Collaborative Research: Effect of Cohesion on Size and Statistics of Avalanches in Granular Systems

1336116PI：Lehman 1336634PI：Dahmen本合作项目旨在了解凝聚力对颗粒材料中雪崩统计的影响，并预测或最大限度地减少灾难性雪崩。用粘聚力可调的简单颗粒堆积桩的试验结果与模拟和理论模型的预测结果进行比较。本项目将系统地研究粘聚力对堆积体滑移雪崩统计的影响。过去的工作主要集中在非粘聚性颗粒材料的动力学上；新的初步结果表明，粘聚性可以导致灾难性的影响，不能将其视为对非粘聚性研究的小扰动。伍斯特学院开发了一种独特的实验设备，利用磁场系统地改变钢珠之间的粘聚力，伊利诺伊大学开发了一个带有一个剪应力弱化参数的解析平均场模型，以模拟粘聚力对颗粒材料的影响。这两个机构之间的合作努力将探索凝聚力对雪崩的普遍(即独立于细节)影响，并将确定决定灾难性大规模雪崩的大小和概率的实验调整参数。实验系统由珠子缓慢地落在偶尔会发生雪崩的圆锥形堆上组成。分析的重点是雪崩的统计特性，如特定雪崩大小和持续时间的概率、雪崩之间的时间以及最大事件的大小和复发时间。所有这些特性都是作为内聚力大小、初始增加的能量大小、桩的大小和其他实验参数的函数来测量的。该项目的分析部分使用相变理论和重整化群的工具来推导出对间歇性雪崩的预测。凝聚力与各种雪崩系统有关，这些结果最终可以用来最大限度地减少这些系统中危险的、灾难性的雪崩的发生。了解凝聚力的影响也将有助于更好地控制粉末、沙子、建材和农业谷物的流动。

1336116PI: Lehman1336634PI: DahmenThis 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.