Effect of Cross-Joints Fin on the Thrust Performance of Bionic Pectoral Fins

Cownose rays have a high forward propulsive performance due to their unique oscillating fin structure (named the cross-bracing structure), which differs from undulatory fish. The cross-bracing structure obtained through anatomy, on the other hand, is extremely complex. Hence, researchers used simple structures to model the biological structure to target the individual factors that affect cownose ray cruising performance. This paper simplified the cross-bracing fin structure to a cross-joints fin (CJF) structure with 18 designs. CJFs had five different joint widths (2 mm, 3 mm, 4 mm, 5 mm, and 6 mm) in both spanwise and chordwise directions, and these had two fin thicknesses (1.5 mm, 2.5 mm). The joint widths of CJF are related to the stiffness of the spanwise and chordwise fins (Fin stiffness increases with joint width). The experiments were conducted in a still water tank (1.5 m × 0.8 m × 0.8 m) with three stroke amplitudes (30^◦, 50^◦, 70^◦) and three flapping frequencies (0.4 Hz, 0.6 Hz, 0.8 Hz) for each fin, making up 162 distinct sets of data. The experimental results showed the following: (1) at low wingtip Reynolds numbers, the high stiffness of the CJF causes a significant reduction in thrust. In particular, high stiffness results in a low thrust averaged from all motion parameters; (2) at high wing tip Reynolds numbers, the effect of changing spanwise stiffness on thrust is more significant than the effect of changing chordwise stiffness. This paper compares the effects of spanwise and chordwise stiffness on thrust performance, indicating that the magnitude of spanwise stiffness should be considered when designing the bionic oscillating pectoral fin structure.