Revista: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICAISSN 00278424 Vol.118 N° 50 2021 págs. e2107827118ResumenWe characterize the dynamics of runners in the famous Running of the Bulls Festival by computing the individual and global velocities and densities, as well as the crowd pressure. In contrast with all previously studied pedestrian systems, we unveil a unique regime in which speed increases with density that can be understood in terms of a timedependent desired velocity of the runners. Also, we discover the existence of an inaccessible region in the speeddensity state diagram that is explained by falls of runners. With all these ingredients, we propose a generalization of the pedestrian fundamental diagram for a scenario in which people with different desired speeds coexist.

Autores: Pongo, Tivadar (Autor de correspondencia); Puzyrev, D.; Harth, K.; et al.Revista: THE EUROPEAN PHYSICAL JOURNAL CONFERENCESISSN 21016275 Vol.249 2021 págs. 04003ResumenSome years ago, Harth et al. experimentally explored the steady state dynamics of a heated granular gas of rodlike particles in microgravity [K. Harth et al. Phys. Rev. Lett. 110, 144102 (2013)]. Here, we report numerical results that quantitatively reproduce their experimental findings and provide additional insight into the process. A system of spherocylinders is heated by the vibration of three flat side walls, resulting in one symmetrically heated direction, one nonsymmetrically heated direction, and one nonheated direction.

Autores: Garcimartín Montero, Ángel (Autor de correspondencia); Guerrero Borges, Bruno Valdemar; Nicolas, A.; et al.Revista: THE EUROPEAN PHYSICAL JOURNAL CONFERENCESISSN 21016275 Vol.249 2021 págs. 03009ResumenFlowing grains can clog an orifice by developing arches, an undesirable event in many cases. Several strategies have been put forward to avoid this. One of them is to vibrate the system in order to undo the clogging. Nevertheless, the time taken to break an arch under a constant vibration has a distribution displaying a heavy tail. This can lead to a situation where the average breaking time is not well defined. Moreover, it has been observed in some experiments that these tails tend to flatten for very long times, exacerbating the problem. Here we will review two conceptual frameworks that have been proposed to understand the phenomenon and discuss their physical implications.

Autores: Wang, J.; Fan, B.; Pongo, Tivadar; et al.Revista: SOFT MATTERISSN 17446848 Vol.17 N° 16 2021 págs. 4282  4295ResumenWe study the outflow dynamics and clogging phenomena of mixtures of soft, elastic lowfriction spherical grains and hard frictional spheres of similar size in a quasitwodimensional (2D) silo with narrow orifice at the bottom. Previous work has demonstrated the crucial influence of elasticity and friction on silo discharge. We show that the addition of small amounts, even as low as 5%, of hard grains to an ensemble of soft, lowfriction grains already has significant consequences. The mixtures allow a direct comparison of the probabilities of the different types of particles to clog the orifice. We analyze these probabilities for the hard, frictional and the soft, slippery grains on the basis of their participation in the blocking arches, and compare outflow velocities and durations of nonpermanent clogs for different compositions of the mixtures. Experimental results are compared with numerical simulations. The latter strongly suggest a significant influence of the interspecies particle friction.

Autores: Echeverría Huarte, Iñaki (Autor de correspondencia); Garcimartín Montero, Ángel; Parisi, D. R.; et al.Revista: JOURNAL OF STATISTICAL MECHANICSTHEORY AND EXPERIMENTISSN 17425468 Vol.2021 N° 4 2021 págs. 043401ResumenWe report experimental results of the speeddensity relation emerging in pedestrian dynamics when individuals keep a prescribed safety distance among them. To this end, we characterize the movement of a group of people roaming inside an enclosure varying different experimental parameters: (i) global density, (ii) prescribed walking speed, and (iii) suggested safety distance. Then, by means of the Voronoi diagram we are able to compute the local density associated to each pedestrian, which is afterward correlated with its corresponding velocity at each time. In this way, we discover a strong dependence of the speeddensity relation on the experimental conditions, especially with the (prescribed) free speed. We also observe that when pedestrians walk slowly, the speeddensity relation depends on the global macroscopic density of the system, and not only on the local one. Finally, we demonstrate that for the same experiment, each pedestrian follows a distinct behavior, thus giving rise to multiple speeddensity curves.

