Wind turbine partial wake merging description and quantification

Abstract

Individual turbine location within a wind plant defines the flow characterisitcs experienced by a given turbine. Irregular turbine arrays and inflow misalignment can reduce plant efficiency by producing highly asymmetric wakes with enhanced downstream longevity. Changes in wake dynamics as a result of turbine position were quantified in a wind tunnel experiment.Scale model turbines with a rotor diameter of 20 cm and a hub height of 24 cm were placed in symmetric, asymmetric, and rotated configurations. Simultaneous hub height velocity measurements were recorded at 11 spanwise locations for three distances downstream of the turbine array under two inflow conditions. Wake interactions are described in terms of the time-average streamwise velocity and turbulence intensity as well as the displacement, momentum, and energy thicknesses. The effects of wake merging on power generation are quantified, and the two-point correlation is used to examine symmetry in the mean velocity between wakes. The results indicate that both asymmetric and rotated wind plant arrangements can produce long-lasting wakes. At shallow angles, rotated configurations compound the effects of asymmetric arrangements and greatly increase downstream wake persistence. KEYWORDS turbulence, wakes, wake merging, wind energy INTRODUCTIONWind energy has grown with the push to provide renewable energy to a growing world population. Wakes have been shown to produce adverse effects on the performance of wind turbines. 1-3 Turbine wakes can be divided into two distinct regions, the near and far wake, based on rotor proximity. In the near wake region, fluid dynamics are dominated by axial forces stemming from mechanical power extraction. 4 Tip vortices generated by the blades decay within the near wake as the velocity regains a Gaussian profile. 5,6 In the far wake region, turbulence intensity decays with distance downstream as the flow recovers energy. 4 In a wind plant setting, turbulence intensity determines the flow induced rotor loads and systematic wake impacts on downstream turbines. 7 Added turbulence intensity has been measured at distances of 15 rotor diameters highlighting the need to describe the development of turbulence intensity from turbine wake interactions throughout the wind plant. 7Efforts to characterize turbine wakes and turbulent wake interactions include numerical models, simulations, and experiments. Numerical models and simulations have demonstrated an increased capacity to represent turbine wakes with the actuator line model forming the basis of numerous studies. 8,9 The actuator line model was simulated through EllipSys3D with two turbines in staggered and inline configurations under various inflow conditions and predicted arrangement-dependent asymmetric wake dynamics. 10 The actuator line has been extended to include multiple turbines for plant optimization. A variety of methods for calculating wake interactions have been developed for large turbine arrays. 11,12 However, numerical models and simulations rely on superposition of v...

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