| Tidal current energy is one type of ocean renewable energy with huge reserves.The development and utilization of tidal current energy can reduce the pressure of the energy crisis and the environmental pollution.The floating platform of the floating vertical axis tidal current turbine(VATT)is more suitable for the offshore deep water and has the advantage of easy maintenance and transportation.The hydrodynamic performance of the floating VATT,as the primary mechanism of tidal current energy utilization,has been studied extensively.However,the hydrodynamic performance of the floating VATT under the excitation of the waves and current in the open sea has rarely been studied.Actually,the impeller is not only rotating under the current but also has six degrees of motion with the floating structures under the wave-current loads.This paper studies the hydrodynamic performance of impeller of the VATT with the oscillatory motion(sway,surge,yaw,roll,pitch)based on the CFD viscous theory and ANSYS-CFX software,as well as the moving mesh and the sliding mesh.The results are achieved using the least square method by the CFD numerical results.The results of this thesis have great scientific significance and engineering value on the impeller rotation and the coupled motion hydrodynamic analysis of VATT and the motion response of the floating structures.At first,the complex model of the VATT moving in the waves is simplified to an impeller moving in the uniform current and superposed with the harmonic motions,for the convince of numerical model establishment and the results analysis.Then,the hydrodynamic loads and the tidal current energy capture efficiency of the single impeller are calculated by coupling the motion of sway,surge,yaw,roll,and pitch respectively.The axial thrust,transverse force,and the power of the impeller for different tip speed ratio and motion parameters(frequency and amplitude)are discussed to achieve the influence of the motion parameters on the hydrodynamic performance and the tidal energy capture efficiency.Through the analysis of the flow field characteristics and the blade forces,the impact principle of the rotation and other coupled motion of the impeller on the change of the flow field of the impeller.Second,the varying characteristics of the load fitting coefficients are analyzed for varying motion parameters through the analysis of the damping(related to the velocity)and added mass(related to acceleration)series coefficients,namely the hydrodynamic loads and characteristics,by the least square method.At last,the coupled motions of the VATT are numerically simulated and analyzed of different span-chord ratio,for the study of 3D effect of the coupled motion(sway and surge)of the impeller of the VATT.The results show that: the amplitude of the envelope lines of the impeller thrust and the transverse forces coefficients are increasing as the motion frequency and amplitude increasing,but the average remain constant.The power efficiency does not change much when the impeller under coupled motions.The yaw can lower the efficiency and the surge can enhance it a little bit.The wake field of the moving impeller is cyclically fluctuating,with the frequency and amplitude increase as the frequency and amplitude of the moving impeller increase.The vortex strength of the wake field is decreasing as the flow is moving towards downstream.A Karman-vortex-street like wake pattern is generated by the impeller oscillating motion.The uniform hydrodynamic part of the thrust is not changing much by the motion frequency and amplitude,but becomes larger as the tip speed ratio increase.The output power efficiency is enhanced by a larger span-chord ratio.When the span-chord ratio is larger than 10,the efficiencies of the impellers are almost the same for in 3D and 2D scenario. |
PDF Full Text Request