Autores: Gella Bitrián, Diego (Autor de correspondencia); Maza Ozcoidi, Diego; Zuriguel Ballaz, IkerRevista: JOURNAL OF FLUID MECHANICSISSN 00221120 Vol.925 2021 págs. A24ResumenThe dynamics of granular media within a silo in which the grain velocities are controlled by a conveyor belt has been experimentally investigated. To this end, the building of coarsegrained 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.

Autores: Pongo, Tivadar; Stiga, V.; Torok, J.; et al.Revista: NEW JOURNAL OF PHYSICSISSN 13672630 Vol.23 2021 págs. 023001ResumenGranular flow out of a silo is studied experimentally and numerically. The time evolution of the discharge rate as well as the normal force (apparent weight) at the bottom of the container is monitored. We show that particle stiffness has a strong effect on the qualitative features of silo discharge. For deformable grains with a Young modulus of about Ym ¿ 40 kPa in a silo with basal pressure of the order of 4 kPa, lowering the friction coefficient leads to a gradual change in the discharge curve: the flow rate becomes filling height dependent, it decreases during the discharge process. For hard grains with a Young modulus of about Ym ¿ 500 MPa the flow rate is much less sensitive to the value of the friction coefficient. Using DEM data combined with a coarsegraining methodology allows us to compute all the relevant macroscopic fields, namely, linear momentum, density and stress tensors. The observed difference in the discharge in the low friction limit is connected to a strong difference in the pressure field: while for hard grains Janssenscreening is effective, leading to high vertical stress near the silo wall and small pressure above the orifice region, for deformable grains the pressure above the orifice is larger and gradually decreases during the discharge process. We have analyzed the momentum balance in the region of the orifice (near the location of the outlet) for the case of soft particles with low friction coefficient, and proposed a phenomenological...

Autores: To, K. W.; Mo, Y. K.; Pongo, Tivadar; et al.Revista: PHYSICAL REVIEW EISSN 24700045 Vol.103 N° 6 2021 págs. 062905ResumenWe study the flow of elongated grains (wooden pegs of length L = 20 mm with circular cross section of diameter d(c) = 6 and 8 mm) from a silo with a rotating bottom and a circular orifice of diameter D. In the small orifice range (D/d < 5) clogs are mostly broken by the rotating base, and the flow is intermittent with avalanches and temporary clogs. Here d (3/2d(c)(2)L)(1/3) is the effective grain diameter. Unlike for spherical grains, for rods the flow rate W clearly deviates from the power law dependence W proportional to (D  kd)(2.5) at lower orifice sizes in the intermittent regime, where W is measured in between temporary clogs only. Instead, below about D/d < 3 an exponential dependence W proportional to e(kappa D) is detected. Here k and kappa are constants of order unity. Even more importantly, rotating the silo base leads to a strongmore than 50%decrease of the flow rate, which otherwise does not depend significantly on the value of omega in the continuous flow regime. In the intermittent regime, W(omega) appears to follow a nonmonotonic trend, although with considerable noise. A simple picture, in terms of the switching from funnel flow to mass flow and the alignment of the pegs due to rotation, is proposed to explain the observed difference between spherical and elongated grains. We also observe shearinduced orientational ordering of the pegs at the bottom such that their long axes in average are oriented at a small angle <theta > approximate to 15 degrees to the motion of the bottom.

