Multiple-LiDAR measurements of wind turbine wakes

Field measurements of the wake flow produced from the interaction between the atmospheric boundary layer and a wind turbine are performed with three LiDARs. The tested wind turbine is a 2 MW Enercon E-70 located in Collonges, Switzerland. First, single LiDAR measurements are performed in order to check accuracy of the mean axial velocity and frequency resolution of velocity fluctuations. Subsequently, two-dimensional measurements with a single LiDAR are carried out over vertical sections of the wake and the mean wake flow is obtained by averaging 2D measurements consecutively performed. The high spatial resolution of the used LiDAR allows characterizing in details wind turbine wakes, e.g. the velocity defect in the central part of the wake and its downstream recovery. Single LiDAR measurements are also performed by staring the laser beam at selected fixed directions and maximizing the sampling frequency in order to characterize wake turbulence. From these tests fluctuation peaks are detected in correspondence of the turbine blade top-tip height, which could represent dangerous fatigue loads for following wind turbines within a wind farm; the magnitude of these fluctuation peaks is reduced by moving downstream. Moreover, the proper characterization of flow fluctuations is proved by the detection of the inertial subrange from spectral analysis of these velocity signals. Simultaneous measurements with two LiDARs are performed over the mean vertical symmetry plane of the wake, while a third LiDAR measures the incoming wind over a vertical plane parallel to the mean wind direction and lying outside of the wake. For these measurements axial and vertical velocity components are retrieved only for measurement points where the two laser beams result to be roughly orthogonal. These 2D measurements confirm the presence of significant flow fluctuations at blade top-tip height in the near wake.