Analysis On Mechanism And Performance Of Bio-inspired Pectoral Fin Propulsion Using Immersed Boundary Method

Aquatic animals,such as fish,have the ability to swim quickly,efficiently and flexibly,which is the constant goal of underwater vehicles with bio-inspired fin propulsors.Currently,the reverse Kármán vortex street produced by flapping foil and caudal fin propulsion has been well reported,and however the key feature of the wake structure is not well-established for pectoral fin propulsion.Present numerical studies on pectoral fins are more confined to rigid pectoral fins or fins with assumed flexible deformation pattern,and experimental investigations on real pectoral fins are relatively scarce due to the difficulties in effective control over live fish,conducting experiments in a repeatable manner and obtaining swimming efficiency quantitatively.In this thesis,a modified immersed boundary(IB)method is proposed,combined with a finite element analysis program for structure,setting up a solution system for fluid-structure interaction(FSI),and the hydrodynamics and FSI problems of bio-inspired pectoral fin propulsion are investigated.For solving the flow field of fish swimming numerically,a modified IB method is proposed,in which an efficient reconstruction algorithm for IB condition is developed.Moreover,a new moving boundary treatment approach is proposed with excellent adaptability to deal with a variety of situations,even if adopting large-step implicit time-marching method and the body moving with large amplitude.Therefore,efficient and accurate numerical simulations of fish swimming on simple Cartesian grids are achieved.Several nominal test cases are carried out to validate the proposed modified IB method widely,including flow past a sphere,an oscillating cylinder in the free-stream and a bio-inspired hydrofoil with large flapping amplitude.For a small-aspect-ratio flapping foil that can be considered as a simplified model of the pectoral fin,the modified IB method is used to study its highly three-dimensional wake structure and to analyze the formation of main vortex structures and their characteristics from the viewpoint of vortex dynamics.On this basis,the effect of kinematic parameters on the wake structure and hydrodynamic performance of the flapping foil is systematically investigated,and the mechanism underlying the variation of the hydrodynamic performance with parameters is discussed deeply.For a complex-shaped pectoral fin with a compound rotational motion,numerical simulations based on the modified IB method are employed to reveal the key wake feature that dominates the pectoral fin propulsion,and the connection between the fin kinematics,vortex dynamics and force production is analyzed in detail.Then a general law characterizing the hydrodynamic difference between the pectoral fin and flapping foil is obtained by broad comparisons with flapping foils,and the mechanism behind the performance difference is explained.Next a systematic parametric study is conducted to discuss the effect of kinematic parameters on the wake structure,hydrodynamic coefficients and propulsive efficiency of the pectoral fin.Finally,suggestion on the design of a bio-inspired pectoral fin propulsor is provided according to the research results.In order to investigate the hydrodynamic performance of a highly-deformable flexible pectoral fin,the modified IB method is integrated with the finite element method for structure to set up a FSI solution system.The comparisons with the numerical simulation and experiment of flexible panels performing unsteady motion are also carried out to validate the accuracy and reliability of the FSI method.Then the deformation caused by the interaction between the flexible pectoral fin and surrounding fluid is analyzed,and the question of how the improvement in the propulsive performance by the flexibility of the pectoral fin is expected to be affected with changes in structural and kinematic parameters is discussed.Finally,a comparative study of between the bio-inspired flexible fin,flapping foil and real pectoral fin is conducted,and the comparisons have revealed the advantage of the pectoral fin over the flapping foil and found that the well-designed flexible bio-inspired pectoral fin propulsor can provide propulsive performance comparable to a real pectoral fin.