The Effect of an Obstacle Before a Bottleneck: Inert Particles, Sheep, and Persons

The placement of an obstacle before the exit has been put forward as a plausible mechanism to alleviate flow interruptions. Whenever a dense flow of discrete particles goes through a bottleneck, arches can form there and restrict the flow. The driving force tends to make the arches more stable. An obstacle before the exit would reduce the load and therefore arches should be easier to break down. We have tested this strategy in different systems. In a two dimensional silo filled with grains, an obstacle can improve by two orders of magnitude the time between clogs. We have also demonstrated that an obstacle can ease the entrance of a sheep herd through a door. Surprisingly, this does not seem to work for people. Although we are still trying to understand the reasons behind this result, some possible explanations are pointed out.