Autores: BlancoRodríguez, R.; Cruz Hidalgo, Raúl; PérezÁngel, G.; et al.Revista: GRANULAR MATTERISSN 14345021 Vol.23 N° 4 2021 págs. 86ResumenWe present a critical comparative analysis between numerical and experimental results of quasitwodimensional silo and hopper flows. In our approach, the Discrete Element Method was employed to describe a singlelayer monodisperse 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 nonmonotonic 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 particleparticle and particlewall friction on the system macroscopic response. We mainly found that the degree of confinement and particlewall 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.

Autores: Echeverría Huarte, Iñaki; Garcimartín Montero, Ángel; Cruz Hidalgo, Raúl; et al.Revista: SCIENTIFIC REPORTSISSN 20452322 Vol.11 N° 1 2021 págs. 1534ResumenWith people trying to keep a safe distance from others due to the COVID19 outbreak, the way in which pedestrians walk has completely changed since the pandemic broke out1,2. In this work, laboratory experiments demonstrate the effect of several variablessuch as the pedestrian density, the walking speed and the prescribed safety distanceon the interpersonal distance established when people move within relatively dense crowds. Notably, we observe that the density should not be higher than 0.16 pedestrians per square meter (around 6 m2 per pedestrian) in order to guarantee an interpersonal distance of 1 m. Although the extrapolation of our findings to other more realistic scenarios is not straightforward, they can be used as a first approach to establish density restrictions in urban and architectonic spaces based on scientific evidence.

Autores: Reddy, A. V. K.; Reddy, K. A. (Autor de correspondencia); Zuriguel Ballaz, Iker; et al.Revista: JOURNAL OF STATISTICAL MECHANICSTHEORY AND EXPERIMENTISSN 17425468 Vol.2021 N° 6 2021 págs. 063201ResumenWe report numerical results aiming to unveil the role that the shape of asymmetric dumbbells has on the emergence of clogging in a two dimensional silo. To this end, dumbbells are designed adjoining two discs of different diameter giving rise to what are known as snowman shaped particles. Then, simulations performed for several outlet widths reveal that the standard case of a dumbbell conformed by two equally sized discs, is the one for which the system clogs more frequently. In this way, as the size difference among the two discs becomes greater, the system evolves from a dumbbelllike to a disclike behavior. This phenomenon correlates very well with the evolution of several properties of the clogged arches, such as regularity or particles pressure. Finally, the analysis of the orientations of clogged dumbbells reveals a strong alignment of their long axis with the exit direction which seems to be barely dependent on the dumbbell asymmetry and the outlet size.

Autores: Méndez, D. (Autor de correspondencia); Cruz Hidalgo, Raúl; Maza Ozcoidi, DiegoRevista: GRANULAR MATTERISSN 14347636 Vol.23 N° 2 2021 págs. 34ResumenIn 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 nonmonotonic 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 analyzed 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.

Autores: Caitano Barbosa Da Silva, Rodrigo; Guerrero Borges, Bruno Valdemar; Gonzalez, R. E. R.; et al.Revista: PHYSICAL REVIEW LETTERSISSN 00319007 Vol.127 N° 14 2021 págs. 148002ResumenThe existence of a transition from a clogged to an unclogged state has been recently proposed for the flow of macroscopic particles through bottlenecks in systems as diverse as colloidal suspensions, granular matter, or live beings. Here, we experimentally demonstrate that, for vibrated granular media, such a transition genuinely exists, and we characterize it as a function of the outlet size and vibration intensity. We confirm the suitability of the "flowing parameter" as the order parameter, and we find out that the resealed maximum acceleration of the system should be replaced as the control parameter by a dimensionless velocity that can be seen as the square root of the ratio between kinetic and potential energy. In all the investigated scenarios, we observe that, for a critical value of this control parameter Sc, there seems to be a continuous transition to an unclogged state. The data can be resealed with this critical value, which, as expected, decreases with the orifice size D. This leads to a phase diagram in the SD plane in which clogging appears as a concave surface.

