Numerical Simulation Of Wind Turbine Wake Interference Effects

Wind turbine wakes lead to velocity deficit and turbulence intensity surge in the downstream area of wind turbines. The former can cause power losses and the latter can result in fatigue load that will decrease the life-span of wind turbines, which will exert a considerably negative impact on the economic benefits of wind farms. In the downstream areas of multiple wind turbines, their wakes superimpose with each other, named wake interference. It has more influence on the performance of downstream wind turbines than free wakes and becomes an essential issue for wind farm wake study. Therefore, three-dimensional CFD simulations have been carried out for a single, two and multiple wind turbines to investigate flow field distributions and their impacts on downstream wind turbines. Main contributions are:(1) Numerical simulation for free wake effectThe study on a single turbine’s free wake is the basis of the research on wake interference effects. Three-dimensional full-scale multi-block structured mesh model was established based on National Renewable Energy Laboratory (NREL) 5MW reference turbine, including the nacelle and tower. Under the power-law wind shear, steady and unsteady numerical simulations were carried out and the CFD results were validated through comparing with the NREL data. The impact of mesh structures and simulation methods on the numerical results were investigated. Besides, the effects of different inflow, determined by various wind velocity at hub height, on the wake development were also analyzed. The results show that:structured mesh with high quality has obvious superiority in investigating wake related issues; compared with unsteady simulation, steady simulation cannot obtain the detailed flow field, but the distinctions of wind turbine parameters and velocity distributions between them are acceptable to satisfy the engineering needs; higher inflow velocity contributes to the more effective wake recovery.(2) Numerical simulation for full and partial wake interference effectsWake interferences of two wind turbines represent the simplest form of wake interference, which also reflect two typical interference effects-full wake interference and partial wake interference. Three-dimensional CFD models of two NREL 5MW reference wind turbines were established and the two turbines were positioned in a line with and without an offset Unsteady simulations were carried out under wind shear inflow. The two typical wake interference were compared and analyzed to clarify the differences between their effects on downstream flow field and turbines’performance. The results show that:full wake interference has more influence on wind turbines’ power outputs; partial wake interference affects the load on turbines more severely, and the periodicity of thrust coefficients against azimuth angle was destroyed and could not be portrayed as a "three leaf clover"; both the two kinds of wake interference effects aid the expansion of the wake region.(3) Numerical simulation for wake interference effect of multiple wind turbines in a lineWake interference produced by multiple wind turbines in a line is the most common form of wake interference. When wind turbines are arranged in a line, the wake effect on downstream wind turbine is considerably serious. Wind turbine model offered by Iowa State University (ISU) was selected as the object and three-dimensional numerical model based on 5 in-line ISU wind turbines were established. Three inflow conditions-high, medium and low turbulence inflow, considering the exponent of power-law wind shear and inflow turbulence intensity, were simulated. Their impacts on wake interferences were compared and validated through comparing with the wind tunnel data. Moreover, the performance of downstream wind turbines at different locations was discussed. The results show that:higher turbulence inflow with correspondingly larger exponent of power-law wind shear leads to smaller power losses of downstream turbines, as well as larger thrust coefficients; higher turbulence inflow help control the velocity deficit and promote the wake recovery; the locations of wind turbines has a significant impact on velocity distributions and turbines’ performance, and compare with other downstream wind turbines, the second one is highly affected.