Study On Hydrodynamic And Energy Extraction Performance Of A Tidal Energy Extraction System Based On An Oscillating Hydrofoil

In order to reduce the fossil fuel consumption and environmental pollution, the development and utilization of renewable energy is getting more and more countries’ concern over the whole world. Due to the advantage in shallow sea zone, the tidal power generation technology based on an oscillating hydrofoil is widely used in river bed, coast areas. In that case, it is significant to investigate the hydrodynamic and energy extraction performance of this novel power generation technology. Moreover, it is beneficial to develop tidal current energy extraction devices with high efficiency and stability.Generally, the hydrofoil motion is regarded as a coupling motion of pitching motion and heaving motions, in this process, the flow kinetic energy will be converted to hydrofoil mechanical energy. In order to study the impact and impact mechanism of geometrical parameters, motion parameters as well as the viscosity parameters to the hydrodynamic and energy extraction performance of oscillating hydrofoil, the following aspects are investigated in the thesis.In this thesis, the basic theory of energy extraction system based on an oscillating hydrofoil is introduced, and the relevant physical model is also built. In the meantime, the kinematic model of an oscillating hydrofoil that undergoes prescribed simple harmonic pitch-heave motion is presented, and the parameters that can influence energy extraction performance are analyzed.In this study, the theory used to solve the unsteady incompressible Navier-Stokes equations for the turbulence flow around an oscillating hydrofoil is researched, and both CFD and Fluent software are also used to build numerical calculation model. The oscillating hydrofoil model based on dynamic mesh technique and sliding mesh technique is built in the thesis. Moreover, the parameters and algorithm used to calculate Navier-stokes equation is also presented. The algorithm based on the secondary development function of UDF is proposed, and the program of numerical model for the unsteady flow around the oscillating hydrofoil was validated. The results show that the proposed model can calculate vortex structure characteristics around oscillating hydrofoil, the hydrodynamic and the energy extraction performance accurately. Moreover, computation period, turbulence model, boundary size of computational domain, mesh density and time step are chosen appropriately, and it can ensure numerical calculation efficiently and accurately.Besides, in this thesis, the tidal energy extraction mechanism of oscillating hydrofoil is analyzed quantitatively. The impact of geometric parameters, motion parameters and viscosity parameters to hydrodynamic as well as the energy extraction performance is also studied systematically. Moreover, the impact mechanism of relative parameters to hydrodynamic and energy extraction performance are deeply analyzed from the point of vortex structure characteristics. The results show that all of these parameters can affect the vortex structure characteristics on the boundary layer of oscillating hydrofoil. In the process, if these parameters are changed, it might lead to the phenomenon of boundary layer separation and vortex shedding, and then the vortex shedding could cause the phenomenon of dynamic stall and the break of instantaneous hydrodynamic. If the energy extraction efficiency of the oscillating hydrofoil system is higher than30percent, the radio of energy extraction efficiency to drag coefficient can be higher than0.2, and the instantaneous hydrodynamic performance has no mutation, the comprehensive performance of the tidal energy extraction system based on the oscillating hydrofoil is much better. The oscillating hydrofoil meets the high comprehensive performance requirements with a hydrofoil profile of NACA0018, a phase difference between the pitching and heaving motions of90°, a pitching axis at the third chord, a heaving amplitude of one chord, a pitching amplitude from70°to80°, and a reduced oscillating frequency of0.14. In this point, when the Reynolds number is equal to500000or higher, the pitching amplitude is75°, and in that case, the energy extraction efficiency of oscillating hydrofoil could reach40%.