Autores: Hernández Delfín, Dariel; Pongo, Tivadar; To, K.; et al.Revista: PHYSICAL REVIEW EISSN 24700045 Vol.102 N° 4 2020ResumenVery recently, To et al. have experimentally explored granular flow in a cylindrical silo, with a bottom wall that rotates horizontally with respect to the lateral wall [Phys. Rev. E 100, 012906 (2019)]. Here we numerically reproduce their experimental findings, in particular, the peculiar behavior of the mass flow rate Q as a function of the frequency of rotation f. Namely, we find that for small outlet diameters D the flow rate increased with f, while for larger D a nonmonotonic behavior is confirmed. Furthermore, using a coarsegraining technique, we compute the macroscopic density, momentum, and the stress tensor fields. These results show conclusively that changes in the discharge process are directly related to changes in the flow pattern from funnel flow to mass flow. Moreover, by decomposing the mass flux (linear momentum field) at the orifice into two main factors, macroscopic velocity and density fields, we obtain that the nonmonotonic behavior of the linear momentum is caused by density changes rather than by changes in the macroscopic velocity. In addition, by analyzing the spatial distribution of the kinetic stress, we find that for small orifices increasing rotational shear enhances the mean kinetic pressure < p(k)> and the system dilatancy. This reduces the stability of the arches, and, consequently, the volumetric flow rate increases monotonically. For large orifices, however, we detected that < p(k)> changes nonmonotonically, which might explain the ...

Autores: Lopez, D.; Hernández Delfín, Dariel; Cruz Hidalgo, Raúl; et al.Revista: PHYSICAL REVIEW EISSN 24700045 Vol.102 N° 1 2020 págs. 010902(R)ResumenWe 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 pipetoparticle diameters. This behavior 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.

Autores: Echeverría Huarte, Iñaki (Autor de correspondencia); Zuriguel Ballaz, Iker; Cruz Hidalgo, RaúlTítulo: Pedestrian evacuation simulation in the presence of an obstacle using selfpropelled spherocylindersRevista: PHYSICAL REVIEW EISSN 24700045 Vol.102 N° 1  1 2020ResumenWe explore the role that the obstacle position plays in the evacuation time of agents when leaving a room. To this end, we simulate a system of nonsymmetric spherocylinders that have a prescribed desired velocity and angular orientation. In this way, we reproduce the nonmonotonous dependence of the pedestrian flow rate on the obstacle distance to the door. For short distances, the obstacle delays the evacuation because the exit size is effectively reduced; i.e., the distance between the obstacle and the wall is smaller than the door width. By increasing the obstacle distance to the door, clogging is reduced leading to an optimal obstacle position (maximum flow rate) in agreement with results reported in numerical simulations of pedestrian evacuations and granular flows. For further locations, however, a counterintuitive behavior occurs as the flow rate values fall again below the one corresponding to the case without obstacle. Analyzing the headtimes distribution, we evidence that this new feature is not linked to the formation of clogs, but is caused by a reduction of the efficiency of the agent's instantaneous flow rate when the exit is not blocked.

Autores: Darias, J. R.; Gella Bitrián, Diego; Fernandez, M. E.; et al.Revista: POWDER TECHNOLOGYISSN 00325910 Vol.366 2020 págs. 488  496ResumenHoppers 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 solidfraction and the yelorities of the particles. To this end, we use a quasitwodimensional system which allows a precise characterization of the profiles of these variables at the orifice. Using these experimental results, we compute the flowdensity 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.

