Study On Wake Characteristics Of Wind Turbines Based On Actuator Line Model

The wake effect of wind turbines, which has a serious effect on power output and safety of wind farms, could reduce power and increase fatigue load of the downstream wind turbines.The wake effect is very important for the micrositting of wind farms. Wake effects of wind turbines are affected by inflow wind conditions, terrain of wind farms and placement of wind turbines. In this paper, the characteristics of wakes of wind turbines operating in various wind conditions including uniform inflow, shear inflow, turbulent inflow and interacting wakes between multiple wind turbines were studied by wind tunnel experiments and numerical simulations. The study would help to understand more accurately development and change of wakes of wind turbines and would contribute to improving wind farm micrositting and wind power forecasting.The numerical study of wake of wind turbines was carried out with actuator line model. The model could lower the difficulty of meshing of blades and accelerate the computation, since the blade was represented by body force instead of actual geometry. In this paper, the actuator line model was developed based on ANSYS FLUENT. The nacelle and tower which were modled using body force were to be considered firstly in actuator line model developed and improved the computational accuracy. The main factors were determined according to the aerodynamic parameters and wake flow field of wind turbines. The regulared parameter of three-dimensional gaussian distribution of body force should not be lesser than twice cell side length and the time step should be confirmed according to Courant-Friedrich-Levy-number(CFL number). The actuator line model developed could be used to simulate wake flow fields of wind turbines by comparing numerical results and experimential results of model wind turbine in wind tunnel in IET and comutational results of NREL phase VI wind turbine.For uniform inflow, the characterisitics of wake of NREL phase VI wind turbine were studied using actuator line model and the wake parameters of a model wind turbine in IET wind tunnel were measured with the hot-wire anemometer. Resultes showed that:the blocking effect of tower would increase the velocity deficit and turbulent intensity of wake flow under the center of wind turbine.The effects of naceller and tower on the near wake could not be neglected. The power and thrust force of wind turbine would increase as the number of blades increased, at the same time, the velocity deficit and turbulent intensity of wakes wolud increased. The velocity deficit and turbulent intensity in wakes would decrease with a increase of the distance from wind turbine. Then, higher power output and smaller fatigue load would be got through choosing proper spacing of wind turbines.For shear inflow, the characterisitics of wake of NREL phase VI wind turbine were simulated using actuator line model. Resultes showed that:the velocity deficit of upside of wake was greater than that of downside and the velocity of wake flow field was asymmetric about wind turbine due to wind shear effect and blocking effect of wall. The wake vorticity was skewed because of velocity difference between upside and downside.For turbulent inflow, the turbulent inflow conditions self-maintened were set up by modifying turbulent model and shear stress of walls, then the characterisitics of wake of NREL phase VI wind turbine were computed. Resultes showed that:the recovery of wake of wind turbines in turbulent wind was faster than it in uniform and shear wind conditions because of higher turbulent intensity. The turbulent intensity of wind was the most important factor about wake of wind turbines.For interacting of wakes inflow, the numerical simulations were carried out with actuator line model. Resultes showed that:the velocity deficit was maximum and power of wind turbine was minimum when the downstream wind turbine was fully in wake. At the same time, velocity deficit and turbulent intensity increased and the wake region increased because of interaction of two wake regions of wind turbines. The power and thrust force of wind turbine was greater when the wind turbine was partly in wakes, but the fluctuation increased. The maximum fluctuated force of wind turbine was got when the centre of wind turbine and the boundary of wake overlaped.For tropical cyclones whic have severe impact on wind farms in southeast coastal area, the tropical cyclone atmospheric boundary layer was generated firstly using spires and hex bolts and the characterisitics of wake of model wind turbine in IET wind tunnel were measured. Resultes showed that:the velocity in wake field was asymmetric about wind turbine, but the velocity deficit was symmetric. The wake had already recovered mainly when the distance was triple diameter of wind turbine in tropical cyclone condition.