Research On Hydrodynamic Characteristics Of Robotic Fish Synergically Driven By 3-DOF Pectoral Fins And Flexible Body

The bionic robotic fish,as a new type of underwater transportation,observation and measurement platform,which has the advantages of low resistance,high mobility,and high concealment,It has been extensively studied in recent years.At present,it is difficult to establish an accurate robotic fish dynamics model.Therefore,the simulation analysis of the robotic fish hydrodynamic characteristics by means of the Computational Fluid Dynamics method can provide an important reference for the design and performance improvement of the bionic robotic fish.In order to optimize the cooperative propulsion mode and obtain more efficient motion performance,it is particularly important to analyze the characteristics of the surrounding flow field and its dynamic performance in different propulsion modes.This thesis takes the three-degree-of-freedom pectoral fin/line-driven flexible body to imitate the carangidae robotic fish.By establishing a fish-body-fluid coupling CFD calculation model,the flow field structure around the fish body and its hydrodynamic performance in each movement mode are calculated.After calculation and analysis,the following results are obtained:(1)Without considering the pectoral fin effect,the 2D / 3D flexible body CFD models of biomimetic robotic fish were established respectively,and the hydrodynamic performance of the robot fish was analyzed when the flexible body self propelled straight swimming.The simulation results show that the self propulsion swimming process of flexible body can be divided into acceleration stage and average constant fluctuation patrol stage.Compared with the case of 1Hz swing frequency,the acceleration of flexible fish body increases when the frequency increases to 2Hz.The maximum cruising speeds of 2D and 3D models are 1.18m/s and 0.87m/s respectively.Under the condition of the same flexible deformation law and frequency,due to the existence of the propulsion mechanism of the robot fish,the first third part of the three-dimensional model is deeper than the two-dimensional model,and the contour flow linearity is worse,which makes it suffer more resistance and reduce the propulsion efficiency.Therefore,for the shell design of carangidae robot fish,attention should be paid to reducing the proportion of head volume and making it closer to the slightly flat spindle shape.(2)In the two-dimensional model,the vortex structure and the interaction between the flapping pectoral fins and the swing of flexible body are analyzed.The simulation results show that the motion of flexible body can produce vortices with opposite direction of rotation,and the strength of the high-strength vortices initially produced will weaken in the backward shedding process,and the two vortices with the same direction of rotation in the far flow field will gradually merge due to the strength weakening.When the pectoral fin is flapping back and forth,the pectoral fin will produce high-intensity vortices with the same rotation direction.When the pectoral fin flapping back,it can continuously fall off to the tail,expanding the viscous boundary layer around the fish.The results showed that the flapping of pectoral fin increased the time of keeping the positive extreme value of thrust,and increased the thrust;at the same time,the forward flapping of pectoral fin increased the resistance of the fish,so the contact area between pectoral fin and water should be reduced.Comparing the swimming performance of the flexible body with different wavelengths,it is found that ?=1.0 has better swimming stability and less acceleration efficiency;?=2.0 has higher propulsion speed and higher swimming efficiency in acceleration stage.(3)In the three-dimensional model,under the coordinated propulsion mode of the threedegree-of-freedom pectoral fin and the flexible fish body,the effect of the single-degree-offreedom and three-degree-of-freedom coupled motion of the pectoral fin on the swimming performance and hydrodynamic performance of the robotic fish is analyzed.The simulation results show that when the phase difference between the pectoral fins on both sides changes with an interval between,the lateral force difference of the pectoral fins on both sides can assist the turning movement of the fish.When the phase difference is 60,the lift of the two pectoral fins is opposite,so the lift of the two pectoral fins counteracts each other,which can reduce the fluctuation of the fish and make it swim more smoothly.In the three degree of freedom lift mode,in the return stroke of T 4-T 2,the combination of the swing wing greatly reduces the area of the upstream flow,greatly reduces the reverse thrust,and effectively improves the lift of the fish;In the resistance mode,the resistance of pectoral fin increases at 5T 8-6T 8,and the lift of the fish increases significantly at T 4-3T 4.Therefore,the three degree of freedom coupling motion of pectoral fin can make up for the deficiency of single degree of freedom motion and improve the efficiency.