Numerical Simulation Of Multiple Wind Turbines Wake Based On Actuator Disc Method

Due to the absorption of inflow energy during the operation of the wind turbine,the downstream wind speed is reduced and the turbulence intensity is increased.This phenomenon is called the wake effect.The wake effect will cause power loss and load rise for downstream wind turbines.Therefore,in-depth study of the wake effect is beneficial to the micro siting and safe operation of the wind farm.Compared with the traditional linear method,the nonlinear CFD method has high precision and can capture complex flow conditions.The actual wind farm range is large,and the size of the wind turbine is in relatively small scale,which increases the span of the grid resolution,and also increases the amount of calculation.The RANS method along with k-? turbulence model and the Actuator disc method are adopted to calculate the wake flow field of single and multiple wind turbines,which can balance the calculation time and accuracy requirements in calculation of the wake flow.The standard k-? turbulence model and the actuator disc method are modified to accommodate the simulation of single and multiple wind turbine wakes.For the problem of fast recovery of wind velocity when calculating the wake using the standard k-? turbulence model,an improved method of adding both the turbulence kinetic energy source term and the dissipation rate source term,as well as adopting a parabolic distribution of the dissipation rate source parameter is proposed to improve the accuracy of single wind turbine calculation.For the problem that the fixed turbulence model source term cannot deal with the change of the flow condition,a variable source term method is proposed to improve the accuracy of multiple wind turbine calculation.The flat terrain is took as the research object,the boundary condition equations and turbulence model parameters that agree with homogenous flow field are discussed and the influence of grid condition changes on the flow field are analyzed.The wind turbine is in the atmospheric boundary layer flow field,and accurate simulation of the atmospheric boundary layer flow field is a guarantee to improve the simulation accuracy of the wind turbine wake.Three kinds of inflow boundary conditions and four sets of turbulence model parameters are discussed,and the boundary condition equations and turbulence model parameters that agree with homogenous flow field are determined.It is found that the result of the flow field is not sensitive to the variation of the first layer grid height and its proportion in the vertical direction in the flat terrain cases.The single wind turbine is took as the research object and calculated using improved k-? turbulence model.The mesh division method for ensuring the simulation accuracy of the actuator disc method is determined by the grid independence calculation.The wake flow of the four wind turbines Nibe-B,Dawin,Sexbierum and GH are calculated under various flow conditions and operating conditions using the above mentioned calculation conditions and the improved k-?turbulence model.The results show that the average error of the improved k-?turbulence model is lower than that of the similar k-? turbulence model and one kind of k-? SST turbulence models by 0.6%?5.25%,which is close to the LES method.Then,the characteristics of the wake flow velocity and the turbulent kinetic energy of the two wind turbines in the tandem and staggered arrangement are qualitatively analyzed.The multiple wind turbine is took as the research object and the variable source term method are used for calculation.the influence of the actuator disc thickness and the grid ratio of the flow direction on the number of grids is discussed,and the division method of the flow direction grid for the multi-wind turbines is confirmed.For the problem that unable to determine the inflow for downstream wind turbine,a method based on the average speed of the actuator disc is proposed,and the wind speed is derived according to the power curve.The wake flow of the Horns Rev and the Lillgrund offshore wind farm are calculated under various flow conditions and operating conditions using the variable source term method and the improved k-?turbulence model.The results show that the proposed method can accurately calculate the inflow and the average error of the adaptive source term method is lower than that of the similar k-? turbulence model by 0.49%?3.55%,which is close to the LES method.