High-fidelity Numerical Simulation And Analytical Modeling Of Wind Turbine Wakes In Complex Atmospheric Conditions

The process of generating electricity by a wind turbine is to convert the kinetic energy in the air into mechanical energy through the rotation of the rotor,and then into electrical power through the generator.The extraction of energy from the air by the rotor causes the speed of the area downstream to decrease and the turbulence intensity to increase.This phenomenon is called wake effect.The wake effect reduces the power production of the wind turbine downstream,and increases the fatigue load,which leads to a decrease in the generating capacity of the entire wind farm and an increase in operating costs.In order to increase the profitability of wind farms,it is necessary to conduct in-depth studies on wake effects using high-precision methods.Wind turbines extract energy from the atmospheric boundary layer.The wind shear and turbulence intensity of the atmospheric boundary layer are different under various surface roughness and atmospheric stability conditions.They have different effects on the wind turbine wake recovery speed,fatigue load and wind farm power generation.Based on the above knowledge,this paper has carried out four aspects of studies about the wake effects.This paper first studies the characteristics of the boundary layer flow field under different environmental conditions based on the open source software OpenFOAM platform.A second-order hybrid difference scheme suitable for the large eddy simulation of wakes is constructed,and applied to the study of the wake flow of wind turbines under different conditions,and finally a new MOST-Gaussian wake model is developed based on the in-depth understanding of the wake evolution.These four aspects are detailed as follows.Firstly,this paper studies the atmospheric boundary layer under different roughness and stability conditions.To simulate the real atmospheric boundary layer flows,the thermal convection effect of the surface and atmospheric boundary layers is considered through the potential temperature transport equation.Then the flow characteristics of the atmospheric boundary layer under different surface roughness and atmospheric stability conditions are studied,and the effects of these two factors on the average velocity,turbulence intensity,skewness and flatness factor in the boundary layer are analyzed.With the flow characteristics of the atmospheric boundary layer,reliable initial and inflow conditions can be provided for the Large eddy simulation of wind turbine wakes.Secondly,a hybrid second order scheme is developed for the Large eddy simulation of wind turbine wakes.The unstructured finite volume method usually has secondorder accuracy.The second-order central difference scheme used in the study of wind turbine wakes can easily lead to unstable solutions,and the dissipation of the secondorder upwind scheme is too large.Based on the above considerations,a second-order hybrid difference scheme is constructed in this paper,which better balances the numerical stability and solution accuracy.By comparing with the wind tunnel experiments,the effectiveness of the second-order hybrid scheme is proved.By comparing and analyzing the three second-order schemes,it is found that the second-order hybrid scheme has better stability and can also capture small-scale turbulent structures in the flow field.This indicates the hybrid scheme can be used to simulate wind turbine wakes.Thirdly,based on the study of the atmospheric boundary layer and the second-order hybrid scheme,the effect of different atmospheric environmental conditions on the wind turbine wakes is carried out.The large eddy simulation results of atmospheric boundary layer were used as the initial field and inlet boundary conditions of wind turbine wake large eddy simulation.The second-order hybrid scheme is used to discrete convection terms to study the effects of surface roughness and atmospheric stability on wake development.By comparison,it is found that the greater the surface roughness,the faster the speed recovery,and the more unstable in the atmospheric boundary layer,the faster the speed recovery.The speed of recovery in the wake region is closely related to the turbulence intensity in the boundary layer,which provides the necessary foundation for the further development of the analytical wake model.At last,based on an in-depth understanding of the evolution of wind turbine wakes,a MOST-Gaussian wake model based on Monin-Obukhov similarity theory was developed,which includes the effects the surface roughness and atmospheric stability.The new model establishes a relationship between wake recovery velocity and spanwise turbulence intensity,and more accurately reflects wake recovery characteristics.By comparing with the Large eddy simulation results and wind farm test results,it is verified that the MOST-Gaussian wake model has good performance under different conditions of surface roughness and atmospheric stability.The comparison results indicate that the MOST-Gaussian wake model developed in this paper has very good potential for improving the efficiency of wind farms layout optimization and increasing wind farm power generation.