Development of the near wake behind a horizontal axis wind turbine - including the development of a free wake lifting line code

A lot of research has been carried out in the past on wind turbine wake aerodynamics. Models exist for both the near field and the far wake. The near wake is governed by a typical vortex structure that gradually decays due to viscous and turbulent mixing effects, forming the less structured far wake region. The formation of the far field from the rotor region and near wake is not completely understood yet. In this thesis an attempt is made to develop a simple model that tries to capture the decay of the vortex wake within the first two diameters downstream of the rotor. A free wake lifting line vortex model has been developed. Viscous effects have been added to the vortex code in the form of vortex core models and a simple model that describes the turbulent decay of circulation as a function of ambient turbulence intensity. Furthermore models for simulating wind shear and the presence of the nacelle have been implemented. A thorough validation study of the model has been carried out by means of hot-film and Particle Image Velocimetry (PIV) measurements. Close to the rotor the model compares well with the induced velocities measured with a hot-film. PIV measurements show that the model captures the position of the tip vortex quite well within the first diameter downstream. The measured velocity field can however not be reproduced correctly by the code. No measurements were available that can validate the turbulent decay of the vortex wake; instead suggestions are made for an experiment in the Open Jet Facility of the faculty of Aerospace Engineering The new model is only valid in the region of the wake where the vortex structure of the wake is still in tact which is typically between one or two diameters downstream. Further downstream the outputs from the model may be used as an input for a Reynolds Averaged Navier-Stokes (RANS) model that describes the far wake of the rotor.