"Faster is slower": Effect in granular flows

Abstract

We investigate the faster is slower (FIS) effect for a granular flow system which consist of a quasi two-dimensional hopper placed on a vibrating inclined plane. Increasing the angle of the plane (heta) is similar to increasing the driving force in the social force model. We measured the distribution of the time intervals between two successive particles (dt) and found that for narrow exits (3 particle diameter) it displays a power-law tail with a negative exponent alpha, where alpha < 2, indicating that the mean dt is not defined. Hence, we proposed to use alpha as a measure of the ability of the system not to develop long-lasting blockages and in this sense, the FIS effect was observed for the experimental granular system. This is the first experimental evidence of the FIS effect in granular flows. Moreover, our results suggest that a different approach might be necessary to quantify the evacuation time for pedestrians since its mean may not be defined for highly competitive crowd systems.