Focusing on grains, researchers solve a mystery of rivers' flow | 3/29/2017 | Staff
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Taking a simpler approach to a complex problem, Yale researchers have an answer for why large grains move more easily than smaller ones when driven by fluid flow along a riverbed—a question that has confounded scientists for decades.

Much of the natural world is shaped by flowing water that moves sediment, sand, pebbles, and other grains. Understanding exactly when and why grains begin to move in response to complex fluid forces would have major applications in ecology, agriculture, and other fields.

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In the research group of Corey O'Hern, associate professor of mechanical engineering & materials science, physics, and applied physics, researchers developed computer simulations to observe how grains move and interact in a fluid flow over a granular bed—for instance, a river running over a bed of sand or gravel. Their results are published March 28 in Physical Review Fluid.

Determining how grains are carried along with the fluid flow is extremely complicated due to the many variables involved—including grain size, grain-grain friction, non-spherical grain shape, and fluid turbulence. When studying such a complex process, scientists often focus on the aspects of the problem they think are the most important and simplify other aspects as much as possible. Much of the prior research in this field has focused on the fluid forces that cause resting grains to start moving, but the Yale study suggests that more attention should be focused on what's happening while the grains are already in motion.

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"Researchers have traditionally overemphasized the fluid mechanics and treated the grains more like a static bed," said Yale researcher Abram Clark, lead author of the study. "Our approach considers the problem of sediment transport from the other direction, focusing more on the granular bed—especially moving grains—and treating the fluid in a simple way. Instead of thinking about when static grains will first start to...
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