Hydrodynamic Performance of an Undulatory Robot: Functional Roles of the Body and Caudal Fin Locomotion

Both body undulation and caudal fin flapping play essential locomotive roles while a fish is swimming, but how these two affect the swimming performance and hydrodynamics of fish individually is yet to be known. We implemented a biomimetic robotic fish that travel along a servo towing system, which can be regarded as “treadmill” of the model. Hydrodynamics was studied as a function of the principal kinetic parameters of the undulatory body and caudal fin of the model in a self‐ propelled condition, under which the time‐averaged measured axial net force becomes zero. Thrust efficiency was estimated from two‐dimensional digital particle image velocimetry (DPIV) measurements in the horizontal and mid‐caudal fin plane. The Single‐Row Reverse Karman wake (2S) is commonly observed in many previous studies of live fish swimming. However, we show that a Double‐Row Two‐Paired vortices (2P) wake was generated by the robotic model for most kinetic parameter combinations. Interestingly, the 2S wake emerged within the results of a narrow range of robotic caudal fin pitch angles (0≤θ≤10°), occurring concurrently with enhanced thrust efficiency. We also show that, compared with the effect of body wavelength (λ), the wake structure behind the robotic swimmer is more sensitive to the Strouhal number (St) and caudal fin pitch angle (θ).