Taking the measure of water’s swirl

A tornado won’t just whirl you around, it will pick you up. It has what fluid dynamicists call helicity: Its flow winds simultaneously around and along a central vortex line like a corkscrew. Since its introduction into the fluid mechanics parlance nearly 50 years ago, helicity has proven itself to be a valuable theoretical construct. It helps explain how dynamos generate magnetic fields and can help predict the formation and duration of cyclonic thunderstorms. It’s known to be a conserved property of inviscid fluids such as superfluids. But because calculating a flow’s helicity seemingly calls for a full, three-dimensional description of the flow field, the quantity has eluded experimental measurement.

Now researchers led by William Irvine of the University of Chicago have found a way around that problem. By accelerating 3D-printed hydrofoils in water, the group could create vortex rings, in which the vortex core—the centerline of the “tornado"—wraps around on itself to form a loop. Thanks to topological constraints, the helicity of the flow can be inferred entirely from the velocity along the vortex core and the circulation around that core. Irvine and his colleagues could measure the circulation by watching how micron-sized nylon particles swirled as they encountered the vortex ring. To measure the velocity along the core, the researchers seeded it with blobs of fluorescent dye, which could be tracked using high-speed laser tomography.

The team tested its method on a flow comprising a pair of interacting vortex rings, each several centimeters in diameter. The image shows the dye-blob trajectories (yellow) for the concentric, copropagating rings (white). Each vortex ring is several centimeters in diameter; the inner, helical one is endowed with helicity whereas the outer, circular one is not. As the system evolved, flow interactions caused the helical ring to writhe and twist. But, importantly, the helicity remained constant, just as it would in an inviscid fluid. Irvine and his colleagues expect that future analyses of more complicated vortex interactions may help answer long-standing questions about how energy is dissipated in turbulent flows. (M. W. Scheeler et al., Science 357, 487, 2017 .)