| With the urgent need to develop a marine strategy,the exploitation and utilization of marine resources are further expanded.Underwater vehicles have emerged to play a major role in military and civilian needs.The traditional propulsion device of underwater vehicles has been mainly the propeller,which has defects in terms of poor flexibility and low efficiency.Over thousands of years of evolution,fish in nature have excellent swimming ability,thus the fish-shaped bionic fish underwater vehicle has gradually become a hot spot for scholarly research in recent years.In order to realize the bionic fish vehicle to swim as flexibly as the real fish,the swimming mechanism of fish must be studied fundamentally.In recent years,with the enrichment of computational fluid dynamics theory,numerical simulation has become the main means to simulate the swimming of fish.This paper uses the numerical simulation method and combines the experimental data of biological fish swimming.By writing User-Defined Functions(UDF)combined with the dynamic grid technology,based on the shape model of Carangiform fish,the body and/or Caudal Fin(BCF)propulsion mode fish body,body size,movement characteristics,external wall environment,and clustering effect are studied.The results of this paper aim to provide a reference for the design of bionic robotic fish,and the main conclusions are as follows.(1)The autonomous swimming process of BCF fish is divided into three stages:initiation-acceleration-cruising.In the initial stage,the fish body fluctuates,forming a relatively dense wake vortex with opposite directions after the fishtail,and the acceleration reach the peak;In the cruising phase,the positive and negative wake vortices at the fishtail are arranged in a straight line along the midline of the wake,the acceleration is stable at about 0,the net thrust is almost 0,and the hydrodynamic performance and propulsion performance show periodic characteristics.(2)In terms of body size,a streamlined and small fish body can reduce drag and obtain a higher cruising speed.When the same kinematic parameters are held constant,the cruising swimming speed of crucian carp gradually increases with decreasing thickness-to-length ratio,and this phenomenon is more significant in the countercurrent environment.Regarding body length,assuming that fish at different growth stages(with different body lengths)have the same swimming mode(the same tail fin swing mode,swing frequency,and swing amplitude),the longer the body length,the faster the cruising speed.When the fish reaches a length of 10 cm,the cruising speed is not significantly affected by body length.(3)In terms of motion characteristics,the swing amplitude,swing frequency,and propulsion wavelength of the crucian carp are positively correlated with the cruising swimming speed in the design of the working conditions in this paper,among which the swing frequency has a more significant effect on acceleration.Analyzed from the perspective of St number,when 0.25 < St number < 0.35,the fishtail vortex forms an anti-Kamen vortex street,which has high propulsion efficiency.When the kinematic parameters are unified,comparing the swimming ability of Carangiform and Anguilliform reveals that Anguilliform has a larger acceleration in the acceleration stage.Therefore,it can produce greater thrust and reach the dynamic stable cruise stage faster.(4)In the wall environment,the wall effect occurs when the fish is close to the wall,resulting in a reduction in the thrust of the crucian carp and a decrease in the cruising speed.This effect is more significant in the bilateral wall.The unilateral wall effect disappear at 0.5 L from the fish body,and the bilateral wall effect will disappear at 0.8L from the fish body.When the fish body is 0.2 L away from the double side wall,the wall effect will leads to a large area of turbulence in the flow field around the fish body,and the interaction between the wake vortex and the wall will lead to the instability of the fish body velocity.(5)In the line-up mode of fish cluster swimming,both the line-up spacing and the line-up mode will have different effects on the fish body.The smaller the spacing of the fish queue,the overall cruising speed of the fish in the queue mode will be reduced to different degrees,but the shorter the time it takes for the fish to reach the cruising speed,the propulsion efficiency is improved compared with the single fish swimming,which can reduce energy consumption.In the example of this paper,in the queueing mode with small spacing,the diamond queueing mode is the most efficient,followed by the triangular mode,and the single parallel mode and the serial mode swimming are less efficient. |
