Median fins of fish-like swimmers play a crucial role in linear acceleration and maneuvering processes. However, few research focused on untethered robotic fish experiments. Imitating the behaviour of real tuna, we developed a free-swimming bionic tuna with a foldable dorsal fin. The erection of dorsal fin, at proper conditions, can reduce head heave by 50%, enhance linear acceleration by 15.7%, increase turning angular velocity by 32.78%, and turning radius decreasing by 33.13%. Conversely, erecting the dorsal fin increases the wetted surface area, resulting in decreased maximum speed and efficiency during steady swimming phase. This finding partially explains why tuna erect their median fins during maneuvers or acceleration and fold them afterward to reduce drag. In addition, we verified that folding the median fins after acceleration does not significantly affect locomotion efficiency. This study supports the application of morphing median fins in undulating underwater robots and helps to further understand the impact of median fins on fish locomotion.

翻译：鱼类游动体的中央鳍在直线加速与机动过程中起着至关重要的作用。然而，针对无缆机器鱼的实验研究尚少。通过模仿真实金枪鱼的行为，我们研制了一种配备可折叠背鳍的自由游动仿生金枪鱼。在适当条件下展开背鳍，可使头部垂荡幅度降低50%，直线加速度提升15.7%，转向角速度增加32.78%，同时转向半径减小33.13%。反之，展开背鳍会增大浸湿表面积，导致定常游动阶段的最大速度与推进效率下降。这一发现部分解释了为何金枪鱼在机动或加速时会展开中央鳍，随后又将其折叠以减少阻力。此外，我们验证了加速后折叠中央鳍不会显著影响运动效率。本研究支持了形态可变中央鳍在波动推进水下机器人中的应用，并有助于进一步理解中央鳍对鱼类运动的影响。