| Wind turbines operate entirely in the atmospheric boundary layer(ABL),which has multi-scale and high-energy turbulent motion and is usually dominated by large spatial coherent structures.Large scale motion(LSMs)and very large scale motion(VLSMs),as the main coherent structures in the atmospheric boundary layer,have been widely studied in the scientific community in the last decade.These structures have flow-direction dimensions up to(0.3-3)δ and(>3δ),respectively,where δ is the thickness of the near-wall ABL,also known as the atmospheric surface layer thickness.It is found that more than 65% of kinetic energy and 60% of Reynolds stress in ABL are contributed by LSM and VLSM.More specifically,more than 99% of the energy absorbed by wind turbines is provided by LSM and VLSM.Therefore,LSM and VLSM are crucial to the structural evolution of ABL and the operation of immersive wind turbines.However,the flow field characteristics of wind turbines under turbulent atmospheric boundary layer conditions,especially the influence of multi-scale coherent structure on the performance of wind turbines,are still unclear.It is very important for micro-site selection,wind turbine power prediction,wind turbine design optimization and stable operation.Therefore,it is of practical significance and application value to study the influence of turbulent structure and pulsation characteristics of actual atmospheric boundary layer incoming flow on aerodynamic performance of wind turbines for power prediction,planning,design and optimization of wind farms.Therefore,based on the field observation data in Northwest China,the structure characteristics of turbulence are studied systematically,and the power and blade root load of small wind turbines are studied.The main research contents and achievements of this paper are as follows:Large scale motion(LSMs)and very large scale motion(VLSMs)are the main coherent structures in the atmospheric boundary layer(ABL),but these structures have not been fully considered in the analysis of wind turbine power and load,and their effects on wind turbine load have not been revealed.In this paper,the structure characteristics of turbulence and the effect of coherent structures on aerodynamic loads of small wind turbines are systematically studied by using field observation data in Northwest China.The turbulent wind generated by the Kaimal model(IECKAI wind),which is commonly used in wind turbine design,is also compared.The results show that the measured wind flow velocity fluctuates with low frequency and high amplitude.These low-frequency coherent structures appear in the band of 0.005 ～ 0.01 Hz,and the average flow direction scale is(6 ～ 10)δ.In contrast,no very low frequency structures are observed in IECKAI wind,and the coherent structure scale is about(2-3)δ.Specifically,LSMs and VLSMs coexist in the measured atmospheric boundary layer,while only LSMs exists in the IECKAI simulated wind.The load analysis shows that there is a very high positive correlation between the flow direction wind speed and the load fluctuation,and low frequency LSMs and VLSMs can significantly increase the slope between the load and the velocity fluctuation.In order to further clarify the influence of different scales,especially super-large scales,on the aerodynamic performance of wind turbines,this paper extracts the turbulence coherent structure signals of different scales from wind speed signals through multi-scale wavelet decomposition,and compares and analyzes the influence of turbulence coherent structure of different scales on the aerodynamic performance and output power of wind turbines.It is found that VLSM turbulent structure dominates the pulsation amplitude of wind turbine power and load,while LSM and small-scale turbulent structure dominate the pulsation frequency of wind turbine power and aerodynamic load.The contribution of VLSM turbulence structure to the power,thrust and flapping moment of wind turbine is 85%,57% and 46%,respectively,while the power and load pulsation of large-scale and small-scale turbulence is less than 20% on the whole.The load response of wind turbine is caused by the combined action of turbulence structure scale,the energy carried by turbulence structure and the inertia of the mechanical components of wind turbine.Under the action of turbulence structure of different scales,the load response of wind turbine has a certain frequency doubling relation with the wind wheel rotation frequency.Moreover,due to the scale difference,when the turbulence structure passes through the wind turbine,the force imbalance of blades is caused.The cyclic load changes at different frequencies,and the load response peaks at lower frequencies under large and small scales,while the load response frequency is larger under very large scales.Therefore,under the action of super-large scale structure,the aerodynamic load cycle change period of wind turbine is long,while under the combined action of large and small scale,the load cycle change is faster and the period is short.The wind turbine is subjected to both random turbulence and intermittent turbulence in operation,which can cause fatigue load and transient load of wind turbine.At the same time,in actual wind farms,wind turbines are often affected by turbulence at different scales.When the length scale of turbulence is equivalent to the characteristic length scale of wind turbine components,the dynamic response characteristics of small wind turbines may be affected.Therefore,this paper further studies the intermittent characteristics of turbulence at different scales,and the response of wind turbine aerodynamic load to intermittent turbulence.The results show that,at t = 0.02 s time scale,the flow wave has strong intermittent statistics,and the intermittent behavior is strongest at small scale conditions.At the same time,the hyperscale structure will suppress the probability of large oscillation of wind speed,but increase the possibility of small oscillation.Under the super-large scale structure,the increase of wind speed exceeding 9σ is inhibited.Under the combined action of large and small scale structures,the probability of ±6σ wind speed fluctuation is about 11 times that under the action of very large scale only,where σ is the root mean square of the pulsation parameter,which is used to describe the fluctuation amplitude of the characteristic parameter.At the same time,the intermittency of wind turbine load is closely related to the spatial and temporal scales of turbulent structure.Under ultra-large wind conditions,the power,thrust,flapping moment and pendulum array moment of wind turbine have a large increment of more than 11σ,8σ,17σ and 5.6σ,respectively,which are inhibited by ultra-large scale structure.However,for the increment of power,thrust and flapping moment of wind turbine less than ±5σ,The occurrence probability of ultra-large scale wind turbine is 16 times,14 times and 12 times of that of small-scale wind turbine,respectively.It can be seen that ultra-large scale structure can not only restrain the probability of large oscillation of wind turbine aerodynamic performance,but also increase the possibility of small oscillation.The peak values of intermittent parameters of power,thrust,flapping moment and pendulum array moment all occur at 10 s,10s,12 s and 16 s at small and large scales,while the peak values at 101 s,25s,140 s and 115 s at very large scales. |
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