Velocity measurements in the near wake of a horizontal axis wind turbine

Single film hot-film measurements in the near wake of a horizontal axis wind turbine are performed in the Delft University of Technology Open Jet Facility in order to capture the blade azimuth dependent velocity vectors. A novel approach to determine the average, blade azimuth dependent, velocity vector is proposed. It makes use of the asymmetric response of hot-films that have the film parallel to its probe. Contrary to the traditional approach in which the direction of velocity components cannot be determined and which poorly predicts relatively small velocity components, the proposed method yields velocity direction as part of the solution and consistently predicts relatively small velocity components. An uncertainty analysis is performed on the velocity magnitude and flow angle, quantifying the data quality. Furthermore, comparisons with previously published data on the same rotor, in the same wind tunnel are made. The comparisons increase confidence in the data and reveal the advantages of the proposed velocity determination method. Qualitatively, the velocity signals derived with the proposed approach are in agreement with expectations from a general vortex wake model, also the relatively small tangential and radial velocity components.