Investigations On Unsteady Aerodynamic Characteristics Of Floating Offshore Wind Turbine

The energy demand growth and the concept of sustainable development promote the use of renewable clean energy.As an essential part of renewable clean energy power generation,wind power has excellent development potential and scientific research value.Offshore wind power has unique advantages in resource distribution,space and ecology,which is the inevitable trend of large-scale wind power development.Offshore wind turbines are mainly divided into fixed and floating wind turbines.Floating offshore wind turbines are usually used in deep sea areas to provide better space conditions for the large-scale development of wind turbines.During the operation of floating offshore wind turbines,the floating platform has six degrees of freedom(6-DOF)motion under the effects of incoming wind,wave and current,and makes the wind turbine to move together.As for the floating offshore wind turbines,the movement of the wind turbine carried by the platform will significantly impact the aerodynamic load and structural stress.It is of great significance to analyze and study the aerodynamic performance and structural response of the wind turbine under the platform motion,so as to provide some reference for the design and optimization of offshore wind power.Under the platform motion,the floating offshore wind turbine will occur 6-DOF motion,among which the surge motion and pitch motion have the most significant impact on the aerodynamic performance of the wind turbines.NREL 5MW,the typical offshore wind turbine,are selected as the research object.The unsteady aerodynamic characteristics of single degree of freedom(surge or pitch motion)and two degrees of freedom(coupled surge-pitch motion)of floating offshore wind turbine,with different amplitudes and frequencies,are numerically simulated by using CFD method with coupling technique of dynamic and sliding mesh and four kinds of sinusoidal translational motion model and angular motion model.Compared with the results of blade element momentum theory and vortex method,the model rationality and numerical method is verified.The overall performance,cross-section aerodynamic load,angle of attack distribution and flow characteristics of wind turbines under different platform motions are studied.The influence of platform coupled surge-pitch motion with different phases on the aerodynamic performance of wind turbines is also studied.It is found that the increase of amplitude and frequency aggravates the fluctuation of the overall aerodynamic performance of the wind turbine,and the severe platform motion leads to a significant increase in the average force of the wind turbine;under the platform coupled surge-pitch motion,the normal force and tangential force of the blade are mainly affected by the pitch motion;under the platform moderate motion,the aerodynamic load of the inner and middle span of the wind turbine blade is larger,while the outer span is larger There is a large fluctuation of aerodynamic load in the high amplitude platform motion.Platform motion affects the lift and drag coefficients distribution of each section airfoil by changing the spanwise attack of angle distribution,and finally affects the torque and power output.The severe and complex platform motion increases the velocity gradient near the hub,increases the complexity of the three-dimensional flow around the hub,and is accompanied by the complex vortex motion at the blade tip.The existence of atmospheric boundary layer will affect the vertical wind speed distribution above sea level and change the shape of wind profile under wind shear.This change will not only affect the output power of the wind turbine,but also increase the spanwise aerodynamic load of the blade.Firstly,the motion of wind turbine platform under the fixed wind shear index of ?=0.1 is studied and compared with the uniform flow.Considering the differences of sea-level roughness,wind speed and atmospheric stability in different sea areas,the platform motions induced by different wind profile indexes(?=0.1,?=0.2,?=0.3)are compared and analyzed.The results show that,compared with the uniform flow,the power output of the platform in pitch and surge motion will be reduced under the wind shear condition,but the average thrust has no obvious change,that is,under the same fatigue load,the wind shear effect will reduce the power generation.Wind shear will increase the normal load amplitude of spanwise section and increase the fluctuation,but it will reduce the tangential force related to power output.The wind shear delayed the phase of the normal and tangential load peaks in the spanwise section of the platform during the surge motion.The complex wind conditions on the sea will make the wind turbine motion more complex and have a significant impact on the blade structure.Considering the large size of the blade,it is particularly important to analyze the deformation and stress of the floating wind turbine blade.Taking the NREL 5MW wind turbine as the research object,the calculation models of fluid domain and solid domain are established respectively to check the modal of the solid structure of the blade.Firstly,the load performance along the blade spanwise direction was studied.Then,based on the simulation results of the aerodynamic performance of the floating wind turbine group,the aerodynamic results are loaded into the solid model to obtain the aerodynamic load changes along the blade spanwise and the blade flapping and shimmy deformation;finally,the equivalent stress is analyzed to discuss the stress distribution of the blade under different platform motions.The results show that the solid shell model can be used to simulate the blade structure accurately.Under the guidance of complex platform motion,there is a great difference between the forward and backward motion,which will significantly increase the fatigue load of the blade.Induced by platform motion,the stress concentration appears on the pressure surface of the middle lobe.The wind shear effect increases the stress on the spanwise section of the blade,increases the fluctuation of the blade deformation with the phase angle,and intensifies the fatigue stress concentration of the blade.Due to the multi degree of freedom motion caused by complex offshore wind conditions,it is necessary to strengthen the structure of stress concentration area of wind turbine blade.When floating wind turbine is in operation,it will be affected by wind,wave and current loads.Firstly,the wave diffraction and radiation characteristics are studied based on the frequency domain analysis method,and then the motion response of the platform is studied based on the time domain analysis method.The frequency domain analysis process is mostly linear solution or quasi linear solution,mainly including the analysis of platform response amplitude operator(RAO),first-order wave force,second-order average drift force,added mass and radiation damping coefficient.However,in real sea conditions,the motion characteristics of the floating wind turbine platform are often nonlinear,such as shaking and swaying,so time domain analysis is needed:simulate the unsteady characteristics of the floating wind turbine platform under given sea conditions to better reflect the motion response of the floating wind turbine platform under real sea conditions.The dynamic model of the floating wind turbine is established to solve the motion response of the platform.The results show that the variation of Rao with wave frequency under the three kinds of platform motions of surge,pitch and heave is in good agreement with the experimental data in the pool,which verifies the rationality of the model of floating wind turbine with semi-submersible platform.For the floating wind turbine semi-submersible platform,the first-order wave excitation force corresponding to the low frequency band fluctuates strongly,and when the wave incidence angle is 0° and 180°,the excitation force is larger and decreases rapidly with the increase of the incident wave frequency;the excitation force corresponding to the high frequency band fluctuates gently and finally reaches stability.The platform motion response under the combined action of wind,wave and current is obtained,with summing up the influence of mooring dimension on platform motion response and mooring force.