| Unmanned underwater vehicles (UUVs) have played an important role in the ocean exploitation and scientific investigation. Large marine animals, such as turtles and penguins, have evolved with unique physical structure and motion pattern after billions of years of evolution. Owing to these gifts, they can move freely in water with good flexibility, low noise and high efficiency. The significance obviously is there for the improvement of the motion performance of UUVs if we can study the flapping-foil propulsion mechanism of the marine animals and apply it to the UUVs.Currently, underwater flapping-foil propulsion research is in the start-up stage and the propulsion mechanism is unclear. It’s challenging for us to use this new propulsion mode on the UUVs. The motion character and manipulation mechanism of flapping-foil propelled UUVs remains to be investigated. In this paper, the flapping-foil propulsion mechanism, the relationship between the flapping motion and the propulsion performance, the fluid dynamics and the maneuverability of the flapping-foil propelled UUV are investigated with a combined method including numerical simulations, theoretical predictions and experimental studies.The main research contents and innovative works can be summarized as follows:1. Computational fluid dynamics (CFD) simulations of underwater biomimetic flapping-foil.(1) A3degrees of freedom (DOF) motion model is established according to turtle forelimb movement characteristics.(2) With the sliding mesh method and dynamic mesh method, two-dimensional CFD studies are performed on the1DOF,2DOF and3DOF flapping foils, respectively. The thrust generation mechanism is illustrated.(3) For1DOF flapping-foil movement pattern, the influence of foil thickness, foil camber, rotation axis position, and the time-asymmetric movement pattern on the hydrodynamics performance of the flapping-foil is studied. The optimal foil thickness and the rotation axis position are given. The variation of flapping-foil lift and thrust characteristics with the influencing factors is studied.(4) For3DOF flapping-foil movement pattern, the relationship between motion parameters, thrust and propulsion efficiency is studied. The propulsion efficiency of1DOF,2DOF, and3DOF motions are compared quantitatively.2. Shape design and hydrodynamic characteristics of the flapping-foil UUV. (1) A new flapping-foil UUV shape is designed after the biomimetic research of sea turtle morphology.(2) Using sliding mesh technique, the thesis calculates hydrodynamic performance of the1DOF flapping-foil thruster. The aspect ratio corresponding to optimum propulsive efficiency is given. The fitting functions of average thrust coefficient, average thrust and flapping-foil motion parameters are given by full factorial experimental simulation. Through the response surface analysis, this paper gives the size of the main effects and interaction effects which affecting the flapping-foil propulsion performance. On this basis, respectively, the relationship between advance ratio, bias angle and propulsion performance of the thruster is analyzed.(3) The thesis considers flapping-foil thruster and UUV’s body as a whole to study the unsteady hydrodynamic interference with each other, and focuses on the hydrodynamic interference with different inflow velocity and dimensionless gap between the thruster and UUV’body.(4) Respectively, the thesis calculates hydrodynamic characteristics of the flapping-foil UUV’s tri-axial ellipsoid body at high angles of attack and sideslip, and the hydrodynamic expression of drag, lift and moment coefficients change with the two are given. By simulating the cantilever pool experiments, rotary derivatives are calculated.3. Dynamic modeling and motion characteristics simulation of the flapping-foil UUV.(1) A6DOF mathematical model of the UUV motion is developed based on the Newton-Euler equations. An accurate model for the hydrodynamic forces of the flapping foil is established. The flapping foil model is validated with the CFD results.A motion simulation platform is established in the Matlab-Simulink environment. The motion characteristics of the flapping-foil UUV in the longitudinal and the horizontal planes are simulated respectively. The influence of numbers of thrusters and other thruster motion parameters on the motion characteristics of the UUV is studied.4. Experimental study on the hydrodynamics performance of flapping-foil(1). An experiment platform for the measurement of flapping-foil unsteady hydrodynamic forces is build.(2). An experiment on the1DOF motion of a NACA0012foil is performed. The relationship between the motion parameters, thrust and propulsion efficiency is given.(3). The influence of section shape, chord length and span length of the foils on the hydrodynamics performance of flapping foils is studied.(4) Chosen specific situations, the CFD simulation result is validated by comparing it with the experiment data.5. Flapping-foil UUV prototype development and experiment(1) A prototype of flapping-foil UUV is developed. The overall control hardware and software systems of experimental prototype is designed according to collaborative movement characteristics of experimental prototype and the requirement of multi degree of freedom movement in space.(2). The navigation performance experiments of the prototype is conducted in open water.The influence of flapping-foil frequency and amplitude on the UUV velocity and acceleration is studied. The maneuverability of three different motion modes is compared. The feasibility of heave motion is validated by controlling the bias angle of flapping-foil thrusters. Chosen specific situations, the dynamic model and simulation method of the flapping-foil UUV motion is validated by comparing the result with the experiment data. |
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