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Reunión de Usuarios y Desarrolladores de Métodos de Simulación de Aragón

Flow and Clogging in Bottlenecks: simulations and experiments

Prof. Ignacio Pagonabarraga, Barcelona
Prof. Iker Zurigel, Navarra

DATE OF EVENT : 17/09/2014       DURATION : 3 day(s)

LOCATION : ZCAM Campus Actur C/ Mariano Esquillor s/n Edificio I+D 50018 Zaragoza
URL : http://www.cecam.org/workshop-1018.html


SUMMARY :

Controlling the development of clogging in bottlenecks is crucial in fields such as engineering, medicine and architecture. Clogging of granular materials in conduits or industrial silos may lead to stop a line of production causing significant economic loss. In the same way, clogging of suspended particles is a major issue in oil and gas transport through pipelines. Indeed, floating gas hydrate particles can plug oil lines with the consequent safety and environmental hazards. In a much smaller spatial scale, clogging leads to intermittent flow when a dense suspension of microparticles passes through a constriction in a microchannel [1, 2]. Therefore, a more deep and fundamental understanding and use of clogging will be most beneficial. As an example, during the last years, clogging with microparticles has been used in medicine to provoke embolization of blood vessels to shrink a tumor or block an aneurysm. Minimizing clogging in suspensions is also of critical importance in ecological engineering. Nowadays, the use of subsurface flow treatment in wetlands is common worldwide for removing pollutants from wastewaters. This solution has important advantages due to its low cost of maintenance and its mechanical simplicity; the major drawback of this technology is its unpredictable life time, mostly limited by clogs that obstruct the pores [3]. Probably, the most dramatic example of clogging is the one occurring when crowds in panic are evacuated through emergency exits that cannot absorb the amount of people approaching the doors [4]. Sadly, in the last century clogging in narrow passages has provoked hundreds of deaths like in Hillsborough stadium (Sheffield, England) in 1989, or the most recent one happening during Halloween 2012 in Madrid Arena (Spain).

All these examples of clogging occur in systems where both, the interactions among particles and the interaction between particles and the surrounding media, are rather different. For the case of inert grains, gravity and contact forces are the only at play. For particle suspensions, however, the hydrodynamics of the flowing fluid as well as the capillary effects should be also taken into account. Dynamics of crowds through bottlenecks are even more difficult to approach theoretically, yet a social force model has been proved to adequately reproduce the observed behaviour. In the last decade, more than one hundred works with clogging as the key ingredient have been published, which constitutes a gauge of the interest and relevance of this phenomenon. Nevertheless, a reasonable understanding of the physical mechanisms behind clogging is lacking.

Several arguments can be given to justify the lack of fundamental understanding in clogging; probably the most important stands on its local character when compared, for example, with the global nature of jamming [5]. This fact seems to complicate the definition of global extensive variables within the system which could be used to characterize the phenomenology of clogging. As a consequence, most of the researchers working in multiple-particle systems have approached the problem of clogging and flow through bottlenecks as a collateral investigation. In addition it is remarkable that most scientists treat clogging as a particular problem occurring in a given media (inert particles, suspensions, active matter…) focusing primarily in the properties of the media and overlooking the general aspects of the clogging itself. Therefore, there is a need to develop common methodologies to understand, explain and predict the structural and dynamical properties of clogging. Indeed, in the last years, several striking analogies between clogging in different systems have been identified. For example, having a sufficiently high density of particles per unit area near the bottleneck has been shown to be necessary to observe clogging in suspensions [1,6], silos [7] and crowd dynamics [8]. Pressure is known to play a relevant role in clogging; for example, clogs of humans are exclusively reported in panic situations, when people impulsively push each other in their wish to reach the exit. A practical generic solution which seems to release such pressure is the placement of an obstacle just in front of the outlet. Indeed, this strategy has been proved to be efficient in clogging prevention for both silos [9] and room evacuation [10,11,12]. These are only mere examples indicating that a general framework for clogging should be achievable.

Computer simulations are ideally positioned to complement experimental efforts in situations where real conditions are not easily achievable. Indeed, one of the most important difficulties arising in the crowd evacuation situation is the difficulty of performing experiments without putting people in danger. We anticipate the key role that simulations will have in the forthcoming years, but they are still far from being completely reliable in this area.