This collaborative project aims at understanding the effect of cohesion on the avalanche statistics in granular materials, and at predicting or minimizing catastrophic avalanches. Experimental results obtained with simple granular bead piles with tunable cohesion will be compared with the predictions of simulations and theoretical models.
The project will systematically study the effects of cohesion on the statistics of slip avalanches on bead piles. Past work has focused on the dynamics of non-cohesive granular materials; new preliminary results show that cohesion can lead to catastrophic effects that cannot be treated as a small perturbation to the non-cohesive studies. The College of Wooster has developed a unique experimental apparatus utilizing magnetic fields to systematically vary the cohesion between steel beads, and the University of Illinois has developed an analytical mean-field model with one shear-stress weakening parameter to model the effects of cohesion on granular materials. The collaborative effort between the two institutions will explore the universal (i.e. detail-independent) effects of cohesion on avalanches and will identify the experimental tuning parameters that determine the size and probability of the catastrophically large avalanches. The experimental system consists of beads that are slowly dropped onto a conical pile that occasionally avalanches. The analysis focuses on statistical properties of the avalanches, such as the probability of particular avalanche sizes and durations, the time between avalanches, and size and recurrence time of the largest events. All of these properties are measured as a function of the amount of cohesion, the amount of initially added energy, the size of the pile, and other experimental parameters. The analytical part of the project uses tools from the theory of phase transitions and the renormalization group to derive predictions for the intermittent avalanches.
Cohesion is relevant to a wide variety of avalanching systems, and these results could ultimately be used to minimize the occurrence of hazardous, catastrophic avalanches in these systems. Understanding the effect of cohesion will also allow better control over the flow of powders, sands, building materials, and agricultural grains.
