Fig. 2. A schematic drawing of a vortex boundary in a flow. Circles with inscribed arrows indicate vortex cores and their rotational sense. A pair of adjacent 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 FTLE at the boundary. Trajectories can be followed in backward-time (A) to reveal attracting LCS, and in forward-time (B) to reveal repelling LCS boundaries.