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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

When a group of discrete particles pass through a narrowing, the flow may become arrested due to the development of structures that span over the size of the aperture. Then, it is said that the system is clogged. Here, we will discuss about the existence of a phase diagram for the clogged state that has been recently proposed, arguing on its usefulness to describe different systems of discrete bodies ranging from granular materials, to colloidal suspensions and live beings. This diagram is built based on the value of a flowing parameter which characterizes the intermittent flow observed in all these discrete systems provided that there is an external or internal energy supply. Such requirement, which is necessary to destabilize the clogging arches, is absent in a standard static silo, which is therefore examined as a particular case. This view will help to understand some a priori inconsistencies concerning the role of driving force in the clogging process that have been found in the last years.

We report an experimental study on the flow of spherical particles through a vertical pipe discharged at constant velocity by means of a conveyor belt placed at the bottom. For a pipe diameter 3.67 times the diameter of the particles, we observe the development of hanging arches that stop the flow as they are able to support the weight of the particles above them. We find that the distribution of times that it takes until a stable clog develops, decays exponentially. This is compatible with a clogging probability that remains constant during the discharge. We also observe that the probability of clogging along the pipe decreases with the height, i.e. most of the clogs are developed near the bottom. This spatial dependence may be attributed to different pressure values within the pipe which might also be related to a spontaneous development of an helical structure of the grains inside the pipe.

Recently Janda et al. [Phys. Rev. Lett. 108, 248001 (2012)] reported an experimental study where it was measured the velocity and volume fraction fields of 1 mm diameter stainless steel beads in the exit of a two-dimensional silo. In that work, they proposed a new expression to predict the flow of granular average in silos which does not explicitly include the particle size as a parameter. Here, we study if effectively, there is not such influence of the particle size in the flux equations as well as investigate any possible effect in the velocity and volume fraction fields. To this end, we have performed high speed motion measurements of these magnitudes in a two-dimensional silo filled with 4 mm diameter beads of stainless steel, the same material as the previous works. A developed tracking program has been implemented to obtain at the same time both, the velocity and volume fraction. The final objective of this work has been to extend and generalize the theoretical framework of Janda et al. for all sizes of particles. We have found that the obtained functionalities are the same as in the 1 mm case, but the exponents and other fitting parameters are different.

Nowadays, the common method to pack granular materials is to tap the ensemble against the gravity. Despite the apparent simplicity of that method, the asymptotic states reached by the tapped systems have strongly dependences on parameters like the shape of the tapping pulse, the container geometry or the ratio between lateral and axial dimensions. Beyond the restrictions imposed by the system boundaries, the particle shape (like rods or tetrahedrons) plays a central role in the evolution and the final state of the ensemble. In this work, we introduce an unconventional method for compacting granular ensembles by applying a sequence of alternating counterrotating pulses or ¿twists¿. By using spherical particles we analyze the efficiency of this method to achieve highly packed configurations.

The effect of the filling mechanism on the packing of faceted particles with different aspect ratios has been examined. We have experimentally measured the particle angular distribution and the packing fraction of ensembles of faceted particles deposited in a two-dimensional box. The granular system has been numerically simulated using a two-dimensional model of faceted particles. We found that increasing the feeding rate results in an enhancement of the disorder in the final deposit and, consequently, in a reduction of the number of particles oriented in their most stable configuration. In this regime, the final packing fraction monotonically decreases as the feeding rate increases. The correlations between the final packing morphology and the stress transmission were examined by describing the micromechanical properties of the deposits. For the case of elongated particles, increasing the feeding rate leads to an enhancement of the stress transmission towards the sides of the box. On the contrary, for the case of square particles, increasing the feeding rate promotes vertical transmission of the stress.

Transport of material through pipes or channels in mines or gravel quarries seems to be a simple and economic form of conveying blasted ore between different levels. Despite the apparent advantages of moving the material by means of the gravity force, there exists an important problem that makes the applicability of this method more difficult: the election of the pipe diameter to prevent clogging of the stones. It was R. Kvapil in the sixties who extended the ideas of granular flows in silos to underground mining. Nevertheless, after his pioneering works there are only a few manuscripts focused on this topic, and many questions remain unsolved. In this work, we present experimental results about the flow of particles (gravel) driven by gravity through tilted tubes. The amount of material discharged between clogs shows that the probability of clogging can be estimated by the same procedures introduced for silos. Finally, by changing the ratio between the tube diameter and the typical particle size, we discuss about the existence or not of a critical size beyond which clogging is not possible.

We present an experimental study of the effect that an obstacle above the outlet of a silo has on the clogging probability. Both, the size of the orice and the obstacle position are varied for a chosen obstacle size and shape. If the position of the obstacle is properly selected the clogging probability can be importantly reduced. Indeed, as the outlet size is increased ¿ and we approach the critical size above which there is not clogging ¿ the obstacle effect is enhanced. For the largest outlet size studied, the clogging probability is reduced by a factor of more than one hundred. We will show, using numerical simulations, that the physical parameter behind the reduction of the silo clogging seems to be the decrease of the vertical pressure at the outlet proximities.

The role of density and velocity profiles in the flow of particles through apertures has been recently put on evidence in a two-dimensional experiment (Phys. Rev. Let. 108, 108, 118001). in a two-dimensional experiment (Phys. Rev. Lett. 108, 248001). For the whole range of apertures studied, both velocity and density profiles are selfsimilar and the obtained scaling functions allow to derive the relevant scales of the problem. Indeed, by means of the functionality obtained for these profiles, an exact expression for the mass flow rate was proposed. Such expression showed a perfect agreement with the experiential data. In this work, we generalize this study to the three dimensional case.We perform numerical simulations of a 3D silo in which the velocity and volume fraction profiles are determined. Both profiles shows that the scaling obtained for 2D can be generalized to the 3D case. Finally, the scaling of the mass flow rate with the outlet radius is discussed. outlet radius is discussed.