Propulsion Mechanism And Experimental Study On Active/Passive Deformation Fish Fin

With the change of the world economy and energy situation, national marine security has become more and more important nowadays. On the one hand, marine reserves infinite resources which could meet the needs of country’s economic development. On the other hand, marine occupies an extremely important military status. It would be conducive to the protection of national security to obtain military superiority of the ocean. Currently, human mainly rely on the conventional propeller propulsion to explore the ocean, however, it already can’t meet the urgent needs of ocean exploration due to its low efficiency propulsion, poor performance of stealth and other shortcomings. People’s attention naturally turned to fish, which has been perfectly adapted to the marine environment and developed underwater robotic fish. The robotic fish attracts researchers’big concern because of its high swimming efficiency, high maneuvering ability and good stealth performance,In this thesis, the high propulsion performance of the fish fin is focused. We first analyze the kinematics performance of the pectoral fin of Koi Carp, and then investigate the fin flexibility’s effect on the propulsion performance systematically. We also design active variable surface area robotic fin and active deformation robotic fin and conduct experiments on these two fins to study their thrust force and efficiency, which will be useful for the developing of future underwater vehicles. The main contents and contributions of this paper are listed as follows:(1) We study the Koi Carp and take pictures of its pectoral fin in its different swimming postures with a high-speed camera system. Then we analyze the pectoral fin surface characteristics by using digital image processing methods, and calculate the surface area change in a motion cycle. We also use the singular value decomposition to decompose the three dimensional complex motions of pectoral fin, and obtain its four basic movements:cupping, bending, expansion and undulation, which will be a guide for the design of biomimetic robotic fin.(2) We adopt the Fluid-structure interaction method to study the effect of fin flexibility on the propulsion performance systemically. We first build a simplified model of the fin and then analyze and compare three models, namely, rigid fin, flexible fin and variable flexibility fin. It is found out that the flexible fin demonstrates a delay response and exhibits a totally different fluid filed structure, and the variable flexibility fin possesses the best performance which coincides with the real fish fin structure. Then we study the effect of Young’s modulus, the thickness and the bending stiffness. The results demonstrate that the flexibility occupies a complex relationship with the propulsion performance. Optimal flexibility exists to achieve the best performance under the certain kinematic condition. Meantime, we build a two-links fin model and study the effect of link flexibility and hinge stiffness. We investigate their pressure contours, structure deformations and hydrodynamic forces and find that the rigid-flexible fin can generate the largest thrust force with larger hinge stiffness. However, when the hinge stiffness decreases, the medium flexible-medium flexible fin behaves the best.(3) Based on the bionic research on the pectoral fin, we find that the fin exhibits complex three dimensional motions and the fin surface characteristics change significantly during fin motions. We come up with the idea of active deformation underwater propulsion and then elaborate the concept. Then inspired by the surface area instantaneous change in a motion cycle, we design two variable surface area fin models and come up with four control strategies for the surface area change. We measure the hydrodynamic forces and calculate the efficiency based on an experimental system. The first fin model is semi-active variable area fin, whose surface area change is driven by the fluid force. It is found out that the fin with larger surface area in out-strokes can generate larger thrust force. The second fin model is fully active variable fin, whose surface area change is totally under artificial control. We detailed compare the thrust force and efficiency of four control strategies and focus on the performance of the modes of large surface area in in-strokes and out-strokes. We analyze the area change parameters’effect, such as the surface area change ratio and time and their coupling relationship with the kinematic parameters, for example, the pitching frequency and amplitude. The results show that larger surface in in-strokes can achieve relatively larger propulsion performance under certain parameters, which providing inspiration for the future underwater propulsion mechanism designs. It can also be found out from the results that the performance of variable surface area fin is closely connected with the morphological parameters such as surface area change ratio, surface area change time and kinematic parameters such as frequency and amplitude of the pitching motion. In order to obtain a better propulsion performance, these parameters should be chosen carefully.(4) To achieve high-speed, high efficiency and high maneuverability at the same time for the underwater robot, we adopt the active deformation concept to the caudal fin and design a novel caudal fin which can realize steady-state morphological changes. It can change from lunate tail to trapezoidal shape fin, then to fan shape caudal fin which is to mimic the high swimming-speed of tuna, the high acceleration performance of pike and the high maneuverability of banded butterfly fish. Then we set up an experimental system and analyze the propulsion performance and efficiency of the novel active deformation caudal fin with different kinematic parameters.