We simulate and analyze the head-on collision between vortex rings at
ReΞ0ββ= 4,000. We utilize an adaptive, multi-resolution solver, based
on the lattice Green's function, whose fidelity is established with integral
metrics representing symmetries and discretization errors. Using the velocity
gradient tensor and structural features of local streamlines, we characterize
the evolution of the flow with a particular focus on its transition and
turbulent decay. Transition is excited by the development of the elliptic
instability, which grows during the mutual interaction of the rings as they
expand radially at the collision plane. The development of antiparallel
secondary vortex filaments along the circumference mediates the proliferation
of small-scale turbulence. During turbulent decay, the partitioning of the
velocity gradients approaches an equilibrium that is dominated by shearing and
agrees well with previous results for forced isotropic turbulence. We also
introduce new phase spaces for the velocity gradients that reflect the
interplay between shearing and rigid rotation and highlight geometric features
of local streamlines. In conjunction with our visualizations, these phase
spaces suggest that, while the elliptic instability is the predominant
mechanism driving the initial transition, its interplay with other mechanisms,
particularly the Crow instability, becomes more important during turbulent
decay. Our analysis suggests that the geometry-based phase space may be
promising for identifying the effects of the elliptic instability and other
mechanisms using the structure of local streamlines. Moving forward,
characterizing the organization of these mechanisms within vortices and
universal features of velocity gradients may aid in modeling the turbulent
cascade.Comment: 34 pages, 11 figure