Leapfrogging of two thick-cored vortex rings

Leapfrogging of two vortex rings of thick cores is studied in laboratory experiments. Quantitative flow measurements show that, during leapfrogging, vorticity at the outer portion of the two cores diffuses together. However, the two cores remain clearly differentiable with distinct peaks. Vortex circulation decreases at a faster rate when the two rings are aligned radially and the passing ring is closer to axis touching due to vorticity cancellation. When two consecutive passages occur, vorticity cancellation is reduced in the second passage because the passing ring is further away from axis touching compared with that in the first passage. The study also finds that when the interval between generations of two rings is small, the front ring strongly influences the formation of the rear ring, whose core has an elongated tail of vorticity on its formation. The rear ring later sheds vorticity, whose amount increases with a smaller initial separation. The shedded vorticity, when strong enough, counters the effect of the front ring and prevents leapfrogging.