Study On The Effect Of Environmental Flow And Motion Mode On The Propulsive Performance Of Flapping Foil

With evolution and natural selection for hundred million years,aquatic animals,like fish,dolphin and cetaceans,have an outstanding ability to swim,especially those are propelled by body and caudal fin(BCF)that are efficient,high-speed and low-noise.By contrary,the traditional unmanned underwater vehicles(UUVs)always employ many srew propellers for propulsion and maneuvering that occupy large space and produce high noise with propulsion inefficiency.These disadvantages extremely limit the operating capacity and working range of UUVs.So,imitating fish caudal fin,the flapping foil propulsion system is studied widely and many fish-like UUVs are developed.This thesis focuses on the hydrodynamic performance of flapping foil propulsion system in different environmental flow filed and following different motion mode.By adopting computational fluid dynamics(CFD)method,we analyze the propulsion performance of the flapping foil propulsion system.According to the flow filed and motion mode for flapping foil,this thesis includes three parts as follows:(1)Against the background of Wave Glider Propulsor driven by wave energy,the propulsion performance of tandem asynchronous flapping foils(TAFFs)propulsion system is studied in the flow filed interacted between them by using theoretical and experimental methods.In theoretical research,based on RANS equations,the hydrodynamic performances of each foil and the whole propulsion system are calculated with different flapping angle and distance between foils.Combined with vortices structure,the results show that the flow filed interaction between TAFFs has an obvious influence on the propulsion performance of TAFFs and each foil.In experimental study the tandem foils propeller is built based on the theoretical results and using wave engery as its input power.Then the experiments are carried on in tank to test TAFFs' propulsion performance with different wave length and height of regular waves.After comparing the computational and experimental results,it can be seen two methods are all validated and higher wave height can improve the thrust of TAFFs while longer wave length can improve the efficiency.(2)Against the background of performances for flapping foil are influenced by wave,the propulsion performances are studied for flapping foil and caudal fin in regular wave.Using velocity inlet boundary method and damping wave absorber method,the 2D and 3D numerical wave tank are built.After that,the propulsion performances of 2D rigid and flexible foil are calculated in RANS method and then that of 3D crescent flapping caudal fin with airfoil section is also calculated under waves.Meanwhile,the effects of motion parameters for foils,caudal fin and wave as well as coupling parameters between them on propulsion performance are analyzed.Combined with the vortices structure,the results indicate that it can reduce the adverse effect of wave and even utilize wave energy for propulsion by adjusting relevant parameters when the biomemetic propulsion system operates under waves.Moreover,the propulsion performances of foil in flexible and caudal fin in heave motion are higher than that in rigid and in sway motion respectively.(3)Against the background of the fish-like UUV propelled by flapping foil,the hydrodynamic performances are studied for fish-like UUV and its flapping caudal fin in different motion mode.The propulsion performance of single caudal fin is calculated in RANS method and immersed boundary method(IBM)and the results show it can get higher thrust with higher Strouhal number.Moreover,3D vortex of caudal fin shows that different Strouhal number and Reynolds number result in different appearance and shape vortex.The numerical results of self-propelled swimming performaces of fish-like UUV with different swimming form,flapping amplitude of caudal fin and wavelength of body undulations show the reasons for different swimming performance of fish-like UUV are the pressure and viscous forces,thrust and drag,power loss for undulation are different in swimming process with different condition.From above studies and results,it can be seen that CFD method is an effective and powerful tool in analyzing propulsion performance of flapping foil propulsion system in unsteady flow filed and swimming performance of fish-like UUVs.This thesis provides a basis for practical application of biomemetic propulsion system and a technical support for development of fish-like UUVs.Futher,it lays the foundation for studying the swimming mechanism of fish.