Lagrangian flow visualization of multiple co-axial co-rotating vortex rings

This paper, for the first time, experimentally observes the detailed interacting phenomena of multiple co-axial co-rotating vortex rings using the method of finite-time Lyapunov exponent field. Besides the most attractive leapfrogging in dual vortex ring flows, several distinct phenomena are also found. The merger of squeezing is first observed in multiple vortex rings, resulting from the strong axial compressive induced effect. The inner vortex ring becomes axis-touching and cannot recover to the previous status. The merger due to elongation is already found in the previous studies. The inner vortex ring is elongated and distorted. The detachment of several independent vortex rings indicates that vortex merger has its limit, which is also a newfound phenomenon.Graphical abstract

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