From Rings to Smoke: Visualizing the Breakdown of Colliding Vortex Rings*

The turbulent cascade, or the means by which the energy of a flow is conveyed from large to small scales, is governed by the interactions between vortices over many scales. In order to better understand the mechanisms that govern the close-range interactions between vortices, we experimentally examine the head-on collision of two vortex rings. By seeding the vortex rings with fluorescent dye and imaging their collision with a high-speed scanning laser sheet, we visualize the breakdown dynamics of the flow in full 3D. For weak collisions at low Reynolds numbers, the colliding rings stretch radially, develop long-wavelength perturbations, and reconnect into a tiara of secondary vortex rings. Conversely, for violent collisions at high Reynolds numbers, the rings rapidly develop short-wavelength perturbations as they stretch radially before erupting into a turbulent cloud of fine-scale vortex filaments. Initiated by these instabilities, the colliding vortices break down through various distinct processes and lead to the generation of small-scale flow structures. Thus, the close-range interactions of the colliding vortices could provide new insights into the mechanistic underpinnings of the turbulent cascade.