September 27, 2013
WOOSTER, Ohio — What happens when a bank of snow becomes unstable and triggers an avalanche? What causes a pile of sand or gravel to lose its consistency and flow unchecked in all directions?
Susan Lehman, associate professor of physics at The College of Wooster, and Karin Dahmen, professor of physics at The University of Illinois at Urbana-Champaign, hope to provide some insight into those and other questions after receiving a three-year, $355,973 collaborative grant from the National Science Foundation.
“One never knows what can prompt a major event,” says Lehman, who is the Principal Investigator for the project. “Even a tiny perturbation of the system could cause a response, large or small.”
Lehman and Dahmen will be looking specifically at the role of cohesion in the process. For example, wet snow or wet sand is more cohesive, allowing the build-up of larger amounts of material, so that any disturbance could produce a larger, more catastrophic outcome. The main objective is to understand and explain the effect of cohesion on the avalanche statistics in granular material, particularly predicting or minimizing catastrophic avalanches, according to Lehman. “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,” she says. “Understanding the effect of cohesion will also allow better control over the flow of powders, sands, building materials, and agricultural grains.”
The grant is designed to support a new collaboration between the experimental project at Wooster and theoretical simulations at the University of Illinois. In order to study the process, Lehman and her student assistants will use a simple granular pile as a model system to test the effect of cohesion on avalanche behavior. In their experiments, beads will be 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 characterized as a function of the amount of cohesion, the amount of energy that the dropped bead carries with it, and the size of the pile, according to the researchers.
“The experimental results are critical for the development of a new theoretical framework that links non-cohesive granular materials to cohesive materials, and to solids, in a single dynamic phase diagram,” says Lehman. “The theory uses tools from the theory of phase transitions and the renormalization group to derive predictions for the intermittent avalanches, and to determine their range of applicability to other systems.”
When a granular system responds to an incremental stimulus, it can switch from stability to collapse. The collapse could involve just a tiny portion of the pile, or it could involve nearly the entire system, so that the largest collapses can be catastrophic, according to Lehman and Dahmen. “This type of behavior happens both in natural settings,” they say, “with hazards such as landslides and snow avalanches, and also in industrial and agricultural situations involving the transport of granular materials that require the grains to flow freely. High humidity or electrostatic forces can cause cohesion between the grains; the grains lock together and then suddenly give way in large avalanches. The results can be costly and even devastating.”
As a result, Lehman and Dahmen are hoping to determine ways to predict the effects of cohesion in order to decide in advance whether a process needs to be modified or whether collapses will be small enough to pose little risk. “Understanding the effects of cohesion can provide key insights for industrial applications and prevention of natural hazards,” says Lehman. “This careful investigation of cohesion effects is needed in order to identify the key parameters of granular dynamics in many different geometries. This will then allow us to develop a new framework connecting the dynamics of granular materials to that of cohesive materials and solids.”
This collaborative synthesis of experiment and theory will provide new ideas, approaches, and frameworks that will be applicable to a range of situations, according to Lehman. “This interdisciplinary project trains both undergraduate students and graduate students in modern methods from a variety of modern research areas, ranging from statistical physics, computational physics, materials science, and engineering, to earth science and soil mechanics,” she says. “This is a great opportunity for our undergraduate students to interact closely with graduate students and work together on an important project.”
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