· · · Institutional Repository

The offshore wind energy is gaining more and more importance in the scenario of the European renewable energies. Due to high costs of installation and maintenance, it is important to have a good assessment of the wind speed profile. The wind speed at the turbine hub level is used for energy yield evaluations and the knowledge of the wind shear helps estimating turbine structure loads. The wind speed profile in a marine environment is investigated using the data provided from the German offshore research platform FINO-1, the meteorological mast of the Dutch offshore wind park Egmond aan Zee and the weather forecast model COSMO-EU of the Deutscher Wetterdienst DWD. The data are compared to the Monin-Obukhov Similarity Theory using the Richardson Bulk Method, the Richardson Gradient Method and the Profile Methods. The results show that the models do not predict the wind speed profile well and large scatter is present. The weather forecast model COSMO-EU for offshore wind energy purposes is validated using FINO-1 measurements and the results are promising.

The Diffuser Augmented Wind Turbine (DAWT) has been studied periodically over the last five decades. It has already been established by the scientific community that the DAWT is superior to conventional bare wind turbines. In spite of this, the DAWT has not gained popularity worldwide due to high manufacturing cost of the diffuser. There are two possibilities to make a DAWT more lucrative; lowering the manufacturing cost or increasing the performance. The present thesis is concerned with the second approach and considers the hypothesis that the generating power of a DAWT can be increased by turbulent mixing of the wake and free stream flow. This mechanism should decrease the diffuser’s exit pressure and consequently increase the mass flow and power. In the present investigation this turbulent mixing is established by placing vortex generators on the diffuser trailing edge. The hypothesis was tested through a series of full scale wind tunnel experiments. The experiments were conducted in the open jet facility of Delft University of Technology in collaboration with Donqi Urban Windmills. It was found that the application of vortex generators on the diffuser trailing edge lead to an increase in power of 9%. Furthermore, in the pursuit of a better understanding of the flow behavior, an inviscid singularity model was formulated. The model uses a surface vorticity technique to simulate the behavior of the diffuser, supplemented with a lifting line approach to model the rotor. It was found that the inviscid model did capture the behavior of the DAWT reasonably well, although when compared to the measurement results it was observed tobe overly optimistic.

The installation of floating wind farms in deeper water is encouraged by the stronger and steadier wind, the lower visibility and noise impact, the absence of road restrictions, but also the absence or shortage of shallow water. In the summer of 2009, the first large-scale floating wind turbine "Hywind" was installed. Hywind is a spar-buoy concept with three catenary mooring lines. Offshore floating wind turbines are a completely novel concept. The experience with modeling such turbines is still limited. A new basic model has been developed for a spar-type floating wind turbine. The requirement of the model is that it incorporates the most significant physical processes so as to be able to provide insight into the dominant physical behaviour of spar-type floating wind turbines. Various verification methods show that A.T.FLOW simulates load cases as expected and is a useful tool for assessing the physical behaviour of spar-type floating wind turbines. The coming two years the body forces and behaviour of the operating full-scale Hywind demo project is monitored. This data should be used to further test and validate A.T.FLOW and to guide further development of the model.

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.

With the increasing size and complexity of wind turbines, also the risk of failure of a crucial component increases. The expected fatigue life of certain subcomponents is an important design requirement. Fatigue damage is triggered by a periodic loading of the construction. The number of load-cycles (and thus the lifetime) and the amplitude of the periodic loading dominate the fatigue behavior. The lifetime greatly increases if the load amplitude is decreased. One of the possible ways to reduce the periodic behavior of the loading is by applying flaps to the blades of a Horizontal Axis Wind Turbine (HAWT). The flap deflections can be preprogrammed to counteract the periodicity of aerodynamic phenomena like wind shear or yawed operating conditions. In the future, it should even be possible to add a sensing system, calculating in real-time the required flap deflections. With this 'Smart' autonomous system, possibilities open up to even use the control surfaces for non-periodic peak loads occurring due to turbulence or wind gusts. In this master thesis, an aerodynamic model (Vortex Panel Code) is used to investigate the loads on the blades of the HAWT. The simulated wind turbine is an experimental rotor which will be tested in the Open Jet Wind Tunnel Facility (OJF) at the TUDelft. The loads for three different cases are calculated. In a first simulation, the reference condition is analyzed, i.e. the rotor is operating in pure axial inflow. Secondly, the turbine is operating under yaw misalignment. This simulation gives the uncontrolled (and mostly unwanted) periodic behavior of the blade loads. Finally, the effect of a prescribed flap deflection is analyzed. When the output of the flap deflection case is compared to the output of the yaw misalignment case, it is possible to make an estimation of the required flap control in order to reduce or even eliminate the periodic behavior of the loading. The obtained estimations indicate that the smart rotor concept is useful to compensate for periodic structural loadings, but it is not very suitable for applications where the purpose is to level a periodic output of the rotor power.