Autores: Souzy, M. (Autor de correspondencia); Zuriguel Ballaz, Iker; Marín, A.Revista: PHYSICAL REVIEW EISSN 15393755 Vol.101 N° 6 2020 págs. 060901ResumenWhen suspended particles are pushed by liquid flow through a constricted channel, they might either pass the bottleneck without trouble or encounter a permanent clog that will stop them forever. However, they may also flow intermittently with great sensitivity to the necktoparticle size ratio D/d. In this Rapid Communication, we experimentally explore the limits of the intermittent regime for a dense suspension through a single bottleneck as a function of this parameter. To this end, we make use of high time and spaceresolution experiments to obtain the distributions of arrest times (T) between successive bursts, which display powerlaw tails (alpha Talpha) with characteristic exponents. These exponents compare well with the ones found for as disparate situations as the evacuation of pedestrians from a room, the entry of a flock of sheep into a shed, or the discharge of particles from a silo. Nevertheless, the intrinsic properties of our system (i.e., channel geometry, driving and interaction forces, particle size distribution) seem to introduce a sharp transition from a clogged state (alpha <= 2) to a continuous flow, where clogs do not develop at all. This contrasts with the results obtained in other systems where intermittent flow, with powerlaw exponents above two, were obtained.

Autores: Gella Bitrián, Diego (Autor de correspondencia); Maza Ozcoidi, Diego; Zuriguel Ballaz, IkerRevista: POWDER TECHNOLOGYISSN 00325910 Vol.360 2020 págs. 104  111ResumenBy means of an experimental analysis we study the granular flow in a twodimensional 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.

Autores: Feliciani, C. (Autor de correspondencia); Zuriguel Ballaz, Iker; Garcimartín Montero, Ángel; et al.Revista: SCIENTIFIC REPORTSISSN 20452322 Vol.10 N° 1 2020 págs. 15947ResumenAlthough 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 stateoftheart definitions for crowd dynamics. In particular, the socalled 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.

Autores: Zuriguel Ballaz, Iker (Autor de correspondencia); Echeverría Huarte, Iñaki; Maza Ozcoidi, Diego; et al.Revista: SAFETY SCIENCEISSN 09257535 Vol.121 2020 págs. 394  402ResumenWe 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.

Autores: Huang, K. (Autor de correspondencia); Hernández Delfín, Dariel; Rech, F.; et al.Revista: SCIENTIFIC REPORTSISSN 20452322 Vol.10 N° 1 2020 págs. 3207ResumenProjectile impact into a light granular material composed of expanded polypropylene (EPP) particles is investigated systematically with various impact velocities. Experimentally, the trajectory of an intruder moving inside the granular material is monitored with a recently developed noninvasive microwave radar system. Numerically, discrete element simulations together with coarsegraining techniques are employed to address both dynamics of the intruder and response of the granular bed. Our experimental and numerical results of the intruder dynamics agree with each other quantitatively and are in congruent with existing phenomenological model on granular drag. Stepping further, we explore the 'microscopic' origin of granular drag through characterizing the response of granular bed, including density, velocity and kinetic stress fields at the meanfield level. In addition, we find that the dynamics of cavity collapse behind the intruder changes significantly when increasing the initial speed . Moreover, the kinetic pressure ahead of the intruder decays exponentially in the comoving system of the intruder. Its scaling gives rise to a characteristic length scale, which is in the order of intruder size. This finding is in perfect agreement with the longscale inertial dissipation type that we find in all cases.

Autores: Adrian, J.; Amos, M.; Baratchi, M.; et al.Revista: COLLECTIVE DYNAMICSISSN 23668539 Vol.4 N° A19 2019 págs. 1  13ResumenThis article presents a glossary of terms that are frequently used in research on human crowds. This topic is inherently multidisciplinary as it includes work in and across computer science, engineering, mathematics, physics, psychology and social science, for example. We do not view the glossary presented here as a collection of finalised and formal definitions. Instead, we suggest it is a snapshot of current views and the starting point of an ongoing process that we hope will be useful in providing some guidance on the use of terminology to develop a mutual understanding across disciplines. The glossary was developed collaboratively during a multidisciplinary meeting. We deliberately allow several definitions of terms, to reflect the confluence of disciplines in the field. This also reflects the fact not all contributors necessarily agree with all definitions in this glossary.

