Research On Hydrodynamic Characteristics And Propulsive Mechanism Of Oscillating Tuna-tail

Tuna, dolphin and shark exhibit excellent hydrodynamic performance with high cruising speed, high efficiency and low noise. It is important to apply the propulsive mode imitating these aquatic animals to underwater vehicle. The aim of present study was to identify the principal character of wake vortices generated by oscillating rigid and flexible tuna-tail. Thus, a computational fluid dynamic (CFD) simulation has been developed to investigate the hydrodynamic characteristics and propulsive mechanism of a three-dimensional oscillating tuna-tail.Firstly, the hydrodynamic performances of the hydrofoil with steady state were calculated under the influences of grid system and solution parameters. And corresponding flow fields were also displayed. Meanwhile, unsteady, viscous flows over 2D rigid and flexible hydrofoil in plunge motion were computed. The computed unsteady flow fields clearly revealed the formation and evolution of a pair of leading edge vortices along the body of the rigid and flexible foil as they undergo oscillating motion.Secondly, the unsteady hydrodynamic characteristics of oscillating rigid and flexible tuna-tail in viscous flow-fields are analyzed, vortical pattern generated by oscillating tuna-tail and corresponding propulsive mechanism were obtained. Experimental results of tail-fin propulsive system were also researched. The tuna-tail was considered as a lunate fin oscillating with the mode combined swaying and pitching in uniform flow. For the numerical computation, the grid system of solution domain was drawn and divided into the kernel and non-kernel districts, and the size of the cells at least should be one twentieth to one thirtieth of the characteristic chord length of the tail. The dynamic mesh technique was employed to model the flapping motion of tail using" Spring-based smoothing" method, and two methods can be adopted to remove the wrong motion of the grid on the surface of the flexible tail, when using grid partitioning of parallel processing. Thrust characteristics of oscillating tail-fin were calculated by solving the Reynolds-averaged Navier-Stokes equations. The effects of various swimming speeds, flapping amplitudes and frequencies, and flexure amplitudes on the hydrodynamic characteristics of the rigid and flexible tuna-tail were also investigated. Meanwhile, the hydrodynamic performances between rigid and flexible tuna-tail were compared in the same input power coefficient with different oscillating frequencies.At last, we employed the form of the "FangSheng-â… ", developed by the Underwater Vehicle Technology Lab (UVTL) of Harbin Engineering University and measured the locomotion of fish swimming, to build the computational model of tuna-like body. We calculated the hydrodynamic performance of tuna-like body when tuna swim in a uniform velocity, and compared the input power coefficient, output power coefficient and propulsive efficiency of the oscillating tuna-tail with or without body vortexes shedding. Additionally, the load distribution on the body, flow features and vorticity structures around the body were demonstrated. The effects of interaction between the body-generated and the tail-generated vortices on the hydrodynamic performance can be obtained.According to above computational results, we gave a conclusion that the present study confirms CFD methodology as an effective and powerful problem-solving tool in the fields of bio-locomotion and biomimetics. More detailed research should be done on the special swimming method of fish.