Modeling And Performance Analysis Of Underwater Bionic Propulsion Based On Numerical Simulation

In the aquatic environment,due to the difference of ecological niche and survival stress,fish have developed a rich biological diversity through millions of years of evolution.The remarkable swimming skills and abilities to adapt to the aquatic environment of fish are the optimization results of long evolution.For the sake of the environmental adaptation and swimming performance improvement,fish have developed different morphological structures and swimming styles.In addition,there are other ways for fish to further improve swimming performance,such as the intermittent swimming gait and the collective locomotion.Compared with the traditional propeller,the natural propulsion methods give the fish increased efficiency,high mobility and low noise.Researchers have been intrigued by the mechanisms of fish swimming during the past decades.The development of underwater bionic vehicles and robots has been the subject of intense investigation.Bionic robot fish is a kind of underwater robot which can imitate the swimming of a particular species of fish.It is helpful for the better designs of robot fish to study the performance optimization mechanisms of fish swimming.In recent years,with the development of computer technology and computational fluid dynamics(CFD),the numerical simulation technique has become an indispensable means of bionic engineering research.Based on the engineering application background of underwater bionic propulsion,in this dissertation,a numerical study has been made on the modeling and performance analysis of fish swimming.This study begins with four different aspects: morphological structure characteristics,kinematic characteristics,intermittent locomotion and collective locomotion.The main works and research findings of this dissertation are as follows.(1)The morphological structure characteristics of fish have a significant impact on its swimming performance.As the natural counterparts of many bionic robot fish,thunniform swimmers have the advantages of high speed and high efficiency during the long-range swimming.Tuna is a typical example of thunniform fish,which has a streamlined body and a high aspect ratio caudal fin.Compared with the other fish species,the caudal fin of tuna is separated from its body due to the narrow necking at the caudal peduncle.The effects of separated caudal fin on the propulsive performance are studied by two-dimensional numerical simulation from the view of morphological structure optimization.Both the structural relationship and the distance between the fish body and caudal fin are considered.Numerical simulation results show that the leading-edge of the caudal fin is essential to the improvement of thrust.The interaction between the vortices of incoming flow and the leading-edge of the caudal fin can greatly influence the thrust of caudal fin.(2)The kinematic characteristics of fish swimming can strongly affect the propulsive performance.According to the way of thrust generation,fish swimming exhibits two fundamental patterns: BCF(Body and/or caudal fin)mode and MPF(Median and/or paired fins)mode.Both the swimming modes can be further divided into several submodes.Based on the observation of the swimming large-mouthed catfish,in this dissertation,a novel combined undulating-motion pattern is proposed.Unlike existing swimming modes,the features of both BCF and MPF swimming modes can be found in the swimming of large-mouthed catfish.Specifically,the large-mouthed catfish obviously undulates its body and long-based anal fin during swimming.Based on the kinematic analysis of a living specimen,the modeling and simulation of the novel combined undulating-motion pattern are carried out.Simulation results show that the phase-angle difference between the undulations of the fish body and the ribbon anal fin has a signifcant impact on the thrust and heave forces.Particularly,an in-phase combined undulating-motion pattern,which is commonly found in the swimming of actual large-mouthed catfsh,can dramatically improve the thrust.In addition,the effects of the angular amplitude of ribbon fin and the swimming frequency on the the propulsive performance are considered as well.(3)Despite the optimization of morphology and kinematics,there are some special behaviors of motion planning for the further improvement of fish swimming.Many species of fish are more likely to swim intermittently during a long-distance swimming.Using the muscle-contraction model of pre-strains,the intermittent swimming gait of fish is numerically simulated.Both the continuous swimming and the intermittent swimming are compared from the aspects of swimming speed and power consumption.Besides,the effects of the duty cycle and the tail swing number in a burst phase are also emphasized.The results show that,with a smaller duty cycle,fish can save more energy to obtain the same swimming speed and a smaller tail swing number in a burst phase is more helpful to improve the energy utilization rate.Compared with the medium speed situation,the intermittent swimming has more benefts at the lower and the higher average swimming speeds.This conclusion coincides with the fact: the intermittent swimming gait of fish is more common in the low-speed swimming and high-speed swimming.(4)Collective motion is a widespread phenomenon in biological systems,especially in the flying and swimming animal groups.In this dissertation,the tandem locomotion of a single pair of fish is numerically studied with the muscle-contraction model of pre-strains.For the simplicity of simulation,a PID control algorithm is used to make both fish have the trend to swim horizontally in a straight line.On this basis,the effects of phase difference and initial separation between the two fish on the swimming process are studied.Simulation results show that,the swimming process can be divided into two stages: the earlier unsteady stage and the later steady stage.Under different conditions of phase difference and initial separation,both fish can reach the same swimming speed at the steady stage.At the same time,a same flow field structure can be found between the downstream swimmer and the wake vortices of the upstream swimmer.Further,the same phenomenon will occur with a decreased contraction amplitude of muscle for the downstream swimmer.An analysis of energy-saving effect shows that the follower can save a lot of energy through a voluntary reduction of muscle contraction.The same flow field structure found in the simulations is proved to be beneficial in the energy exploiting of the upstream wake vortices for the follower.The research on the modeling and performance analysis of underwater bionic propulsion has certain directive significance and application value for the understanding of fish swimming and the performance improvement of bionic robot fish.