Bottlenecks and traffic jams

When a granular medium is discharged from a silo, spontaneous arching or vaulting over the orifice causes training intermittency in the flow which can lead to a clogging. To restart the flow, an external disturbance is applied to the system, typically a vibration. In our team, we investigate both the problem of the trainingof jamming as well as their destruction by vibration.. We know that trainingof jams is a Poissonian process in which the probability that a stable arc is generated is constant over time. In contrast, arc breaking is a process with reportin which the probability of breaking decreases as time progresses, giving rise to an anomalous statistic in the distribution of breaking times. This dual behaviour has been used to define a jamming diagram based on the fact that the breakdown time distribution may not converge if the hole size or vibration intensity is sufficiently small. We are currently investigating the physical origin of the anomalous statistics found for the rupture time distributions.

Another interesting question that remains to be resolved is whether, in a static silo, there is a critical orifice size above which clogging is not possible. Recent work in this field seems to suggest that such a transition to a fluid state does not exist, implying that even enormously large orifices will clog if one waits long enough. However, there is no fully conclusive physical explanation for this, so we are working in different directions to try to find one. One of them is to decouple the geometric and dynamic contributions to the jamming. In addition, we have been working on the role of several variables in the clogging process such as the width of the silo, or the surprising reduction in the probability of clogging found if an obstacle is placed on top of the outlet orifice.