After some casual discussion, he boarded a train back to his hotel, during which he recalls "having a moment where I thought, 'I think our model could work.'" Last summer, Kamrin paid a visit to researchers in France who had carried out earlier experiments on secondary rheology. The researchers weren't sure if the model would also apply to secondary rheology, where motion at a primary location affects movement at a secondary, removed region. Kamrin and Henann originally devised the mathematical model to predict scenarios of primary flow, such as the flow field for sand flowing through a chute, or a circular trough. It has converted a granular solid (a material that has no trouble supporting the weight of the ball) to a granular fluid in which any object denser than the granular pile will sink. The ball is acting like a force probe, showing that the response of the grains has switched from solid to fluid. In a paper published in the journal Physical Review Letters, Kamrin and his former postdoc David Henann, now an assistant professor at Brown University, applied their existing model of granular flow to the problem of motion-induced quicksand, replicating the Couette cell geometry.īy spinning the turntable at the bottom of the bucket, the turntable "liquifies" the entire granular assembly, even the material very far from it. While others have observed this effect in experiments, there hasn't previously existed a model to predict such behavior. "How the sand responds to stress has changed entirely." "It looks like the mechanical behavior of the sand itself has changed because something was moving far away," Kamrin says. If, however, the cell's inner ring is rotating, the rod will move through the sand with even the slightest push-even where the sand doesn't appear to be moving.
In a stationary Couette cell, the rod will not budge without a significant application of force. In experiments, researchers have filled a Couette cell with sand, and attempted to push a rod horizontally through the sand. Researchers have observed this effect in a number of configurations in the lab, including in what's called an "annular Couette cell"-a geometry resembling the bowl of a food processor, with a rotating ring in its base. "So, for example, if you're walking in the desert and there's a sand dune landslide far away, you will start to sink, very slowly.
"The moment you start moving sand, it acts like fluid far away," Kamrin says. Now Kamrin has applied a recent granular model, developed by his group, and shown that it predicts a bizarre phenomenon called "motion-induced quicksand"-a scenario in which the movement of sand in one location changes the character of sand at a distance. Ken Kamrin, an assistant professor of mechanical engineering at MIT, studies granular materials, using mathematical models to explain their often-peculiar behavior.
0 kommentar(er)
