Symmetrical collision of multiple vortex rings

In this work, we investigate the motion, interaction, and simultaneous collision between many initially stable vortex rings arranged symmetrically in two initial configurations, three and six rings making an angle of 60 and 120 between their straight path lines, respectively. We report results for laminar vortex rings in air obtained through numerical simulations of the ring velocity, pressure, and vorticity fields, both in free flight and during the entire collision. Each collision was studied for small Reynolds numbers Re < 10(3) based on both the self-induced velocity and diameter of the ring. The case of three rings produces secondary vortical structures formed by laterally expanding dipolar arms with top and bottom secondary vortex rings. The case of six colliding rings produces, as secondary structures, two big rings moving in opposite directions, a process that reminds us of the head-on collision of two rings [T. T. Lim and T. B. Nickels, Instability and reconnection in the head-on collision of two vortex rings,Nature 357, 225-227 (1992)] under a hypothetical time reversal transformation. Both collisions display a characteristic kinetic energy evolution where mean collision stages can be identified within the range of Reynolds numbers investigated here. Published by AIP Publishing.