Study On The Mechanical Characteristics Of Juvenile Grass Carp Tail Swinging Based-on Particle Image Velocimetry Techology

In the fast developing 21 st century,biomechanics often brings many new enlightenment to the development of human technology.The research shows that the propulsion efficiency of fish in swimming is generally higher than that of current underwater vehicles.Improving the propulsion efficiency of the underwater vehicles will bring huge competitive advantage to the country.In nature,most fishes use their tail fin to generate thrust on the surrounding fluid,thus obtaining the reaction force of the fluid.This process contains extremely rich and unsolved knowledge of mechanics.In this paper,firstly,the experiment of flow around a cylinder had been completed by using the particle image velocimetry system made in the laboratory.The pressure field was reconstructed and the reconstructed accuracy was analyzed.Thus,a method for obtaining the transient pressure field suitable for the PIV device in this paper was determined.Then,the tail swing process of juvenile grass carp was analyzed by PIV,and the response relationships between the tail swing in static water and free flow were constructed.Finally,the mechanical characteristics of environmental water applied to fish were deduced.Specifically as follows:(1)Using the PIV system to measure the velocity field behind the cylinder,the pressure field was reconstructed using the finite volume method and the direct integration method.Compared with the Fluent simulation results,the velocity field and pressure field reconstruction error of PIV were analyzed.The results show that the resolution of camera is set to 640×480 pixels2 at 120 frame rate to obtain velocity field error of 6.62% in the PIV system.Based on the Fluent simulated velocity field data,under the first boundary condition,the pressure field reconstructed error of the finite volume method is less than that of the direct integral method.The mean square root errors are 1.73% and 8.99%respectively.Based on the PIV measured velocity field,the pressure field reconstructed error of the finite volume method is greater than that of the direct integral method,and the mean square root errors are 26.58 % and 12.72 % respectively.The precision of pressure field reconstruction can be improved by reducing the velocity field error and accurate boundary conditions.(2)Taking juvenile grass carp as the experimental object(the same below),the kinematics characteristics and the distribution characteristics of the environmental pr essure field were analyzed in the hydrostatic environment.The results show that th e motion can be divided into three stages,including“S” shape(T=0 ms~25±8.33ms;the head of fish clockwise swing while caudal fin counterclockwise swing),“C” shape(T=25±8.33 ms~100±8.33 ms;the caudal fin continues to swing count erclockwise until it reaches the "C" shape apex),and “C” shape swing back(T=100±8.33 ms~200 ms;the caudal fin is gradually clockwise back and forth from th e "C" shape until recovery Straight state).In this process,the grass carp juveniles push the fish forward by the reaction force generated by the jet.The pressure distri bution of the tail profile is related to the swing shape of the fishtail.The negative pressure is mainly distributed along the depression of the tail,and the positive pres sure is mainly distributed along the protrusion of the tail.(3)The time-varying regularity of environmental water pressure during the swin ging cycle of juvenile grass carp was analyzed.The results show that in the "S"-sh aped stage,the tail-flicking process,the resultant force F in the negative pressure z one increases first and then decreases with time T;in the "C"-shaped phase,the re sultant force F of the positive pressure zone and the negative pressure zone gradual ly increases;In the C-shaped returning stage,the resultant force F of the positive p ressure zone and the negative pressure zone is gradually reduced.(4)The pressure exerted on the surface of fish in the surrounding environment was calculated,and the statistical values were expressed as mean±standard deviatio n(the same below).The results show that in the first stage,the caudal fin does no t have continuous force on the upper and lower sides of the fluid.In the x and y directions(the same below),the pressure values are-3.44±1.40 m N/cm,6.64±1.71 m N/cm respectively.In the second stage,the upward movement of the caudal fin resulted in the same trend of fluid above and below the caudal fin,with a pressure of 1.79±0.38 m N/cm and-4.99±0.47 m N/cm.In the third stage,the pressure is-0.68±0.75 m N/cm,-0.06±0.15 m N/cm respectively.(5)The pressure exerted on the surface of fish was calculated under free flow.The results show that in the first stage,the pressure values are-0.80±0.26 m N/cm,3.12±0.43 m N/cm respectively.In the second stage,the upward movement of the caudal fin causes the same trend of the fluid on the upper and lower sides,and t he pressure value are-1.37±0.01 m N/cm,-4.99±0.47 m N/cm respectively.In the t hird stage,the fluid changes its movement tendency due to the C-shaped swing of the caudal fin,generating resistance and resisting the movement tendency of the ca udal fin.The pressure values 0.72±0.32 m N/cm,-0.06±0.10 m N/cm.