Fig. 6. The boundary of the vortex derived from LCS. The left solid line shows the attracting LCS from backward FTLE calculation while the right-hand-side solid line shows the repelling LCS from forward FTLE calculation. Broken lines are spline lines connecting the LCS. The fin (the curve 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 because of the limitation in integration time T. Every other vector is shown.