Random Forces on Obstacles in Channels with Grains: A Mechanical Analogy of Crowd Disasters

Abstract

In this work we have studied experimentally, through sensitive force measurements, the fluctuating forces on a rigid obstacle located at the middle of a horizontal channel, when a two-dimensional (2D) granular forced flow is induced by a wall-piston which moves with constant velocity along the channel. In this piston-like system, the force measured show strong fluctuations which are much larger than the average force, giving rise to intermittent behavior. Two different initial packing factors were employed, showing different flow characteristics. In the well-ordered high packing system, the force on the obstacle is very high, producing force peaks at time intervals almost constants. The main frequency is well correlated with the residence time of each row of grains. However, in case of initially disordered, loose packing systems, there is an initial relaxation time where force on the obstacle is extremely low, thus allowing the free flow of several grain rows. The temporal force traces have a $1/\mathrm{f}^{\alpha }$ character and depend on the initial arrangement of grains.