Detached Eddy Simulation Of Wind Turbine Wake Effects

Wind turbine wakes lead to velocity deficit and turbulence intensity increase in the downstream area of wind turbines.It results in power losses and fatigue load,which have negative impact on the economic benefits of wind farms.Unsteady,massive separation and complex vortex structures are the typical features of wind turbine wakes.In engineering,Large Eddy Simulation(LES)and Reynolds-averaged Navier-Stokes(RANS)are often used to simulate the wind turbine wake.However,the calculation resource and accuracy of the flow filed simulation cannot be guaranteed simultaneously by using LES or RANS.Detached Eddy Simulation(DES)which combines the advantages of LES and RANS is more suitable for complex large flow separation of wind turbine wakes.Consider synthetically of the computing resource and accuracy,based on NREL Phase VI scaled turbine model,DES was adopted to carry out the flow field simulation of a single turbine and two in-line turbines simulation to study the distribution law of the wake and the wake interference area.Main contents and conclusions are:(1)A numerical modeling method for structured grid of wind turbine based on DES is proposed.On the bases of NREL Phase VI wind turbine scaled model,a three-dimensional multi block subdomain structured meshing method is adopted to establish a structured grid numerical model of wind turbine based on DES and RANS.The relative errors between the wind speed simulated by the two methods and wind tunnel experiment are no more than 5%,which verifies the reliability of the numerical model.The time-averaged wind velocity in turbine wake described by the DES and RANS is relatively close,but RANS erases the turbulence pulsation characteristics of the wake field.DES can adequately disclosure the fluctuation characteristics of wind turbine wake,and minutely describe the changing process of vortex in the wake.(2)Turbine wake simulation was developed based on DES to reveal the performances of wake and the influence of the inflow velocity on the field.Wind speed,as the main parameter to reflect the information of the flow field,will largely affect the flow characteristics of wind turbine wake,and it is also of great significance for wind turbine site selection.DES was applied to simulate the flow field under different inflow wind speed.The characteristics of wind speed and vortex structure in the wake was studied,and the influence of inflow wind speed on the wake field was analyzed.Results showed that:kinetic energy is extracted by the wind turbine rotor,which leads to the wind speed decay after the wind wheel,and the lower the inlet wind speed is,the more serious the wind speed decays in the wake area;the position of the tail vortex is proportional to the inflow velocity,that is,the greater the wind speed,the farther the vortex begins to break;according to the evolution of the wake vortex,the wake area can be divided into three region;in the near wake,the width of the wake keeps constant,and has nothing to do with the inflow velocity;starting from the separation zone,the width of the wake is expanding in a linear law,and is inversely proportional to the inflow wind speed.(3)DES method was used to simulate the two in-line turbines wake,and the influence of wake interference effect on the wind speed distribution was obtained.Wake interferences produced by two in-line turbines are the most common form of wake interference,and are the basis of the research on interference effect of multi wake.Three dimensional numerical model based on 2 in-line turbines were established,and the numerical simulation was carried out by DES.The results show that:within 3D after the rotor,the speed after upwind turbine is very close to the single turbine,where the width of the wake area is constant;after 3D,the speed is lower than that at the corresponding wake free zone,and the width of the wake area is increased,due to the influence of the low speed zone of the downstream turbine;wind speed in the interference field is basically recovered to 80%of the reference speed after 12D in the downstream area,due to the energy exchange of the interference region;the wake region is continuous from upwind turbine wake to wake interference,but the variety of the wake interference width is relatively complex,and is not the simple superposition of the width of the free wake zone.