Helicity Decay in Vortex Rings*

The helicity of vortex loops is captured in the loops' linking, writhe, and twist. In viscous fluids, helicity loses its conserved status and is dissipated through only one of these modes -- twist. By generating helical vortices using hydrofoils, we are able to measure their helicity and its decay over a range of viscosities. We report the dissipation and exchange dynamics and interpret them in terms of the evolution of the vortex core geometry and size, which we resolve using a novel method.