Study On Mechanism Of Thunniform Bio-inspired Mode Swimming Under Self Propulsion

As a new growing point on mechanical discipline, biomimetics is of great significance in our national security, ocean development, underwater exploration and other fields. Over the past decade, fishlike robots imitating different bio-inspired modes have been available at home and abroad. The success of developed robot will bring dramatic changes to underwater propulsion for future. The rapid rise of the biomimetics needs the biological mechanics theory to support. At present, the mechanism and the prototype has reached a time in parallel. Any new breakthrough in the mechanism will bring new inspiration to the prototype development. To explore the mechanism, scholars at home and abroad spend a lot of theoretical analysis and experimental methods, attempt to reveal the essence of the problems. Unfortunately, either the prototype-based experimental results, or based on existing kinematic simulation results are far from the expected performance obtained by the biological mode. Therefore, this dissertation starts from the mechanism revelation, and studies the dynamics of fishlike robot, the numerical methods of bio-inspired self-propulsion and fast, efficient swimming mechanism.Fishlike robot dynamics is the basis for the development of new prototype. Not a good solution for the current issue, a new dynamic model of fishlike robot with undulatory swimming is proposed. First, thunniform bio-inspired mode is selected as study object and its morphology and kinematics are described. And on this basis, the dynamic model is developed using Kane method with separating the fishlike robot into multiple discrete links. The validity of dynamic model is verified by means of computing example. Second, the two-joint prototype is developed on the basis of dynamic model and the underwater experiment is promoted to measure propulsive performance. However, the results are not satisfactory. Accordingly, the main reason leading to the unsatisfactory results is that the mechanisms are not clearly revealed.To reveal the propulsive mechanism, the bio-inspired self-propulsion is studied and a new numerical method for solving the self-propulsion is proposed. First, the self-propulsion domain is separated into fluid, fish and moving grid subdomain and the governing equations for each subdomain are established. Second, given the fish kinematics as an input, the locomotion will be transferred to the fluid grid to form incentive on the surrounding fluid. The fluid forces are obtained by solving the fluid equations and will be transferred to fish to be treated as the external forces to drive the fish to generate rigid motion. Then the swimming performance is obtained by solving the fish equations and thus fish and fluid interaction is achieved. Finally, the validity of numerical method is verified by use of two classical examples. The proposed mechod will be applied to the mechanism study of the normal, spanwise flexibility, and burst-and-coast self-propulsion as follows.By using the numerical method, the normal self-propulsion of bio-inspired mode is simulated. The dynamic process is solved over the fish starting from rest, gradually accelerating and finally converging to the steady swimming. The time history rules of kinematic and energetic parameters are revealed. Both the body behavior of tail beat frequency and maximum amplitude, and the caudal fin behavior of maximum angle of attack and phase difference are explored on the self-propelled performance. By extracting the three-dimensional flow structure, the mechanism is revealed physically and superior or inferior performance is judged for bio-inspired self-propulsion.To improve the self-propelled performance, the effect of spanwise flexibility is studied. The bio-inspired mode self-propulsion with spanwise flexibility caudal fin is solved by using the numerical method and is compared with normal self-propulsion. The comparative results show that more superior performance is obtained by using the spanwise flexibility caudal fin over the rigid one. Specifically, using a"bow"fin can significantly reduce the lateral force and power consumption and increase the swimming stability at the premise of small reduction of steady velocity and thrust. Using a"scoop"fin can greatly increase the steady velocity and thrust, and thus increase the propulsive efficiency, while a slight increase in the lateral force and power consumption. Therefore, the appropriate adjustment of the fin shape on the one hand can produce more thrust and higher propulsive efficiency; on the other hand can reduce the lateral force and power consumption.With efficient and energy saving features, to study burst-and-coast swimming mechanism is very important. By using the numerical method, the kinematic and energetic performance of the burst-and-coast self-propulsion is solved by changing the tail beat number and duty ratio. Choose small tail beat number and small duty ratio is benefit for obtaining lower power consumption and high speed power ratio, and thus consumpt the minimal power in obtaining the same steady velocity. On this basis, comparison of the burst-and-coast and the normal swimming shows that the two modes have their own advantages and disadvantages on the kinematics and energetics aspects. For achieving the same velocity, select the nomal swimming better; while swimming the same distance, select the burst-and-coast swimming can greatly reduce power consumption and save energy significantly.