Study On Nonlinear Motion Characteristics Of Combined Trunk And Tail Fins Of Koi Carp

Ocean exploration is an important part of the national strategy and has important economic,technological,environmental protection and national defense significance.Underwater aircraft,as an important tool for exploring the ocean,has an irreplaceable position.Traditional underwater aircraft have shortcomings such as complex structure,large volume,and low advance efficiency.Fish creatures can swim quickly in the ocean and have very prominent ocean adaptability.The way of advancement of fish provides new ideas for the manufacturing of new underwater aircraft,and the research on fish movement characteristics is of great significance for the development of underwater aircraft.This thesis is based on the purpose of studying the characteristics of the koi trunk fin combined with non-linear motion characteristics,and uses CFD to analyze the motion performance of koi.First of all,in order to establish a kinematic model of koi trunk tail fin joint fluctuation,the morphology and kinematics of koi were studied.The biological form of koi was analyzed through market research to obtain the shape proportions of the koi and establish the corresponding physical model and coordinate systems.The motion attitude of koi was fully understood through the motion attitude observation experiment,and the kinematics model of the joint motion of the trunk and tail fin of Koi was established,and the dynamics model of the bionic fish was established by using computational fluid dynamics.Secondly,in order to study the kinematic performance of the combined oscillation of the koi trunk caudal fin,the method of solving the N-S equation was adopted,and according to the complexity of the model and the calculation cost,the method of combining the dynamic grid and the overlapping grid technique was adopted to calculate the kinematics.By writing the autonomous swimming program of the bionic fish and defining the motion equation of the bionic fish,it can calculate the information of any position point,speed and flow field during the movement of the bionic fish.The grid independence was verified to determine the number of grids,and the reliability of the numerical calculation method was verified by comparing with the experimental values in the literature.Thirdly,the combined nonlinear motion characteristics of trunk and tail fin of koi were studied by the two-dimensional section of the physical model of Koi.The single factor method was used to change the motion frequency,wave number,amplitude and incoming flow velocity,and the influence of these parameters on the speed,acceleration,displacement,efficiency and tail vortex of the bionic fish was analyzed.The results show that these motion parameters have great influence on the motion and vortex shedding of koi carp.In order to further understand the information of flow field,DMD method was used to analyze the tail vortices generated when the bionic fish swam.The results showed that the frequency of the mode with the highest energy proportion corresponded to the motion frequency of the bionic fish.Finally,the three-dimensional physical model of the koi was simulated to study the influence of motion parameters on the thrust of the combined motion of the trunk and tail fins of the koi,and the difference of the thrust curve between the two-dimensional model and the three-dimensional model was compared by normalization.The results show that the trend of the two curves is similar.Only when the wave number is large,there is a large deviation between the two curves.The spatial vortex system generated by the threedimensional bionic fish was analyzed.The analysis showed that a pair of type B annular vortices would fall off in each period of the bionic fish’s wave advance,and the surface pressure of the bionic fish reached the maximum when the type B vortices fell off.