The use of robotic apparatus for studying propulsion performance and fluid mechanism of undulatory fish locomotion

Fish swimming results from a complex dynamic balance between drag and thrust force. To better understand the thrust performance and the fluid mechanism of undulatory fish swimming, a biorobotic model that mimic mackerel (Scomber scombrus) was developed. The robotic fish model have been used to investigate the hydrodynamic performance as a function of several principal controllable kinetic parameters: Stouhal number (St), undulatory amplitude (h), body undulatory wavelength (λ) and the caudal fin pitch angle (θ). For each combination of these kinetic parameters, there is a critical self-propulsion towing speed, which was not preset but defined as the threshold value at which the mean net force, measured on the robotic model, becomes zero. We simultaneously measured the power consumption, self-propulsion speed, and wake structure of the robotic swimmer under condition of the self-propulsion. Furthermore, the thrust efficiency was estimated using the vortex dynamic model based on Digital Particle Image Velocimetry (DPIV) technique. Primary experimental results indicated that both the caudal fin pitch angle and the Strouhal number had a significant impact on the propulsion performance and the wake structure. A wedge-like 2P wake structure was observed for most of the kinetic parameter combinations. Although the typical reverse Karman wake structure is commonly observed in many flow visualization tests of the live fishes, it only emerged within the results of a narrow range of robotic caudal fin pitch angles in current experiment, occurring concurrently with enhanced thrust efficiency.