Scaling-based design approach for tailbeat fish-inspired robots swimming at high speed and propulsive efficiency

The synchronous integration of multiple parameters in developing high-speed robotic fish remains a challenge. Consequently, most existing fish-like robots have been created through trial-and-error methods. This paper presents a comprehensive scaling-based design approach for developing a 0.32-m-long robot, KUFish-II, capable of achieving a target speed of 1.75 m/s or 5.5 body lengths per second (BL/s). This speed was predicted based on three physical parameters: Froude number, Strouhal number, and reduced frequency. To design a robot capable of achieving the speed, the proposed scaling law specified the stroke amplitude, fin area, tailbeat frequency, and torque needed for required thrust generation and high swimming efficiency. Subsequent experiments showed good agreement between the measured and predicted thrusts, frequencies, and swimming speeds. Swimming at a maximum speed of 1.79 m/s (5.6 BL/s), with a low Strouhal number and a narrow peak-to-peak amplitude of the tail, KUFish-II can be regarded as one of the few fast-swimming robots capable of achieving efficient locomotion with a low cost of transport. These results confirm the reliability of the proposed approach and highlight its potential use for designing efficient, large-scale, fast-swimming robotic fish.