Autores: Guerrero Borges, Bruno Valdemar (Autor de correspondencia); Chakraborty, B.; Zuriguel Ballaz, Iker; et al.Revista: PHYSICAL REVIEW EISSN 24700045 Vol.100 N° 3 2019 págs. 032901ResumenWe report an experiment on the unclogging dynamics in a twodimensional silo submitted to a sustained gentle vibration. We find that arches present a jerking motion where rearrangements in the positions of their beads are interspersed with quiescent periods. This behavior occurs for both arches that break down and those that withstand the external perturbation: Arches evolve until they either collapse or get trapped in a stable configuration. This evolution is described in terms of a scalar variable characterizing the arch shape that can be modeled as a continuoustime random walk. By studying the diffusivity of this variable, we show that the unclogging is a weakly nonergodic process. Remarkably, arches that do not collapse explore different configurations before settling in one of them and break ergodicity much in the same way than arches that break down.

Autores: Zuriguel Ballaz, Iker (Autor de correspondencia); Maza Ozcoidi, Diego; Janda, A.; et al.Revista: GRANULAR MATTERISSN 14345021 Vol.21 N° 3 2019 págs. 47ResumenIn 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 nonGaussian 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.

Autores: López Rodríguez, Diego; Gella, D.; To, K.; et al.Revista: PHYSICAL REVIEW EISSN 24700045 Vol.99 N° 3 2019 págs. 1  6ResumenWe present experimental results of the effect of the hopper angle on the clogging of grains discharged from a twodimensional 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 flatbottom 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 flatbottom silo can survive in hoppers.

Autores: Maza Ozcoidi, Diego (Autor de correspondencia); Schins, J.Revista: CHAOS SOLITONS AND FRACTALSISSN 09600779 Vol.119 2019 págs. 237  242ResumenWe 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 quasitriangular. 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.

Autores: Gella Bitrián, Diego (Autor de correspondencia); Zuriguel Ballaz, Iker; Ortin, J.Revista: PHYSICAL REVIEW LETTERSISSN 00319007 Vol.123 N° 21 2019 págs. 218004ResumenWe 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 onoff 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.

Autores: Aumaître, S.; Behringer, R. P.; Cazaubiel, A.; et al.Revista: REVIEW OF SCIENTIFIC INSTRUMENTSISSN 00346748 Vol.89 N° 075103 2018 págs. 1  10ResumenA new experimental facility has been designed and constructed to study driven granular media in a lowgravity environment. This versatile instrument, fully automatized, with a modular design based on several interchangeable experimental cells, allows us to investigate research topics ranging from dilute to dense regimes of granular media 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.

Autores: Gella, D.; Zuriguel Ballaz, Iker; Maza Ozcoidi, DiegoRevista: PHYSICAL REVIEW LETTERSISSN 00319007 Vol.121 N° 13 2018 págs. 138001ResumenBased 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.

Autores: Cruz Hidalgo, Raúl (Autor de correspondencia); GoñiArana, A.; HernándezPuerta, A.; et al.Revista: PHYSICAL REVIEW EISSN 24700045 Vol.97 N° 1 2018 págs. 012611ResumenIn this work, we numerically study a dense colloidal suspension flowing through a small outlet driven by a pressure drop using latticeBoltzmann methods. This system shows intermittent flow regimes that precede clogging events. Several pieces of evidence suggest that the temperature controls the dynamic state of the system when the driving force and the aperture size are fixed. When the temperature is low, the suspension's flow can be interrupted during long time periods, which can be even two orders of magnitude larger than the system's characteristic time (Stokes). We also find that strong thermal noise does not allowthe formation of stable aggregate structures avoiding extreme clogging events, but, at the same time, it randomizes the particle trajectories and disturbs the advective particle flow through the aperture. Moreover, examining the particle velocity statistics, we obtain that in the plane normal to the pressure drop the colloids always move as free particles regardless of the temperature value. In the pressure drop direction, at high temperature the colloids experience a simple balance between advective and diffusive transport, but at low temperature the nature of the flow is much more complex, correlating with the occurrence of very long clogging events.

