Cascade leading to the emergence of small structures in vortex ring collisions

Our understanding of turbulence still lacks a mechanistic description for how kinetic energy, added into a flow at large scales, cascades to smaller and smaller scales until it is eventually dissipated via molecular fluctuations. We experimentally show that the head-on collision of two laminar vortex rings into a turbulent cloud uncovers the first direct evidence of a self-similar cascade. When the coherent vortex rings approach each other, the vortex cores deform into tent-like structures, and the mutual strain causes them to locally flatten into extremely thin vortex sheets. These sheets then break down into smaller secondary vortex filaments, which themselves rapidly flatten and break down into even smaller tertiary filaments. By performing numerical simulations of the full Navier-Stokes equations, we also resolve one iteration of this cascade and highlight the subtle role that viscosity must play in the puncturing of a vortex sheet. The concurrence of this observed iterative cascade of instabilities with those of recent theoretical predictions could provide further insights into the latent dynamical mechanisms of turbulent flows, namely whether singularities, or blow-ups, exist in the equations governing fluid motion.