A vortex knot caught on camera

Vortex rings pop up often in nature—perhaps most famously as smoke rings emanating from a smoker’s expert exhale, but also in the exhalations of fire eaters and underwater dolphins. More complex vortex topologies, such as interconnected rings and knots, are thought to figure prominently in turbulent flows, but they’ve proved notoriously difficult to create and study in the lab. Now, Dustin Kleckner and William Irvine of the University of Chicago have come up with a clever way to generate the elusive, self-entangled flow structures. The trick was to use a 3D printer to fashion hydrofoils whose topologies correspond to those desired of the vortices. One such hydrofoil, when placed in a water tank and given a swift tug, shed the trefoil vortex knot pictured here. (A video can be seen here. ) Light-scattering microbubbles, which tend to get trapped along the vortex lines, provided a convenient way to visualize the flow; with high-speed laser tomography, the researchers could track the knot’s evolution at 76 000 frames per second. In an ideal, inviscid fluid, a trefoil knot would never come undone, but in water it unravels via a bit of fluid-mechanical sleight of hand: In a sequence of vortex-line reconnections that takes just a few hundred milliseconds, the knot morphs into two unconnected rings, which then drift apart. (D. Kleckner, W. T. M. Irvine, Nat. Phys. 9, 253, 2013, doi:10.1038/nphys2560 .)