| The excellent swimming performance of fish has always been a vital topic for bionics scholars.Till now,the research on hydrodynamic performance of bionic flapping wings and swinging tail fin propulsion has accomplishedfruitful and valuable results.Although bionics scholars have devised a variety of robot fish based on their biological counterparts,the use of the movement and deformation of the pectoral fin to generate thrust and control the posture to achieve self-stabilized motion adjustment involves vast fundamentalbiological knowledge,so the currently developed prototypes by bionics scholars still show considerably insufficiency compared with real fish.Current study on pectoral fins is mostly limited to rigid pectoral fins or assumed flexible deformation modes,while the influence of flexible deformation on the pectoral fin propulsion performance is poorly understood.As live fish cannot cooperate well with the researchers'intentions in the experiment,the routine is highly irreproducible andthe parametric study is seldom systematic,reflecting the limitations of real pectoral fin experiment.Accurate fluid-structure interaction(FSI)study is required to hopefully reveal the true and reasonable propulsion mechanism of the flexible pectoral fin.Presently,the use of PIV experiment and fluid-structure interaction multiple methods toanalyze the implicit mechanism of the superior performance of flexible pectoral fin is rare in China.In this dissertation,the experimental device for the synchronous measurement of the hydrodynamics and the wake field of the flexible pectoral fins is designed.The parametric analysis of the flexible pectoral fin propulsion performance is systematically carried out.The experimental results and the FSI calculation are mutually verified to explore the complicated hydrodynamic characteristics of the pectoral fins with different stiffness.The vorticity cloud plot of the pectoral fin wake field on different cross sectionsis obtained by PIV measurement andthe detailed pressure distribution is supplemented by FSI simulation to explore the propulsion mechanism of the flexible pectoral fin.The existing analysis of hydrodynamic interference between tandem flapping wings is relatively rich,which generally concentrates on the rigid multi-wing system.On the other hand,the research on the three-dimensional parallel flexible wing is less.In this dissertation,the parameterization of the three-dimensional parallel wing propulsion performance is carried out,including detailed analysis of the effects of the flexible deformation parameters,kinematic parameters,and biplane layout parameters,and the comparison with the single-wing case.The relationship between the three-dimensional wake vortex structure and hydrodynamicsof the flexible wing is also discussed.For the case that the follow-up PIV system in the tank can only measure the cross section of the pectoral fin flow field in the flow direction,the auxiliary mechanism is designed to achieve the PIV shooting of two perpendicular crosssections of the wake field.A self-developed experimental control program is used tocontrol the unsteady movement of the pectoral fin,while simultaneously measure the hydrodynamics and flow field.Finally,the hydrodynamics and wake field synchronous measurement experimental device of the bionic pectoral fin propulsion system isestablished.To study the hydrodynamic performance of the passively deformed flexible pectoral fins made of different materials,the multiple means of direct hydrodynamic measurement and FSI numerical simulation are used to obtain the hydrodynamics and propulsion efficiency data of different flexible pectoral fins.The influence of key motion parameters on the performance of the flexible pectoral fins is discussed.The optimal phase difference of the bionic pectoral fin propulsion and the optimal Strouhal number(St)thatreveals significant difference from the flapping wing and caudal fin propulsion are found.In the experiment,the average thrust coefficient and propulsion efficiency spectrums of the pectoral fins made of each material are established.It provides a reference for the design of high performance bionic flexible pectoral fins.For the limitations of the live fish pectoral fin experiment and the situation that its results are uneasy to practically use under the existing mechanical and material conditions,the PIV system and tracer particles are used to obtain the spanwise vortex and streamwise vortex distribution of the pectoral fin wake field.The influence of St and phase difference on the vortex structure of the flow field is analyzed.The FSI calculation results and the hydrodynamic performance data are combined to construct a flexible pectoral fin propulsion mechanism analysis chain to analyze the wake vortex structure-pressure distribution-hydrodynamic performance.Applications of the results are expected to reduce the gap in the swimming performance between bionic robot fish and biological prototypes. |
PDF Full Text Request