From rings to smoke: visualizing the breakdown of colliding vortex rings

We experimentally probe the head-on collision of two vortex rings at high Reynolds numbers and visualize in real-time how the initially coherent cores rapidly break down into a turbulent cloud. The colliding vortex rings are seeded with fluorescent dye and illuminated with a scanning laser sheet that is synchronized with a high-speed camera in order to visualize the breakdown dynamics of the flow in full 3D. We observe that for collisions at sufficiently high Reynolds numbers, the vortex cores develop perturbations consistent with the elliptical instability and form an array of slender vortex filaments perpendicular to the collision plane that traverse the narrow gap between the colliding vortex rings. The close-range interactions of these perpendicular filaments with both each other and the cores of the vortex rings trigger the rapid breakdown of the vortices, resulting in the generation of fine-scale vortex filaments. This breakdown is mediated by the iterative flattening and splitting of these perpendicular vortices into successively smaller filaments. We find that the breakdown of these colliding vortex rings at high Reynolds numbers could thus provide new insights into the mechanistic underpinnings of the turbulent cascade.