Abstract
This study presents the setup, methodology and results from a measurement campaign dedicated to the characterization of full-scale wind turbine wakes under different inflow conditions. The measurements have been obtained from two pulsed scanning Doppler lidars mounted on the nacelle of a 2.5 MW wind turbine. The first lidar is upstream oriented and dedicated to the characterization of the inflow with a variety of scanning patterns, while the second one is downstream oriented and performs horizontal planar scans of the wake. The calculated velocity deficit profiles exhibit self-similarity in the far wake region and they can be fitted accurately to Gaussian functions. This allows for the study of the growth rate of the wake width and the recovery of the wind speed, as well as the extent of the near-wake region. The results show that a higher incoming turbulence intensity enhances the entrainment and flow mixing in the wake region, resulting in a shorter near-wake length, a faster growth rate of the wake width and a faster recovery of the velocity deficit. The relationships obtained are compared to analytical models for wind turbine wakes and allow to correct the parameters prescribed until now, which were obtained from wind-tunnel measurements and large-eddy simulations (LES), with new, more accurate values directly derived from full-scale experiments.
Highlights
The wind flow around the rotating blades of a wind turbine creates aerodynamic forces that result in a torque on the rotor axis, which generates electrical energy, and an axial thrust force, which pushes back the rotor
This section describes the first measurements used for the analysis of the wake of the wind turbine as well as the conditions which entail a removal of those periods which are not suitable
The study demonstrates that a measurement setup based on two nacelle-mounted lidars can be used to measure different characteristics of the incoming flow via Range Height Indicator (RHI), plan position indicator (PPI) and staring-mode scans, while at the same time perform planar scans of the wake
Summary
The wind flow around the rotating blades of a wind turbine creates aerodynamic forces that result in a torque on the rotor axis, which generates electrical energy, and an axial thrust force, which pushes back the rotor. The reaction force of the thrust creates an axial induction opposite to the air motion direction which reduces the kinetic energy of the flow, causing a reduction in velocity. The reaction torque, instead, creates a tangential induction which causes the flow to spin in the opposite sense of the rotation of the blades. Since the reaction aerodynamic forces have a dynamic nature and they generate important shear locally in the flow, they result as well in increased levels of turbulence. The kinetic energy deficit results in a decrease in energy production [2,3], and second, the higher turbulence levels result in higher fatigue loads and a potential life time reduction [4]
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