Study Of Wake Characteristics Of Wind Turbine Based On Improved Actuator Disk Model

With the rapid development of wind power industry in China,the accurate evaluation of flow field in large-scale wind farm is very important for the development of wind farm.Among many methods of wind turbine wake research,the method of actuator disk model combined with two-equation turbulence model has great advantages because of less computational resources and relatively accurate results.However,because the actuator disk model simplifies the wind turbine,the two-equation turbulence model can not accurately calculate the evolution of the real wind turbine wake,so this method needs to be improved from many aspects to improve the accuracy of wind turbine wake evaluation.In this paper,based on the two-equation turbulence model(standard k-ε model,standard k-ω model and SST k-ω turbulence model),the decay law of turbulent kinetic energy and the corresponding control methods are studied.Based on the standard k-ε model,the method to improve the retention of equilibrium atmospheric boundary layer is studied.The accuracy of the actuator disk model in the prediction of wind turbine wake is improved by improving the turbulence model,the volume force distribution of wind rotor,and the simplified treatment method of nacelle and tower.It provides a reference for the study of wind farm flow field and wind turbine layout.(1)Through theoretical derivation and numerical simulation,this paper studies the turbulent kinetic energy decay law of two-equation turbulence models(standard k-ε model,standard k-ω model and SST k-ω turbulence model)in calculating uniform flow,shear flow and neutral atmosphere.From the determination of closure coefficients of turbulence model,influence factors of diffusion term of transport equation,inlet turbulence variables and grid scale of computational domain,the decay law and essential physical difference of turbulent kinetic energy of standard k-ε model,standard k-ω model and SST k-ω turbulence model in calculating uniform incoming flow are analyzed;The effects of turbulence model,inlet turbulence intensity and viscosity ratio,velocity gradient and diffusion term of transport equation on the decay of turbulent variables in shear flow are analyzed and compared with the numerical simulation results in uniform flow;The effects of different top boundary conditions on the decay of turbulent kinetic energy in neutral atmosphere are analyzed.The results show that there are different degrees of turbulent kinetic energy decay under these three inflow conditions,which are related to the above factors.However,the downstream turbulence kinetic energy decay can be reduced by selecting appropriate turbulence model,inlet turbulence variables and top boundary conditions.(2)Five control methods of turbulence kinetic energy decay in uniform flow are studied.Four methods are proposed to control the decay of turbulent kinetic energy in uniform flow by adding a source term to the right side of the transport equation of the two-equation turbulence model,and the effects of two determination processes of the closure coefficients of the turbulence model on the decay of turbulent kinetic energy are studied.Four kinds of source terms are added to the right side of k equation and ε(ω)equation of standard k-ε model,standard k-ω model and SST k-ωturbulence model,including adjustable supplementary term,adjustable adaptive supplementary term,adjustable dissipation term adjustment term and adjustable dissipation term correction supplementary term.The first method to determine the closure coefficients of turbulence model is based on the conservation relationship between the generation and dissipation of turbulent kinetic energy and the experimental value in atmospheric boundary layer.The second method is based on the empirical value of free shear flow diffusion rate.The results show that the turbulence kinetic energy decay level can be effectively controlled by adjusting the corresponding source term coefficient or turbulence model closure coefficients.(3)Several problems before the improvement of the actuator disk model are discussed.Firstly,the independence of the mesh used in this paper is verified,and then several problems before the improvement of the actuator disk model are studied by using the basic actuator disk model combined with the standard k-ε turbulence model:The influence of the thickness of the actuator disk or the number of grid layers in the disk on the accuracy and stability of the numerical simulation results;The modification methods of grid identification include modifying volume force source term,optimizing grid generation method and the combination of the two methods;Based on the previous studies,seven methods of adding the volume force of wind rotor are deduced,and finally three suitable methods are determined;The influences of roughness height,roughness length,roughness coefficient,wall function and the height of the first layer grid on the application of the actuator disk model are analyzed;The influence of three inlet turbulence parameters calculation methods on the wake prediction of wind turbine with the actuator disk model is analyzed;The effects of five methods and their combinations to improve the retention of the equilibrium atmospheric boundary layer are studied,and the influence of these methods on the prediction results of wind turbine wake is analyzed.It is found that the study of the above problems can improve the stability and accuracy of the calculation process of the actuator disk model.(4)The improved method and effect of the actuator disk model are studied from three aspects: the simplification of nacelle and tower,the volume force distribution of wind rotor and the improvement of turbulence model.The influence of nacelle and tower and its shape,the adding method of volume force and the distribution of volume force on the calculation results of the actuator disk model are studied;A volume force distribution method considering rotation based on axial induction factor and tangential induction factor is proposed and compared with other volume force distribution functions;Combined with the control method of turbulence kinetic energy decay,the improvement method of equilibrium atmospheric boundary layer retention,the dissipation source term and generation source term of turbulent kinetic energy,the improved method of two-equation turbulence model combined with the actuator disk model under uniform and shear flow is studied.The results show that the distribution of the volume force of the wind rotor and nacelle proposed in this paper can improve the accuracy of the numerical simulation,and the accuracy of the wake prediction can be improved by combining the turbulence kinetic energy decay control method and the equilibrium atmospheric boundary layer retention improvement method with other turbulence model improvement methods.