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Fig. 1. Schematic diagram of a vortex boundary in a flow. (A) 3-D sketch of a
vortex ring; (B) the vortex ring on its median-symmetry plane. Circles with
inscribed arrows indicate vortex cores and their rotational sense. Two fluid
particles close to but on different sides of the vortex boundary separate from
each other faster than other arbitrary pairs of fluid particles, giving a
larger value of the finite-time Lyapunov exponent field (FTLE) at the
boundary. Adapted from Peng et al. (Peng
et al., 2007).