Study On Swimming Behavior And Dynamics Of Juvenile Grass Carps Based On Vorticity Principle

Swimming is one of the main life activities of fish.It is of great biological significance for fish to take bait,migrate,gather,escape from enemies and reproduce in water.The swimming behavior of fish often depends on the fluctuation of the body.In order to swim the fish efficiently in the underwater environment,the body's head,tail and pectoral fin and other parts need to cooperate.Therefore,the study on the vorticity and dynamic formation process of grass carp juveniles will help to understand the swimming behavior and propulsion mechanism of scads,and provide a new research method and method for the quantitative analysis of swimming dynamics.In this paper,through the particle image velocity(PIV,Particle Image Velocimetry)device made in the laboratory,the vorticity,flow field and tail vortex structure of the grass carp juveniles in three swimming states of straight line,turning and retreating are completed.At the same time,based on the principle of vorticity calculation,the propulsion force,lateral force and propulsion efficiency of grass carp juveniles under different swimming conditions were analyzed,and the important role of tail vortex structure in the formation of dynamic and efficient propulsion mode was clarified,and the key hydraulic parameters for the efficient propulsion of grass carp juveniles were obtained.The main conclusions are as follows:(1)Based on PIV experiment and static water experiment,the instantaneous flow field and vortex field of juvenile grass carps under static water condition were obtained.By analyzing the vortex structure under different flow and vortex fields,the conclusions were as follows:the structure of the tail vortex is relatively stable when the grass carp juveniles move forward in still water.The structure of tail vortex of grass carp juveniles with accelerated swimming d/D>0.3,and the structure of the tail vortex of the grass carp juveniles that speed up and speed down is distributed in the range of(d/D=0?0.5).(2)Based on the PIV experiment,the vorticity and vortex structure of the juvenile grass carps were analyzed,and the force exerted by the vorticity and vortex structure around the juvenile grass carps under transient conditions was calculated,and the force distribution of the juvenile grass carps under still water conditions was obtained.The main conclusions are as follows:when the grass carp juveniles swim in straight line or in turn,the propulsion force produced by their tail accounts for more than 50%of the total propulsion force,which proves that the tail of the juveniles is the main power source;when they retreat,the grass carp juveniles mainly rely on the middle part of the fish body to push themselves forward.(3)Combined with kinematic parameters,vorticity,vortex structure and other hydrodynamic parameters,the swimming efficiency of juvenile grass carps under the condition of clean water was statistically analyzed,and the conclusions were as follows:the propulsive efficiency of grass carp juveniles in still water is in accordance with the normal distribution of tail vortex structure and head skew angle,and the smaller head skew angle(less than 15°)and tail vortex structure(D/d=0.1?0.4)are more conducive to the propulsive efficiency of grass carp juveniles.(4)The vorticity and vortex structure of juvenile grass carps under the condition of0?2BL·s^-1 adjusting flow velocity were analyzed by PIV experimental device and dynamic water experimental device,and the conclusions were as follows:the vorticity of grass carp juveniles under the condition of dynamic water changes most obviously in the period of0?T/2 tail swinging.The vorticity of grass carp juveniles in this period is distributed between-0.5?3s^-1,and the structure of tail vorticity d/D mainly changes in a concave shape(first decreases and then increases);however,after the period of T/2?T tail swinging,the vorticity of the whole body of grass carp begins to decrease gradually,with an average distribution between-0.1?0.7s^-1.In this period,the structure of tail vorticity presents a convex shape change(decrease one by one).(5)PIV experimental device and hydrodynamic experimental device were used to film the fish body under different flow rates by adjusting the camera shooting frequency,and the hydrodynamics changes of the fish body under different flow rates were analyzed.The conclusions were as follows:when the flow velocity changes in the range of 0?1BL·s^-1,the overall propulsive efficiency of grass carp larvae increases first and then decreases with the increase of flow velocity;when the flow velocity changes in the range of 1?2BL·s^-1,the overall propulsive efficiency of grass carp larvae decreases first and then increases with the increase of flow velocity.