(a) Dependence of the FTLE on the time interval $t1−t0$ for $Re=500,Str=0.05$⁠, S = 0 and two initial points at $t0=−1/4$ with $(x,y)=(−0.16,0.14)$ (blue) and $(x,y)=(0.16,0.14)$ (red). (b) Dependence on $Str$ of the space-averaged FTLE for $(t1−t0)/Str=100$ and Reynolds numbers $Re=1$ (blue), 5 (red), 10 (yellow), 20 (purple), 50 (green), 100 (cyan), and 200 (dark red).