Fig. 12

Z-vorticity contours (− 20 to + 20; red: counter-clockwise, blue: clockwise) for upstream and downstream foils under in-phase (0°) flapping along FT-I and FT-II trajectories showing different wake–foil interaction regimes: (a, b) weak/no wake interaction, (c, d) direct wake-foil collision, and (e, f) partial interaction based on the proximity parameter (Πvf).