Innovative measurement techniques for atmospheric turbulence and wind energy

The measurement of different atmospheric flow quantities is of utmost importance for a correct understanding of most atmospheric phenomena. Researchers and industry in the fields of meteorology and wind engineering demand extensive and accurate measurements of atmospheric turbulence for a better understanding of its role in a wide range of applications such as weather forecast, wind resource evaluation, wind turbine wake, pollutant transport or urban climate. Quantitative measurements of relevant variables are particularly valuable for the development, testing and validation of turbulence parameterizations used in both analytical and numerical models. This thesis focuses in the development of innovative measurement techniques for atmospheric turbulence, particularly suitable for wind energy applications, and it is divided into four different studies.

The first study presents a multirotor UAV-based technique for the measurement of atmospheric turbulence and temperature. The technique is based on the integration of a fast-response multi-hole pressure probe and a thermocouple with an inertial measurement unit (IMU). This technique allows for an accurate measurement of time series of the three components of the velocity vector and temperature at any point in the atmosphere in which the UAV can fly. The technique relies on the correction of the velocity vector measured by the pressure probe on the frame of reference of the UAV -non inertial- with the information provided by the IMU. The study includes a validation of the technique against sonic anemometry and the measurement of the signature of tip vortices shed by the blades of a full-scale wind turbine as an example of its potential.

The second study presents a triple-lidar technique developed for the measurement of atmospheric turbulence at a point in space from synchronous measurements of three intersecting Doppler wind lidars. The laser beams must be non-coplanar so that trigonometric relationships allow the reconstruction of the velocity vector. The technique is validated against sonic anemometry in terms of the instantaneous velocity vector, turbulence statistics, Reynolds stresses and the spectra of the three components of the velocity and the turbulent kinetic energy.

The third study investigates the theoretical accuracy of the reconstruction of a full-scale wind turbine wake in terms of the average and the standard deviation of the longitudinal velocity component by volumetric scans from lidar measurements. To that end, a series of virtual experiments are performed, where synthetic lidar measurements are obtained from LES simulation results. The methodology described quantifies the errors and allows the optimization of the scan pattern so that it balances the different error sources and minimizes the total error.

The fourth study presents a measurement campaign dedicated to the characterization of full-scale wind turbine wakes under different inflow conditions. The measurements are performed with two nacelle-mounted scanning lidars. The first lidar characterizes the inflow while the second performs horizontal planar scans of the wake. The relationships obtained for the growth rate of wake width, velocity recovery and length of the near wake are compared to analytical models and allow to correct the parameters prescribed until now with new, more accurate values directly derived from full-scale experiments.