Numerical Study On The Bionic Fish Propelled By Caudal Fin And Pectoral Fin

Fish have streamlined bodies and fins that can easily control their posture after nature’s elimination and screening,and their swimming performance is very outstanding.Compared with the propeller propulsion system used in the traditional propulsion device,fish has advantages such as small size,high efficiency and good maneuverability,which are the key technical issues that the new bionic propulsion device needs to break through.Traditional experimental methods to explore the swimming mechanism of biomimetic fish are difficult to recover and have low capture accuracy.With the rapid development of computer technology,numerical simulation technology provides a powerful tool for the study of the mimicry fish propulsion mechanism,which can accurately capture the propulsion motion form of biomimetic fish,and has important engineering value for guiding the research and development of a new generation of underwater propulsion devices.In this thesis,numerical simulation tools are used to explore the swimming mechanism of bionic fish and provide theoretical support for the optimization and selection of joint propulsion mode.The numerical simulation method has the advantages of low cost,good repeatability and high accuracy.In this thesis,the Computational Fluid Dynamics(CFD)method is used to simulate the autonomous swimming of the bionic fish in a horizontal direction under various propulsion modes.This thesis focuses on the variation characteristics of the biomimetic fish’s hydrodynamic performance under different motion modes and parameters,which provides certain guiding significance for optimizing the parameters related to the joint propulsion mode to obtain better motion performance.Firstly,the hydrodynamic performance of the relevant parts of the biomimetic fish was numerically simulated by computational fluid method and dynamic grid technology,and the hydrodynamic performance of the fish body,the oscillating hydrodynamic performance of the pectoral fin,and the self-propelled motion performance of the fish body were analyzed.The obtained results were compared with the relevant literature and experimental results,and the mesh convergence and time step convergence were analyzed.It is found that the numerical calculation model adopted in this thesis can effectively solve the parameters of the hydrodynamic force of the fish body,the hydrodynamic force of the pectoral fin and the self-propulsion velocity of the fish body,which verifies the accuracy and reliability of the numerical calculation method adopted in this thesis.Secondly,by placing the fish trevally in the BCF mode in the calculation domain and changing the kinematic parameters such as frequency,wave number and amplitude,the propulsive performance of the body caudal fin in the propulsive mode under different kinematic parameters is explored,with emphasis on exploring and analyzing the changes in the propulsive speed,propulsive efficiency,longitudinal force,lateral force,the pressure distribution on the surface of the fish body and the tail vortex structure.The differences of different frequency,wave number,amplitude kinematics and other characteristic parameters on the swimming propulsion performance of the bionic fish were analyzed to reveal the best swimming propulsion characteristic morphology of the bionic fish.Then,the motion rules of pectoral fins of longocephidae in MPF mode are studied to build a numerical model of pectoral fin oscillation.The swimming speed and longitudinal force of the bionic fish in swimming process are analyzed by changing kinematic parameters such as the average flapping Angle and the average longitudinal Angle under the two-degree-of-freedom lift model and the phase difference between the forward and backward flapping under the three-degree-of-freedom.Focusing on the surface pressure distribution and vortex structure changes of flow field,the propulsion performance of the pectoral fins in the two-degree-of-freedom and three-degree-offreedom motion propulsion modes under lift mode was revealed,and the mechanism of the propulsion performance variation of the pectoral fins under different parameters was clarified.Finally,the joint motion of body caudal fin oscillation and pectoral fin oscillation is studied and analyzed,and the numerical calculation model under the joint propulsion of pectoral fin-caudal fin is established.By constructing the continuous forward motion form of bionic fish under the action of swinging body caudal fin and pectoral fin,the oscillating frequency of different body caudal fin and the swimming speed,swimming efficiency and longitudinal force changes of pectoral fin-caudal combined propulsion under different pectoral fin motion models were analyzed,and the pressure distribution and tail vortex structure on the surface of the fish body were emphatically observed.The source of propulsive force and the difference of propulsive performance were further revealed.The numerical simulation results and relevant conclusions provided certain technical support for the research and development of bionic robotic fish with joint propulsive mode.