Fig. 14

Vortex contour plots (− 20 to + 20; red: counter-clockwise, blue: clockwise) for upstream and downstream foils under in-phase (0°) flapping along FT-III and FT-IV 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).