Volumetric Three Dimensional Wind Measurement Using a Single Mobile Based LiDAR

Abstract

A novel approach to measure the wind flow field in a utilityscale wind farm is described. The measurement technique uses a mobile, threedimensional scanning LiDAR system to make successive measurements of the lineofsight (LOS) wind speed from three different positions; from these measurements, the timeaveraged threedimensional wind velocity vectors are reconstructed. The scanning LiDAR system is installed in a custombuilt vehicle in order to enable measurements of the threedimensional wind flow field over a footprint that is larger than with a stationary scanning LiDAR system. At a given location, multiple series of plan position indicator (PPI) and velocity azimuthal display scans are made to average out turbulent fluctuations; this series is repeated at different locations across the wind farm. The limited duration of the total measurement time period yields measurements of the threedimensional wind flow field that are unaffected by diurnal events. The approach of this novel measurement technique is first validated by comparisons to a meteorological mast and SODAR at a meteorological observatory. Then, the measurement technique is used to characterize the wake flows in two utilityscale wind farms: one in complex terrain and the other in flat terrain. The threedimensional characteristics of the wakes are described in the measurements, and it is observed that in complex terrain the wake has a shorter downstream extent than in flat terrain. A maximum deficit in the wind speed of 20–25% is observed in the wake. The location of the maximum deficit migrates upward as the wake evolves; this upward migration is associated with an upward pitching of the wake flow. A comparison of the measurements to a semiempirical wake model illustrates how the measurements, at fullscale Reynolds numbers, can support further development of wake models.