Driving Instabilities with a Twist

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Imagine that you want to study how two fluids mix when a lighter fluid is pushed into a denser one. Conceptually, it’s a straightforward situation. It would be like having a layer of oil under a layer of water and watching what happens. But how do you do that experimentally? Oil won’t naturally stay under water. If you flip the container over to start the experiment, you’ve added a bunch of extra motion from the rotation. And if you use a barrier to separate the two layers and then pull it out, you’ve added extra shear where they meet.

To deal with challenges like these, researchers at Lehigh University spent five years designing and building the rotating wheel apparatus you see in the video above. Instead of relying on gravity to force the lighter fluid into a denser one, this set-up uses centrifugal force. The test fluids start out on the loading wheel, spinning in their naturally stable configuration. Then once both sides are rotating at the desired speed, the track flips, transferring the experiment onto the other wheel, which rotates in the opposite sense. This gives the fluids a sudden change in the direction of the centrifugal force and, once the apparatus completes shake-down, should give us new insight into the sort of mixing seen in fusion. (Video credit: Lehigh University; see also Turbulent Flow Design Group)

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