Propulsion Curve Optimization Of Bionic Pectoral Fin With Three DOFs Based On CFD

Biomimetic robotic fish is one of the important directions in the research of underwater robots,and has broad application prospects in underwater resource exploration,military investigation,and other fields.Due to the difficulty in observing changes in the flow field during swimming,it is difficult to establish a hydrodynamic model for underwater biomimetic robotic fish.Therefore,determining a reasonable motion curve and stroke ratio for the pectoral fin to achieve higher swimming efficiency and drag reduction performance is a key issue in the research of biomimetic robotic fish.This paper focuses on the collaborative propulsion of robotic fish with 3-DOF pectoral fins,flexible bodies,and tail fins,and mainly studies the cooperative motion laws of efficient pectoral fin propulsion.(1)CFD optimization was conducted on elliptical and "8" shaped oscillation curves,respectively.The hydrodynamic characteristics of the pectoral fin,pressure distribution upstream and downstream of the fin surface,velocity vector of the surrounding flow field during pectoral fin oscillation,and three-dimensional vortex structure were analyzed,when the pectoral fin advances in an elliptical swinging curve,after two optimizations,its resistance peak decreases,but at the same time it loses a certain amount of thrust.The reduction of resistance is mainly achieved by reducing the pressure on the downstream surface.The pectoral fin generates a jet through flapping and generates a backward vortex ring to provide thrust;When the pectoral fin advances in an "8" shaped swing curve,after two optimizations,the resistance of the pectoral fin decreases while its propulsion performance further improves.When the amplitude of the pectoral fin swing is 35°,the robot fish experiences the least resistance in the flow field,resulting in the maximum average swimming speed of the robot fish.During the swing process,the pectoral fin generates three vortex rings in different directions to generate thrust and lift.(2)Determine the proportion of time between the propulsion and recovery travel within a unit propulsion cycle as a measure of the efficiency of pectoral fin propulsion.Based on the swimming sliding behavior of robotic fish under different proportion conditions,the optimal proportion of linear swimming of robotic fish was obtained through tail vortex structure analysis,effectively improving propulsion efficiency.The swimming sliding mode can fully utilize the thrust generated by the back swing of the pectoral fin.The results show that during the pectoral fin propulsion process,a stroke ratio of 0.5:1 can effectively reduce the resistance of the robotic fish during operation;The increase in the duration of the pectoral fin recovery phase prevents premature damage to the vortices and vortex ring structures generated during the propulsion phase,improving the drag reduction performance and swimming efficiency of the robotic fish during swimming.(3)Determine the proportion of the free sliding time of the robot fish in the unit propulsion period as an alternative measure of the swimming and sliding efficiency of the robot fish,which is defined as the duty cycle.The thrust coefficient and velocity under different duty cycle for the cooperative propulsion of 3 DOF sternal fin and flexible body/caudal fin are analyzed,and the results are as follows: The jet and vortex generated by the caudal fin periodically in the propulsion process jointly promote the autonomous swimming of robot fish;The pressure difference between the two sides of the caudal fin is relatively large when swinging,and the backward shedding of the high pressure vortex provides thrust for the forward direction;The complex vortex structure is generated around the flow field during the propulsive process,in which the vortex generated by the thoracic fin gradually falls off and merges into the tail vortex of the caudal fin to strengthen the thrust;The smaller the duty cycle,the greater the peak thrust coefficient,the better the propulsion effect,but the greater the power consumption.