Hydrodynamic Analysis Of Cownose Ray's Swimming And Research Of Bionic Robotic Fish

With the development of ocean resources exploring and marine sovereignty safeguarding, underwater propulsion has been becoming a hot research field concerned by more and more scientists and engineers all over the world. Under such circumstances, some biologically inspired propulsors are suggested and designed to mimic fish swimming modes. One typical case is pectoral lift-based mode inspired by Cownose Ray. Compared to the traditional propellers, this bionic propulsion mode presents some obvious advantages with lower oscillation frequency, higher lift force, greater maneuverability and lower noise. Thus, it is of great significance to improve performance of underwater autonomous vehicles.In this dissertation, kinematics, hydrodynamics and vortex structure of Cownose Ray in swimming are explored by taking advantages of the finite element simulation and experimental validation with the robotic fish. Furthermore, it is ascertained how morphology of Cownose Ray with pectoral lift-based mode is adapted to the complex environment. In particular, three characteristics, in terms of autonomous swimming, spanwise flexibility and asymmetric oscillation, are respectively studied. The main contributions in this work can be summarized as follows:1. Morphology of Cownose Ray adapting to the living circumstance is at first introduced. By observing the swimming kinematics and analyzing the force of Cownose Ray, morphology of Cownose Ray does reduce drag force and increase propulsive force, which is naturally selected during a long period adapting to water surroundings.2. A three-dimensional finite element model is established. Under this computational model, the autonomous swimming mode is simulated and analyzed with the coupling of fluid-structure, by using fin-ray animations to imitate the muscle internal force and using fin-surface combination to mimic the pectoral fins oscillation. Swimming behaviors of Cownose Ray are illustrated by kinematic results, hydrodynamic results and vortex structures. Moreover, swimming performance of pectoral lift-based mode is discussed at length.(1) As all the pectoral fin rays oscillate in specified orders, an autonomous propulsive wave appears from the leading edge to the trailing edge. In a result, the Cownose Ray computational model is propelled, to validate that the propulsive waves of pectoral oscillation can generate effective propulsion and maneuvering as well;(2) The propulsive net force presents double crests in one period. It shows both the upstroke and the downstroke of pectoral fins can produce positive power. This feature of the continuous power indicates high propulsive efficiency of pectoral lift-based mode;(3) Cownose Ray can skillfully control the oscillation of pectoral fins to form the useful con-Karman gait, and therefore induce shed fluid to attain the required propulsive force and lift force.3. An autonomous robotic fish with pectoral lift-based mode is designed and developed, based on the observation of fish swimming and corresponding simulations. The relative contribution of the fin beat frequency, the largest amplitude of fin rays and the phase difference between the adjacent fin rays to the forward velocity is originally investigated:(1) When the pectoral fins flap in frequencies lower than 0.8Hz, the forward velocity of the robotic fish keeps direct ratio to these three variables;(2) When the pectoral fins flap in frequencies between 0.8Hz and 1Hz, the forward velocity has no remarkable relationship with fin beat frequency;(3) When the pectoral fins flap in frequencies between 1Hz and 2Hz, the forward velocity is only dependent on the phase difference between the adjacent fin rays.4. Affection of spanwise flexibility of the pectoral fins on the swimming of the robotic fish is also explored, by measuring and quantitatively comparing the hydrodynamic force and vortex structure of the robotic fish. This prototype can be equipped with both rigid and flexible fin rays. Experimental results show that spanwise flexibility not only increases the forward and backward propulsive force, but also greatly improves the swimming stability.5. Pectoral flapping of Cownose Ray exhibits asymmetric oscillation, which means a slower downstroke following a quicker upstroke in one flapping circle. Asymmetric oscillation with flexible fin rays is quantitatively investigated through comparisons among mathematical model, kinematics and hydrodynamics of the robotic fish. A better asymmetric range and optimum factor are also studied and discovered. In detail, compared to the symmetric oscillation, asymmetric oscillation with a better asymmetric range can improve propulsive performance, and meanwhile swimming becomes more stable; as for the experimental range,, the velocity and the propulsive force of the robotic fish is optimum in the case of 0.56 of asymmetric factor.All the above achievements make a beneficial exploration for the imitation on Cownose Ray swimming from "imitation in shape" to "resemblance in function". In this work, the simulation of autonomous swimming will give a new alternative to the mechanics of pectoral lift-based swimming mode in the viscous fluid. Discussion on the oscillation variables, spanwise flexibility of the pectoral fins and the pectoral asymmetric oscillation should have potential for the design and exploration of underwater autonomous vehicles, theoretically and applicably.