| With the steady development of various technologies in the wind power industry,the size of wind turbines tends to be large,which leads to the actual operation of wind turbine flow conditions becoming more complex.The changing inlet conditions have a great influence on the wake evolution,load of important components and power generation of wind turbines,which makes it difficult to ensure the safe operation and economic benefits of wind farms.However,the current study on the influence of real inflow conditions on wind turbine wake characteristics is not comprehensive,and there is still a lack of understanding and discussion on the wake winding triggering mechanism of large wind turbines.Therefore,it is very important to correctly evaluate the influence of different flow conditions on the wake evolution and meandering characteristics of wind turbines.Firstly,the standard actuator line model is improved.Aiming at the problem of insufficient prediction of near-wake velocity loss by standard actuator line model,the nacelle and tower are modeled on the basis of standard actuator line model.By comparing the experimental data of the"Blind test"with the numerical simulation results,the results show that the improved actuator line model is in good agreement with the experimental data for thrust coefficient,power coefficient and velocity distribution in the near-wake region（1D and 3D downstream of the wind turbine）.Therefore,the model can meet the needs of follow-up research.Secondly,based on three typical surface roughness（z0=0.001 m,z0=0.02 m,z0=0.2 m）,the neutral atmospheric boundary layer is generated by precursor method.Then,the atmospheric boundary layer with different surface roughness was taken as the incoming flow condition,and a single wind turbine and a series wind turbine with different spacing were numerically simulated with the actuator line model,and the influence of different surface roughness on the average and instantaneous characteristics（meandering characteristics）of the wind turbine wake field was analyzed.It is found that with the increase of surface roughness,the turbulence intensity in the atmospheric boundary layer increases,and the mixing between external turbulence and wake becomes more intense,which leads to the wind turbine wake recovering closer to the rotor.In addition,the wake meandering phenomenon is observed under three wind conditions with different surface roughness.The fluctuation of the wake center of a single wind turbine is successfully captured by Gaussian fitting,and it is found that the amplitude of the wind turbine wake meandering oscillation increases with the increase of the distance in flow direction,which is also applicable to tandem wind turbines.For tandem wind turbines with different spacing,the amplitude of the wake meandering oscillation of the downstream wind turbine is greater than that of a single wind turbine under all working conditions,which is related to the deterioration of the inflow conditions of the downstream wind turbine.At the same time,the wake meandering of a single wind turbine and the downstream wind turbine is accompanied by energy changes,which corresponds to the evolution of turbulent kinetic energy in the wake.Finally,under the wind conditions with different surface roughness,two PSD peaks can be observed in the spectrum analysis of fluctuating velocity,and the Strouhal number corresponding to the peaks corresponds to the two mechanisms of wake meandering.Finally,atmospheric boundary layers with different stability,including unstable and neutral atmospheric boundary layers,are generated by precursor method.The numerical simulation of a single wind turbine and a series wind turbine with different spacing was carried out,and the influence of different atmospheric stability on the average and instantaneous characteristics（meandering characteristics）of the wind turbine wake field was analyzed.It is found that the flow direction turbulence intensity of the unstable atmospheric boundary layer is similar to that of the neutral atmospheric boundary layer,but the wind turbine wake recovery speed is much faster than that of the neutral atmospheric boundary layer,indicating that turbulence intensity in three directions should be considered in the analysis of wake field characteristics.In the atmospheric boundary layer with different stability,it can be obviously observed that the amplitude of the wind turbine wake center fluctuation increases with the development of the wind turbine wake downstream,and the meandering oscillation amplitude of the horizontal wake is larger than that of the vertical plane,which is related to the obstruction of the ground.In addition,by analyzing the pulsation velocity spectrum,it is found that the PSD peaks corresponding to the large-scale turbulent structure and wake meandering caused by the instability of the shear layer can be observed in both the atmospheric boundary layers with two stability.In the unstable atmospheric boundary layer,the peak value of PSD representing wake meandering caused by large-scale turbulent structure is larger than that of wake meandering caused by shear layer instability,while in the neutral atmospheric boundary layer,the peak value of PSD corresponding to shear layer instability is larger.Therefore,compared with the neutral atmospheric boundary layer,the main reason for wake meandering in the unstable atmospheric boundary layer is the large-scale turbulent structure in the incoming flow.Combined with the analysis of wake meandering characteristics under different surface roughness wind conditions and different stability wind conditions,this dissertation further confirms the analysis of the causes of wind turbine wake meandering in previous studies.That is,there are two triggering mechanisms for wake meandering:（1）large-scale turbulent structure in the incoming flow;（2）shear layer instability. |
