Effects of pectoral fins' spanwise flexibility on forward thrust generation

Pectoral fins of manta rays have been a great source of inspiration for propulsive mechanism of underwater vehicles in the last decade. In the existing literature, effects of the chordwise flexibility of these fins on the forward thrust generation has been widely studied, while the study on the spanwise flexibility is considerably lacking. In this paper, we aim to investigate the effects of the fins' spanwise flexibility (quantified by bending stiffness) on the forward thrust generation. An experiment was conducted in a water channel to measure the forward thrust generated by 16 fins under free stream condition (0.5 m/s). The fins' bending stiffness ranged from 0.018 N/mm to 0.499 N/mm. All the fins had the same span length and surface area in order to preserve the hydrodynamic integrity of the experiment. The experimental results showed that for the range of the spanwise flexibility considered in this study, the forward thrust generation increases by decreasing the fins' spanwise flexibility until a certain bending stiffness value. After that the rate of the change in the thrust generation becomes very minor, indicating a plateau region.