Dynamic Response And Aerodynamic Characteristics Of Floating Wind Turbines

Floating wind turbine is a novel technology to develop and make use of offshore wind energy. With the rapid development of offshore wind power, worldwide interests are growing in floating wind turbines. Because the floating platform has all the degrees of freedom in the offshore environment, and is always subjecting to the influence of wave, floating wind turbines will keep in significant oscillations all the time. Persistent oscillations will not only introduce dynamic responses, but also strengthen the unsteady effect of aerodynamic loads, which will cause great difficulties for the design and assessment of floating wind turbines. Therefore, in-depth study of the dynamic response and aerodynamic characteristics of floating wind turbines will be of great significance to reveal the characteristics of dynamic response, accurately simulate a variety of loads including aerodynamic loads, and promote the development of floating wind power.In this thesis, study on the dynamic response and aerodynamic characteristics of floating wind turbines was carried out with many simulation methods. Detailed analyses of the dynamic response characteristics under a variety of environmental conditions, and the aerodynamic characteristics of the airfoil, the rotor and the wake, were also performed.During the study of dynamic response characteristics, the dynamic responses of three typical floating wind turbines in a variety of wind and wave conditions were calculated. The means and standard deviations for each dynamic response in every simulation case were calculated. Results analysis showed that:surge is the major motion modes of floating wind turbines; the mean of every dynamic response is mainly dependent on the wind speed, and the amplitude is mainly dependent on the wave height; the power average is almost the same as that of the steady-state value of a fixed wind turbine, but the thrust average has a certain increase. Angle of attack and induction factor analysis showed that the average and amplitude of angle of attack are larger for the thick airfoils inboards, but those of induction factor are larger outboards, indicating that the blade sections inboards are more prone to dynamic stall, but the sections outboards have large possibility to change into the turbulent wake state.In the study of airfoil aerodynamics, the DU35airfoil was selected based on the variation characteristics of the angle of attack obtained in the study of dynamic response characteristics. Firstly, the static aerodynamic characteristics of DU35was studied with CFD method, and the simulation model was verified by comparing the CFD results with the experimental data. Then, with the help of dynamic mesh technology, the unsteady aerodynamic characteristics of pitching DU35was studied, and the results were compared with those of the B-L dynamic stall model. At last, the influences of mean angle of attack, angle of attack amplitude and reduced frequency were studied. It was shown that, under attached or slightly separation flow conditions, the Cl of thick airfoil has no unsteady aerodynamic effect at all, but the Cd and Cm have significant unsteady aerodynamic effect. It was also found that the B-L dynamic stall model, which is built on the basis of thin airfoils, has similar trends with CFD when simulating thick airfoils, but overestimates the unsteady effect of thick airfoils.During the study of rotor aerodynamics, simulations on the rotor of NREL5MW wind turbine in steady and surge state were carried out. The simulations in steady state were carried out with BEM and CFD methods, separately. Results showed that:for variable speed and pitch regulated wind turbines, when the wind speed is smaller than the rated wind speed, the power and the load will both increase with wind speed, and the power and the load mainly come from outboards; when the wind speed is larger than the rated wind speed, the load will decrease significantly, and the major power source will shift inboards; for the BEM method, Reynolds number effect is the primary cause for the increase of deviations. The simulations in surging state were carried out with prescribed surge displacement and under blade reference frame, with the use of CFD. Results showed that, during the surge motion of the rotor, the surface flow of the blade has no apparent changes, but fluctuations of surface pressure of the suction side near the blade tip are significant.In the study of the wake aerodynamics, a new model was set up based on analysis and improvements of existing actuator line methods. After comparing the simulation results of an airfoil and an elliptic wing with those of theories, the accuracy of the new model was verified, and the two methods to improve the accuracy of actuator line model were proposed. Then,with the new actuator line method, the wake aerodynamics of the rotor in fixed and surge stated were studied. Results of fixed rotor showed that:when the wind speed is smaller or equal to the rated wind speed, the strength of the bounding vortex outboards and the tip vortex are very strong; when the wind speed is much larger than the rated wind speed, the strength of the tip vortex will be reduced significantly, and the strength of the bounding vortex inboards and the root vortex will increase. The results of surge rotor showed that:fluctuations of the axial force and the tangential force are very large in the middle of the blade, and the amplitudes would increase significantly when the surge amplitude was increased; when the wind speed is smaller or equal to the rated wind speed, the surge motion of the rotor will accelerate the dissipation of the trailing vortexes, and improve the turbulence intensity of the wake; when the wind speed is larger than the rated wind speed, the wake flow will much less affected by the surge motion of the rotor. Finally, after a comprehensive analysis of existing experiments, an experiment bench for the modeling of the surge motion of floating wind turbines in conventional wind tunnel was designed, and the similarity criteria was established. Then, the aerodynamic characteristics of a scaled NREL5MW wind turbine with a diameter of1.5m in steady and surge state, was studied. Results showed that:because of the Reynolds number effect, and the friction damping between the rotating axis and the bearings, the CP of a scaled wind turbine will smaller than that of the prototype wind turbine; when the turbine is in surge state, the rotor speed and the thrust will both vary with time, and the fluctuation amplitudes will almost not change with the wind speed and the surge frequency.