Autores: Cruz Hidalgo, Raúl (Autor de correspondencia); Szabo, B. ; Gillemot, K. ; et al.Revista: PHYSICAL REVIEW FLUIDSISSN 2469990X Vol.3 N° 7 2018 págs. 074301ResumenWe present an extensive numerical and experimental study, investigating a threedimensional (3D) granular flow of elongated particles down an inclined plane. Similarly to sheared systems, the average particle orientation is found to enclose a small angle with the flow direction. In the bulk, this behavior is independent of the shear rate. At the surface, however, the particles move in more dilute conditions, and the average orientation strongly depends on the shear rate. A systematic numerical study varying the particle aspect ratio and the plane inclination reveals that the particle size perpendicular to the flow direction, d(eff), is an appropriate length scale to define an effective inertial number Ieff, which fully captures the impact of the particle shape on the system's rheology. Like in the case of spheres, density and friction result in welldefined functions of the effective inertial number Ieff,Thus, we quantify and explain the dependence of the rheological parameters on the aspect ratio, based on the micromechamcal details.

Autores: Guerrero Borges, Bruno Valdemar; Pugnaloni, L.A.; Lozano, C.; et al.Revista: PHYSICAL REVIEW EISSN 24700045 Vol.97 N° 4 2018 págs. 042904ResumenWe experimentally explore the vibrationinduced unclogging of arches halting the flow in a twodimensional silo. The endurance of arches is determined by carrying out a survival analysis of their breaking times. By analyzing the dynamics of two morphological variables, we demonstrate that arches evolve toward less regular structures and it seems that there may exist a certain degree of irregularity that the arch reaches before collapsing. Moreover, we put forward that ¿ (the standard deviation of all angles between consecutive beads) describes faithfully the morphological evolution of the arch. Focusing on longlasting arches, we study ¿ calculating its twotime autocorrelation function and its meansquared displacement. In particular, the apparent logarithmic increase of the correlation and the decrease of the meansquared displacement of ¿ when the waiting time is increased reveal a slowing down of the dynamics. This behavior is a clear hallmark of aging phenomena and confirms the lack of ergodicity in the unclogging dynamics. Our findings provide new insights on how an arch tends to destabilize and how the probability that it breaks with a long sustained vibration decreases with time.

Autores: Garcimartín Montero, Ángel (Autor de correspondencia); Maza Ozcoidi, Diego; Pastor, J. M. ; et al.Revista: NEW JOURNAL OF PHYSICSISSN 13672630 Vol.20 2018ResumenThe 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 resultwhich 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.

Autores: Parisi, D. R. (Autor de correspondencia); Zuriguel Ballaz, Iker; Cruz Hidalgo, RaúlRevista: SCIENTIFIC REPORTSISSN 20452322 Vol.8 N° 1 2018 págs. 9133ResumenWe report extensive numerical simulations of the flow of anisotropic selfpropelled particles through a constriction. In particular, we explore the role of the particles' desired orientation with respect to the moving direction on the system flowability. We observe that when particles propel along the direction of their long axis (longitudinal orientation) the flowrate notably reduces compared with the case of propulsion along the short axis (transversal orientation). And this is so even when the effective section (measured as the number of particles that are necessary to span the whole outlet) is larger for the case of longitudinal propulsion. This counterintuitive result is explained in terms of the formation of clogging structures at the outlet, which are revealed to have higher stability when the particles align along the long axis. This generic result might be applied to many different systems flowing through bottlenecks such as microbial populations or different kind of cells. Indeed, it has already a straightforward connection with recent results of pedestrian (which selfpropel transversally oriented) and mice or sheep (which selfpropel longitudinally oriented).

