Fig. 3. The boundary of the vortex derived from Lagrangian coherent structures (LCS). The left solid line shows the attracting LCS from backward FTLE calculation while the right solid line shows the repelling LCS from forward FTLE calculation. Broken lines are spline lines connecting the LCS. The fin (curved with high brightness inside the lines) can be seen embedded inside the vortex. The attracting and repelling LCS do not intersect to give the entire vortex boundary due to the limitation in integration time T. Adapted from Peng et al. (Peng et al., 2007).