Abstract
Abstract. Wake meandering is a low-frequency oscillation of the entire wind turbine wake that can contribute to power and load fluctuations of downstream turbines in wind farms. Field measurements of two Doppler lidars mounted on the nacelle of a utility-scale wind turbine were used to investigate relationships between the inflow and the wake meandering as well as the effect of wake meandering on the temporally averaged wake. A correlation analysis showed a linear relationship between the instantaneous wake position and the lateral velocity that degraded with the evolution of the turbulent wind field during the time of downstream advection. A low-pass filter proportional to the advection time delay is recommended to remove small scales that become decorrelated even for distances within the typical spacing of wind turbine rows in a wind farm. The results also showed that the velocity at which wake meandering is transported downstream was slower than the inflow wind speed but faster than the velocity at the wake centre. This indicates that the modelling assumption of the wake as a passive scalar should be revised in the context of the downstream advection. Further, the strength of wake meandering increased linearly with the turbulence intensity of the lateral velocity and with the downstream distance. Wake meandering reduced the maximum velocity deficit of the temporally averaged wake and increased its width. Both effects scaled with the wake meandering strength. Lastly, we found that the fraction of the wake turbulence intensity that was caused by wake meandering decreased with downstream distance contrary to the wake meandering strength.
Highlights
The wind turbine wake is a flow region of reduced wind speed and increased turbulence that extends downstream of a wind turbine for several rotor diameters
– low signal-to-noise ratio (SNR) of the wake-scanning Doppler lidar led to gaps in the measurement data, which is quantified by a rejection rate of more than 0.5 % at any range gate between xD−1 = 4 and xD−1 = 9 (Sect. 2.2);
Atmospheric field measurements of the wind velocity from two Doppler lidars mounted on the nacelle of a utility-scale wind turbine were used to investigate wake meandering
Summary
The wind turbine wake is a flow region of reduced wind speed and increased turbulence that extends downstream of a wind turbine for several rotor diameters. In wind farms the wake of an upstream turbine becomes the inflow of a downstream turbine, leading to power losses and increased mechanical wear, which translates into an increased cost of the energy. One characteristic of wind turbine wakes is wake meandering, a low-frequency, large-scale oscillation of the entire wake in the lateral and vertical directions (Taylor et al, 1985). Two theories have been presented as the origin of wake meandering: (i) a passive advection of the entire wake by large-scale turbulence of the inflow (Larsen et al, 2008) and (ii) an intrinsic shear instability of the wake characterized by periodic vortex shedding (Medici and Alfredsson, 2006).
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