Scombrid fishes and tuna are efficient swimmers capable of maximizing performance to escape predators and save energy during long journeys. A key aspect in achieving these goals is the flexibility of the tail, which the fish optimizes during swimming. Though, the robotic counterparts, although highly efficient, have partially investigated the importance of flexibility. We have designed and tested a fish-like robotic platform (of 30 cm in length) to quantify performance with a tail made flexible through a torsional spring placed at the peduncle. Body kinematics, forces, and power have been measured and compared with real fish. The platform can vary its frequency between 1 and 3 Hz, reaching self-propulsion conditions with speed over 1 BL/s and Strouhal number in the optimal range. We show that changing the frequency of the robot can influence the thrust and power achieved by the fish-like robot. Furthermore, by using appropriately tuned stiffness, the robot deforms in accordance with the travelling wave mechanism, which has been revealed to be the actual motion of real fish. These findings demonstrate the potential of tuning the stiffness in fish swimming and offer a basis for investigating fish-like flexibility in bio-inspired underwater vehicles.

翻译：鲭科鱼类和金枪鱼是高效的游泳者，能够在长距离迁徙中最大化性能以躲避捕食者并节省能量。实现这些目标的关键在于尾部的灵活性，鱼类在游泳过程中会对此进行优化。然而，尽管机器鱼具有高效率，但其对灵活性的重要性研究尚不充分。我们设计并测试了一种类鱼机器人平台（长度为30厘米），通过在其尾柄处安装扭转弹簧使尾部具有柔性，以量化其性能。我们测量了身体的运动学、受力和功率，并与真实鱼类进行了比较。该平台的摆动频率可在1至3赫兹之间调节，在自推进条件下速度超过1体长/秒，斯特劳哈尔数处于最优范围。研究表明，改变机器人的频率会影响其产生的推力和功率。此外，通过使用适当调谐的刚度，机器人能够按照行波机制变形，该机制已被揭示为真实鱼类的实际运动方式。这些发现证明了调节刚度在鱼类游泳中的潜力，并为研究仿生水下航行器的类鱼灵活性提供了基础。