Vortex Structure And Hydrodynamic Mechanism For Torsional Wave Propulsion

The Ariiform fish Gymnarchus niloticus is able to swim as easily forward as backward just by switching the spread direction of the wave on its long dorsal fin while keeping its body almost straight. Its extraordinary ability on swimming manners is of great inspiration to us on propulsor design. However, the research on the hydrodynamics and propulsive efficiency of torsional wave propulsion are still insufficient at present, and the spatial vortex structure study is particularly short. In this paper, the hydrodynamic force, propulsive efficiency and spatial vortex structure of the torsional wave propulsion by flexible long fin is studied based on series of numerical simulation and relevant experimental measurements. The main contents and contributions of this paper are summarized as follows:The numerical method for hydrodynamic calculation of the torsional wave propulsion has been established based on the dynamic mesh technique including spring-based smoothing model and local grid remeshing. In the present numerical simulation, the maximum wave amplitude of fin ray reaches 85°, and the result is compared with experiment results and validated. Based on the numerical mothod, the paper studies the fluctuation of the fin’s hydrodynamic force with time, and the varitation of the mean hydrodynamic force with respect to frequency, inflow velocity, wavelenght and wave amplitude is also analysized respectively.By numerically analyzing the torque at tiny units selected from the fin surface, we get the variation tendence of input torque with time at each position along the fin axle. After that, integrating of the output power of each fin unit is done to gain the total output power of the fin. The methode is different from that of traditional propellers whose total output power can be calculated directly from the total torque. After a series of numerical calculation, we get the variation of the propulsive efficiency with respect to wave frequency, inflow velocity, wavelength and wave amplitude.Based on the diffusion-based smoothing model in dynamic mesh technique, the paper establishes the numerical method to simulate the spatial vortex structure of the torsional wave propulsion using the large eddy simulation(LES). At the wave amplitude of 30°, we obtain the major spatial vortex structues such as streamwise vortex, crab-like vortex, ringlike vortex and the high-speed jet near the fin as well as their evolution process with wave phase. Further analysis show that the crab-like vortex is mainly related to the grenaration of thrust while the streamwise vortex is related to the heave force. At the same time, PIV experiment has been carried out using phase synchronization technique at wave amplitude of 85°. Through a series of mesearment and analysis of the flow field in the cross-section and mid-sagittal plane, we also get the streamwise vortex, crab-like vortex, ringlike vortex and the high-speed jet as well as their evolution process with wave phase. The numerical results matches the experiments quite well.The works in the paper is not only helpful in studying the hydrodynamic mechanism of the torsional wave propulsion, but also of important application values for its hydrodynamic performance prediction.