Autores: Arietaleaniz, S.; Malgaretti, P.; Pagonabarraga, I.; et al.Revista: PHYSICAL REVIEW EISSN 24700045 N° 98 2018 págs. 042603ResumenIn this work, we study the constitutive behavior of interacting colloidal suspensions for moderate and high concentrations. Specifically, using a lattice Boltzmann solver, we numerically examine suspensions flowing through narrow channels, and explore the significance of the interaction potential strength on the system's macroscopic response. When only a shortrange interaction potential is considered, a Newtonian behavior is always recovered and the system's effective viscosity mostly depends on the suspension concentration. However, when using a LennardJones potential we identify two rheological responses depending on the interaction strength, the volume fraction, and the pressure drop. Exploiting a model proposed in the literature we rationalize the simulation data and propose scaling relations to identify the relevant energy scales involved in these transport processes. Moreover, we find that the spatial distribution of colloids in layers parallel to the flow direction does not correlate with changes in the system macroscopic response; but, interestingly, the rheology changes do correlate with the spatial distribution of colloids within individual layers. Namely, suspensions characterized by a Newtonian response display a cubiclike structure of the colloids within individual layers, whereas for suspensions with nonNewtonian response colloids organize in a hexagonal structure.

Autores: Nicolas, A.; Garcimartín Montero, Ángel; Zuriguel Ballaz, IkerRevista: PHYSICAL REVIEW LETTERSISSN 00319007 Vol.120 N° 19 2018 págs. 198002ResumenGranular flows through narrow outlets may be interrupted by the formation of arches or vaults that clog the exit. These clogs may be destroyed by vibrations. A feature which remains elusive is the broad distribution pð¿Þ of clog lifetimes ¿ measured under constant vibrations. Here, we propose a simple model for arch breaking, in which the vibrations are formally equivalent to thermal fluctuations in a Langevin equation; the rupture of an arch corresponds to the escape from an energy trap. We infer the distribution of trap depths from experiments made in twodimensional hoppers. Using this distribution, we show that the model captures the empirically observed heavy tails in pð¿Þ. These heavy tails flatten at large ¿, consistently with experimental observations under weak vibrations. But, here, we find that this flattening is systematic, which casts doubt on the ability of gentle vibrations to restore a finite outflow forever. The trap model also replicates recent results on the effect of increasing gravity on the statistics of clog formation in a static silo. Therefore, the proposed framework points to a common physical underpinning to the processes of clogging and unclogging, despite their different statistics.

Autores: Zuriguel Ballaz, Iker; Garcimartín Montero, ÁngelLibro: Encyclopedia of complexity and systems scienceISSN 9783642277375 2020 págs. 1  32
Proyectos desde 2018

Título: Comportamiento colectivo emergente en sistemas de muchas partículas (EmerCol)Código de expediente: PID2020114839GBI00Investigador principal: IKER ZURIGUEL BALLAZ, RAUL CRUZ HIDALGO.Financiador: AGENCIA ESTATAL DE INVESTIGACIONConvocatoria: 2020 AEI PROYECTOS I+D+i (incluye Generación del conocimiento y Retos investigación)Fecha de inicio: 01092021Fecha fin: 31082024Importe concedido: 175.450,00 €Fondos FEDER: NO

Título: Flujo Intermitente en el Paso por Estrechamientos de Sistemas de Muchas PartículasCódigo de expediente: FIS201787631PInvestigador principal: IKER ZURIGUEL BALLAZ, RAUL CRUZ HIDALGO.Financiador: MINISTERIO DE CIENCIA E INNOVACIÓNConvocatoria: 2017 MINECO EXCELENCIAFecha de inicio: 01012018Fecha fin: 30092021Importe concedido: 78.650,00 €Fondos FEDER: SI

Título: CALIPERCódigo de expediente: 812638Investigador principal: RAUL CRUZ HIDALGOFinanciador: COMISIÓN EUROPEAConvocatoria: H2020CienciaExcelenteMSCAETN2018Fecha de inicio: 01092019Fecha fin: 31082023Importe concedido: 2.500.000,00 €Fondos FEDER: NO