  The idea that the knottedness (hydrodynamic Helicity) of a fluid flow is
conserved has a long history in fluid mechanics. The quintessential example of
a knotted flow is a knotted vortex filament, however, owing to experimental
difficulties, it has not been possible until recently to directly generate
knotted vortices in real fluids. Using 3D printed hydrofoils and high-speed
laser scanning tomography, we generate vortex knots and links and measure their
subsequent evolution. In both cases, we find that the vortices deform and
stretch until a series of vortex reconnections occurs, eventually resulting
several disjoint vortex rings.
  This article accompanies a fluid dynamics video entered into the Gallery of
Fluid Motion at the 66th Annual Meeting of the APS Division of Fluid Dynamics.
