Research On The Wake Evolution Law Of Horizontal Axis Wind Turbine Based On The Field Environment

Wind turbines operate at the bottom of a turbulent atmospheric boundary layer with a complex structure.With the large-scale of wind turbines,the interaction of wind turbines with the flow of atmospheric boundary layer is more prominent.The wake of the wind turbine,as the product of the interaction between atmospheric turbulence and the wind turbine,shows high unsteady characteristic with the change of atmospheric turbulence,thus affecting the safe and stable operation of the entire wind farm.However,the law and mechanism of the dynamic characteristics of these wakes are still unclear.Therefore,an in-depth understanding of the evolution characteristics of wake in the atmospheric environment is of great significance to the planning,site selection,control strategy,wind power assessment and safety assessment of wind farms.In view of this,the evolution law of the wake of the wind turbine in the external environment was studied based on the wind turbine field experiment combined with the numerical simulation.The main research work is as follows.（1）The evolution law of blade tip vortices near wake in different atmospheric background turbulence was revealed.The research results show that the blade tip vortex near wake shows the locality of the turbulent structure,and the main feature appears at the blade passing frequency（2f _T）.With the increase of the average turbulent kinetic energy（TKE）of the inflow,the TKE of the blade tip vortex also increases.But the amplification is limited,which is mainly determined by the operating state of the wind turbine.When the scale and intensity of inflow turbulence increase(TKE is above 3m²s^-2),the large-scale turbulent structure in the background atmosphere becomes unstable,and many high-frequency small-scale structures appear under the dual action of wind rotor disturbance and blade tip vortex.That is,the high-frequency energy increases significantly with the increase of TKE of the inflow.The increase of TKE of the inflow will intensify the flow of the blade tip vortex of the wake in the horizontal and vertical directions,thereby causing an increase in the influence range of the blade tip vortex.At the same time,the energy in the main flow direction will also be dispersed,resulting in faster recovery of the blade tip vortex.（2）The fusion/recombination law of blade tip vortices near wake in the atmospheric environment was announced.With the increase of tip speed ratio,the thread interval of the helical vortex structure decreases,the interaction between adjacent vortex filaments enhances,and the phenomenon of vortex fusion/recombination occurs in the adjacent blade tip vortices.That is,the characteristic of the blade tip vortex at the blade passing frequency（2f _T）gradually disappears,and the characteristic frequency shifts to the rotor rotational frequency（f _T）.The fusion/reorganization of the structures of the blade tip vortexes increases the horizontal and vertical flow angles of the blade tip vortex,which aggravates the expansion of the blade tip vortex,and then expands the influence range of the wake.Moreover,the fusion/recombination of the structures of blade tip vortexes accelerates dissipation of high frequency turbulent structures.（3）The influence mechanism of the nacelle/tower on the turbulent transport near wake of the wind turbine was disclosed.After the atmospheric turbulence is modulated by the operating wind turbine,The initial position of the inertial sub-region in the wake moves from0.3Hz of the flow to the blade passing frequency of 2f _T.And there is a peak characteristic at the blade passing frequency 2f _T,which is most obvious especially at the corresponding position of the blade tip and the nacelle.The vortex structure shed by the nacelle and the tower interacts with the vortex structure shed on the blade,and the power spectrum is distorted before 2f _T,resulting in a frequency band with relatively stable energy.And this distortion of the power spectrum is more obvious in the wake region of the tower corresponding to the middle section of the blade.With the increase of the tip speed ratio,the fusion of blade shedding vortices still occurs in the wake of the tower affected area,and the fused turbulent structure is dominant in the influence area of the tower.The nacelle/tower effect will exacerbate the expansion and asymmetry of the wake structure.（4）Based on the coupled method of the actuation line and large eddy simulation,the influence of inflow dynamic wind direction on the wake evolution law was carried out.And a rapid assessment method for the wake of the wind farm considering dynamic wind direction is proposed.The research results show that the dynamic wind direction will cause changes in the wake velocity distribution,turbulent intensity distribution,meandering characteristics and wake boundary.The dynamic wind direction angle with the larger variation amplitude or the smaller period leads to faster recovery of the average velocity of the wake.The changes in dynamic wind direction increase the turbulent intensity of the near wake and accelerate the fusion of wake vortex structures.Under the condition of dynamic wind direction,the oscillation phenomenon of the blade tip vortex occurs in advance.And with the increase of the change amplitude of the wind direction angle,the oscillation amplitude becomes larger,and the oscillation position is the turning point of the wake bending motion.The dynamic wind direction will intensify the meandering of the wake.With the increase of the angle amplitude,the degree of the meandering of the wake increases,and the maximum offset of the wake center shows a second power increasing law with the increase of the angle amplitude.The dynamic change of the wind direction angle changes the influence range of the wake in both the horizontal and vertical directions,especially in the horizontal direction of the far wake.In addition,the rapid assessment method considering dynamic wind direction for the wake of the wind farm can more accurately assess the power generation of wind farm.（5）The influence of atmospheric turbulence and atmospheric stability on velocity and meandering characteristics of the wake of the wind turbine were studied.The results show that with the increase of atmospheric turbulence and atmospheric instability,the recovery of the average velocity of the wake is advanced,and the influence range of the wake continues to increase.The meandering of the wake in the near wake is dominated by large-scale high-energy turbulence,while the meandering of the far-wake is affected by the dynamic characteristics of the rotor and the turbulence effect of the inflow.Under the condition of different inflow turbulence intensity,the meandering of the wake has a certain similarity,and shows the form of moving around along the axis of the wind rotor.The changes in atmospheric stability cause the wake center to move away from the ground,and this movement trend becomes more and more obvious with the increase of instability.