| The wind turbine operates in the lowest region of the Atmospheric Boundary Layer(ABL).According to the definition of atmospheric boundary layer by MoninObukhov stratification stability parameter,the three state changes of atmospheric boundary layer are Convective Boundary Layer(CBL),Neutral Boundary Layer(NBL)and Stable Boundary Layer(SBL),and atmospheric stability has an important influence on wind turbine performance,wake expansion and dissipation,blade load and productivity prediction.In this paper,the Large Eddy Simulation(LES)method is used to study the influence of turbulence scale on the aerodynamic performance and wake characteristics of wind turbines under different atmospheric stability conditions.Firstly,the wake characteristics of wind turbines with different tip speed ratios are studied in neutral atmosphere.On this basis,the effects of different turbulence scales on the aerodynamic performance and wake characteristics of wind turbines are analyzed from the energy point of view by Proper Orthogonal Decomposition(POD).Finally,the influence of atmospheric stability on the aerodynamic performance of wind turbine is studied.The research results can provide a basis for deeply understanding the influence of turbulent coherent structure on the aerodynamic performance and wake characteristics of wind turbines under different atmospheric stability conditions,and help to optimize the design,operation strategy and site selection and layout of wind farms of wind turbines.The main research contents and achievements of this paper are as follows:The Large Eddy Simulation(LES)method based on the actuation line model is used to simulate the wake field of wind turbines with different tip speed ratios under neutral atmospheric stability conditions.The effects of tip speed ratio on wake speed loss,wake length and turbulence intensity are studied,and the modal convergence,modal coefficient and spatial coherence structure of POD under different tip speed ratios are studied by proper orthogonal decomposition method.The results show that the tip speed ratio has a significant influence on the velocity distribution in the downstream of the wind turbine for 12D(D is the diameter of the wind turbine),and the influence of the tip speed ratio gradually weakens with the increase of the flow direction distance.In the wake of the wind turbine,the speed loss is the largest at 2D,and the speed is completely recovered near 28.5D downstream;The tip speed ratio has a great influence on the wake turbulence intensity of wind turbine.When λ=2,the wake turbulence intensity is the weakest,while when λ=3,the wake turbulence intensity is the largest,which shows that the mixing effect between wake and atmospheric boundary layer is intense at this tip speed ratio,which is beneficial to wake speed recovery.Tip speed ratio has a significant influence on the scale and energy content of wind turbine wake structure.Whenλ=3,the large-scale structure and its energy content in the wake are the highest,and with the increase of POD modal order,the scale and energy content of wake structure gradually decrease.The wake structure of wind turbine is composed of high-speed and low-speed vortex pairs rotating in opposite directions,and its vertical scale can reach the whole boundary layer thickness,while the wind energy absorption and utilization efficiency of wind turbine is mainly reflected in the scale of wake-spreading turbulent vortex structure.Quantifying the influence of turbulence scale on the wake characteristics of wind turbine aerodynamic performance under different atmospheric stability conditions is of great significance to wind resource evaluation,wind turbine design and selection,wind farm power prediction and site selection.In this study,POD reduced-order model is used to reconstruct the flow field under different atmospheric stability conditions and different turbulence scales from the energy point of view,and the influence of different turbulence scales on the aerodynamic performance and wake characteristics of wind turbines is quantitatively analyzed.The results show that the scale of coherent structure in SBL pulsating flow field is the smallest,and the maximum scale of coherent structure in spanwise and vertical direction is only 2D×3D,while the maximum scale of CBL pulsating flow field and NBL pulsating flow field in spanwise and vertical direction reaches 8D×9D and 4.2D×6D;With the increase of energy content of turbulent coherent structure,the fluctuation degree of wind speed increases,and the shape,size and spatial position of coherent structure in the whole fluctuating flow field also change,which has a deeper influence on the flow field;Turbulent coherent structure in CBL flow field has the greatest influence on wind turbine power and thrust fluctuation,while in NBL flow field,turbulent coherent structure in SBL flow field has the least influence on wind turbine power and thrust fluctuation.The power fluctuation caused by turbulent coherent structure is more obvious than the thrust fluctuation;With the energy content of the turbulent coherent structure increasing gradually,the interaction between the turbulent coherent structure and the wind turbine intensifies,and the wind turbine will bear high-frequency and high-amplitude dynamic loads,which will affect the wind energy utilization efficiency and service life of the wind turbine.It is also found that when the energy content of the turbulent coherent structures of CBL,NBL and SBL is higher than 45%,the interaction between the turbulent coherent structures and the wind turbine is intensified,which promotes the recovery of the wake.However,due to the influence of the multi-scale turbulent coherent structures on the wake field of the wind turbine,the shimmy of the instantaneous flow field of the wind turbine is intensified.In order to further study the influence of atmospheric stability on the aerodynamic performance of wind turbines,the load characteristics of wind turbines under different atmospheric stability conditions are studied by combining large eddy simulation(LES)with FAST aerodynamic calculation and decomposing the scale through continuous wavelet transform.The results show that under different atmospheric stability,the super-large-scale turbulent coherent structure dominates the wind speed fluctuation of the flow field,and the fluctuation amplitude is much larger than that of the large-scale and small-scale conditions;The wind speed of CBL and NBL fluctuates greatly,while that of SBL fluctuates slightly.Turbulent atmospheric motion is the most intense under CBL condition,and the interaction between turbulent coherent structure and wind turbine is the strongest,resulting in the largest power fluctuation;The turbulent atmospheric motion is relatively stable under NBL and SBL conditions,and the power fluctuation is smaller than that under CBL conditions.Atmospheric stability has a great influence on wind turbine load,and the super-large-scale turbulent coherent structure is an important reason for wind turbine load fluctuation,and the wind turbine load has a strong correlation with the incoming wind speed.The wind turbine load fluctuates violently under CBL and NBL conditions,and the load fluctuation is relatively stable under SBL conditions. |
