We present experimental results of pedestrian evacuations through a narrow door under a prescribed safety distancing of either 1.5 or 2 meters. In this situation, flow rate augments with pedestrian velocity due to a complete absence of flow interruptions or clogs. Accordingly, the evacuation improves when the prescribed physical distance is reduced, as this implies shortening the time lapses between the exit of consecutive pedestrians. In addition, the analysis of pedestrian trajectories reveals that the distance to the first neighbor in the evacuation process is rather similar to the one obtained when pedestrians were just roaming within the arena, hence suggesting that this magnitude depends more on the crowd state (desired speed, prescribed safety distance, etc.) than on the geometry where the pedestrian flow takes place. Also, an important difference in pedestrian behavior is observed when people are asked to walk at different speeds: whereas slow pedestrians evidence a clear preference for stop-and-go motion, fast walkers display detouring and stop-and-go behavior roughly in the same proportion.

The emergence of coherent vortices has been observed in a wide variety of many-body 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 vortex-like patterns which, intriguingly, only rotate counterclockwise. By implementing simple numerical simulations, we evidence that the development of swirls in many-particle 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 right-handed 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.

The 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 S-c, 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 S-D plane in which clogging appears as a concave surface.

We report experimental results of the speed-density 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 speed-density relation on the experimental conditions, especially with the (prescribed) free speed. We also observe that when pedestrians walk slowly, the speed-density 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 speed-density curves.

With people trying to keep a safe distance from others due to the COVID-19 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 variables-such as the pedestrian density, the walking speed and the prescribed safety distance-on 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.

Flowing 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.

We 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 time-dependent desired velocity of the runners. Also, we discover the existence of an inaccessible region in the speed-density 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.

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.

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.

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.

In a recent work [C. Lozano et al. Phys. Rev. Lett 114, 178002 (2015)] segregation in an horizontally shaken granular layer was studied by analysing the particle-particle interactions in the simplest case possible of a two particles cluster. There, it was found that all clusters are transient (they eventually split if one waits long enough) and the probability distribution function of the separation times displays a power law tail, indicating that the splitting probability is not constant over time. Here, we extend this study to clusters of 3, 5, 10 and 20 particles where we also observe the power law decay of the distribution of cluster splitting time. In addition, we observe a weak increase of the average cluster splitting time with the cluster size, suggesting that interaction forces are non-additive. Finally, we show interesting statistics on the way in which clusters break suggesting that escaping of individual particles in the cluster borders is more likely than cluster breakage in subclusters of similar size.

Granular flow through an orifice can be suddenly halted by the formation of arches in the vicinity of the outlet, which are stable under the action of gravity. They may be broken when an external driving (for instance, vibration) is applied. With the aim of shedding light on the dynamics of arch destruction, we built an experiment consisting of a vertical two-dimensional silo filled with monodisperse beads, to which a constant vibration is applied. It was previously found that an important parameter to predict the robustness of the arch is the angle between consecutive beads. We focus on long-enduring arches and study the angles among the beads along time. We have found that in many cases the dynamics of the largest angle determines the breaking of the arch; it does not only determine where the ¿weakest link¿ is, but also the process that leads to the final destabilization. This is interesting because it can provide information about whether the flow will resume in a well-defined time or not, which is especially useful for industrial processes that have to constantly deal with the possible emergence of clogs.

A distinctive feature of discrete solids is their ability to form arches. These mechanically stable structures alter the isotropy of granular packings and can arrest the motion of grains when, for example, they flow through a bottleneck. Breaking arches can be achieved by means of an external vibration, which effectively eliminates clogging. Indeed, these phenomena and procedures are quite common in industrial applications. Nevertheless, there are not rigorous, well founded criteria to determine the most efficient way to break arches and restore the flow of grains. This happens in part because it is not known which are the relevant characteristics that boost the arch strength. In the experiment presented here, we have carried out a statistical analysis of the arches that block the exit orifice at the bottom of a two dimensional silo, and described their geometrical properties. We then submit the silo to an external vibration. We find that the larger the outlet size, the weaker the arches that clog it. This dependence is just the outcome of a more complicated process that involves geometrical defects in the arch. The defects are quantitatively defined in terms of contact angles and we show that this is a key factor regarding the endurance of arches.

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.