| Wind energy is clean and renewable,and it has developed rapidly in recent years.At present,the installed capacity in the world has exceeded 837GW,and it will continue to increase globally in the next few decades.Wind turbine have long been in turbulent environment with long period and high fluctuation intensity,the interaction between atmospheric turbulence coherent structure and wind turbine becomes more and more complex with the increase of wind farm construction scale and installed capacity,which has adverse effects on wind turbine wake characteristics,power generation efficiency,wind turbine load,service life and grid integration of wind power.However,people’s understanding of the influence of turbulent coherent structure of atmospheric boundary layer on the aerodynamic performance and wake of wind turbine is not comprehensive,mainly due to the randomness of turbulent motion of atmospheric boundary layer and low accuracy of numerical simulation,which is difficult to describe quantitatively and needs further in-depth research and exploration.In this paper,the influence of turbulent coherent structures with different scales in the atmospheric boundary layer on the aerodynamic performance and wake characteristics of wind turbines is studied.Firstly,the extraction method of turbulent coherent structures and the key factors affecting their accuracy are studied by using the proper Orthogonal Decomposition(POD),and the prediction models of Power coefficient(C_p)and wake length of wind turbines are established based on the energy angle.Finally,the influence rules of different scale turbulent coherent structures on wind turbine power fluctuation,aerodynamic load and wake characteristics are obtained.The research results can provide a basis for the extraction of turbulent coherent structure,the prediction of wake length and velocity distribution,and the understanding of the influence of turbulent coherent structure on wind turbine power generation efficiency,power fluctuation and wake development.The main research conclusions and results of this paper are as follows:The wake field of wind turbine in neutral atmospheric boundary layer is calculated by Large Eddy Simulation(LES)based on Actuator Disk Model(ADM),and the convergence characteristics of POD snapshot sampling frequency,energy ratio,calculation cost and its influence on the reconstruction accuracy of wind turbine flow field are studied by POD method.The results show that,sampling frequency has little effect on the convergence of POD modes and the energy content of low-order modes,while low-order modes represent large-scale turbulent coherent structures and high-order modes represent small-scale turbulent coherent structures,which indicates that sampling frequency mainly affects small-scale turbulent coherent structures.When the sampling frequency is 0.015T_f,the accumulated energy of the first 100 modes of different snapshot sampling frequency examples has little difference.However,when the mode order is greater than 25,the sampling frequency will affect the energy proportion of each mode and the accumulated energy at higher POD mode number(>100 order),thus affecting the accuracy of flow field reconstruction.When the same number of POD modes are used to reconstruct the flow field,the accuracy of the flow field obtained by the example with low sampling frequency is obviously higher than that obtained by the example with high sampling frequency,and when the same accuracy of flow field reconstruction is achieved,compared with the minimum example,the calculation cost of the maximum sampling frequency example can save about 86%.For a certain number of POD modes,the lower the sampling frequency,the higher the accuracy of flow field reconstruction.It is also found that the large-scale turbulent coherent structure in the wake of wind turbine consists of pairs of counter-rotating vortices in the streamwise direction,and its vertical scale can reach the order of atmospheric boundary layer thickness.Power coefficient and wake length are important indexes of wind farm design and operation control,and accurate estimation of the coefficient is of great significance to the design of wind energy conversion system.In this study,the large eddy simulation method based on Actuator Line Model(ALM)is used to simulate the wake field of wind turbine with different tip speed ratios.A new method for estimating the power coefficient and wake length of wind turbines is proposed by POD method,and the corresponding prediction model is established.The results show that the maximum error between the power coefficient calculated by the new prediction model and that calculated by the conventional method is 1.7%,which proves that the prediction model of power coefficient proposed in this study is reliable.The wake velocity profiles of wind turbines at different tip speed ratios verify that the wake length model proposed in this study has great accuracy,and the wake length of wind turbines at different tip speed ratios is about 28.5D;According to the energy difference between the front plane and the rear plane of the wind turbine,it is found that more than 99.5%of the energy absorbed by the wind turbine is provided by the 1st POD mode.It is proved that large-scale turbulent coherent structure is the main energy source of wind turbine.Based on POD method and weight function,a POD reduced-order model(POD-WROM)is proposed,and the influence of different scale turbulent coherent structures in the neutral atmospheric boundary layer on the aerodynamic performance and wake characteristics of wind turbines is calculated by this model.The results show that the turbulent coherent structure is the main factor that determines the turbulent structure,wind speed fluctuation,aerodynamic performance and wake characteristics in the atmospheric boundary layer.When the energy content of turbulent coherent structure exceeds 1%,the fluctuation amplitude of wind speed in neutral atmospheric boundary layer increases with the increase of energy content of turbulent coherent structure.When the energy content of turbulent structure exceeds 35.4%,the wind turbine bears high-frequency dynamic load and power,with the lowest thrust amplitude of 2.4%and the highest thrust amplitude of 13.9%.The lowest power amplitude is 4.5%and the highest is 28.6%.When that energy content of coherent structure increase to 45.2%,the average power generation of wind turbines increased by 26%.Research of wake shows that when the energy content of turbulent coherent structure exceeds 14.2%,the spanwise width of wake expands to 2.5D,and the offset reaches 2D,and the starting position of wake vortex breaking dissipation zone reaches 7D from the wind turbine,and the vertical width of wake extends to the height of atmospheric boundary.Turbulent coherent structure will accelerate the recovery of wake velocity and momentum exchange between atmospheric boundary layer and wind turbine wake. |
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