The wind turbine wake is a downstream region of kinetic energy and velocity deficit, higher turbulence and has a complex helical vortex structure. The wind turbine wake’s stability depends mainly on the ambient turbulence and the wind turbine tip- speed ratio. Researchers have developed several wake control techniques to mitigate this issue, such as static axial induction control by torque or pitch control, dynamic axial induction control by pitch control and actuating flaps or other actively controlled add- on devices. Some gaps are present in these methods, in terms of applicability of these techniques in terms of full scale validation and practicality to have the devices as add on to existing blades.

To address these research gaps, this study focuses on designing and evaluating segmented Gurney flaps (in line with the ECN (now TNO Wind Energy) patent by Edwin Bot and Arne Van Garrel) for wind turbine blades to enhance the wake recovery by inducing turbulence in the wake to excite the tip vortices. 4 Gurney flaps were attached in the tip region of each blade of a GE 3.8MW research wind turbine. Field tests were conducted in this study for the wind turbine wake (using a scanning LiDAR to scan a sector up to 5.5D downstream at different altitudes; with a scan time of roughly 3 minutes) and performance analysis (using 10 minute averaged measurement data). Free vortex wake simulations were conducted to validate the faster wake breakdown by the change in lift distribution upon addition of segmented Gurney flaps. Simulations using dynamic blade element momentum theory with IEC NTM inflow conditions from cut in to cut out wind speed were conducted to assess the structural impacts on the retrofitted wind turbine.

The field tests’ wake analysis was quantified with different wind speed, turbulence intensity, wind shear, wind direction conditions. Post processing of LiDAR data involved filtering, creating bin averaged data set, using Gaussian process regression and retrieval of the required wind component. The results show a consistent increase in wake recovery, generally at all downstream distances. The retrofitted configuration results are associated with a higher standard error because of a shorter testing period (due to increased noise levels) than the baseline configuration. The wake simulations indicate an earlier wake break down position, by roughly 2D.

The simulations indicate AEP increase for the retrofitted wind turbine to be roughly +0.2% (+50MWh), at the expense of increased blade and tower structural loads. The wind speed weighted damage equivalent blade flapwise bending moment was found to increase by roughly +2% to +4%; with peaks observed in partial load region of the wind turbine, associated with the operating angle of attack in this region.

A 3D panel method code, named AWSM3D, is developed as an extension to the AeroModule aerodynamic software suite of TNO which includes a Blade Element Momentum theory (BEM) and a Lifting Line (LL) code. The underlying panel code method is presented along with a gradual process of simulation validation culminating in validating the code against the New MEXICO experimental dataset and an established panel code. In view of exploring the avenue of sweeping wind turbine blades as a way of increasing wind turbine efficiency, the capabilities of the three proposed aerodynamic models in solving flows about swept geometries are studied. The unmodified BEM model is incapable of adequately modeling flows over swept geometries. The limits of the cross-flow theory which the LL simulation hinges on for the simulation of swept flows are put to the test. Both a fixed wing and a conventional Horizontal Axis Wind Turbine case show that the LL solution in proximity to body extremities where the cross-flow component is prominent fails to capture smaller-scale local effects caused by said cross-flow. Despite this, the larger-scale effect is reproduced well. Although AWSM3D requires no airfoil polars and produces an accurate inviscid flow solution for more complex body geometries, it currently excludes viscosity and has a much higher computation time in comparison to an LL. A number of further AWSM3D development and research into the effects of sweep suggestions are formulated.

Wind turbines experience a wide variety of load cases, both when in operation and when parked. While a turbine in operating conditions often experiences larger loads than in parked conditions, uniquely large inflow angles may occur in parked conditions resulting in less well-understood aeroelastic conditions. Vibrations under these conditions are referred to as stall and vortex-induced vibrations (SVIV). While there has been some success in simulating these conditions, there has been a lack of experimental studies in the literature.

To address this gap, this study analyses the experimental data obtained from a 3.8 MW research wind turbine. This turbine was equipped with strain gauges at the root of 2 blades and in the tower, as well as pressure sensors on one blade and accelerometers in the nacelle and tower. Several experiments were performed with a focus on simulating various conditions where SVIV may occur. This included pitch traverses of a single blade, yaw traverses, and a traverse of the azimuth angle at a yaw angle of 90 degrees to determine the effect of the inclination angle.

Severe stall-induced vibrations were identified at the yaw angle of 110 degrees, when a single blade pointing up in the sky was pitched 180 degrees, and the other two blades were in vane positions at 85 degrees pitch. The severity of these vibrations strongly varied with the wind speed, at an average wind speed of 19.5 m/s the test needed to be stopped early for the safety of the wind turbine, while at 16.6 m/s, the vibrations appeared to reach a limit that was still considered to be safe. The first tower mode experienced the most severe increase in magnitude during these conditions of SIV. In other conditions, no severe case of SIV was identified. However, under several conditions, smaller increases in vibration magnitude were identified. These are likely the result of slightly reduced aerodynamic damping.

No vortex shedding was identified in the data obtained from these experiments. This means that the tested turbine/blade design either did not experience vortex shedding or that it simply could not be measured by the installed instruments. Having pressure sensors installed in multiple locations along the blade would likely help in identifying vortex shedding for future experiments.

Simulations from the experimental conditions were performed using the aeroelastic software PHATAS together with the aero module from ECN (now part of TNO). This simulation used beam-based structural modeling with either a BEM or free vortex wake aerodynamic model. These simulations confirmed the need for realistic turbulent inflow conditions and the need for a good dynamic stall model. However, the structural model failed to simulate all the natural frequencies with the expected accuracy. This should be improved upon for future use of this structural model in parked conditions.

The battle against climate change begins with maximizing the share of renewable energy in the total energy mix, such that the cost of energy will be minimum. Wind energy will play a vital role in maximizing the share of renewables in the total energy mix. In order to reach the LCOE projection of 0.09 Euro/KWh a decrease in the material cost involved in the manufacturing of blades should play an important role. The drive to make wind energy cheaper pushed me to dive deeper into the subject of wind turbine aerodynamics and thus, I chose to work on this thesis project. The research will help the wind energy community to make the wind turbine design process robust, safe and cost-optimal. The thesis focuses on improving the general practice (based on the IEC standard) for wind turbine blade design by incorporating a more advanced aerodynamic model, called as the vortex wake model.
The objective of this thesis is to evaluate differences in BEM and vortex wake models for IEC load case calculation. With respect to these differences, the thesis also aims to recommend a practice for fast and more accurate results. Initially, the IEC load cases are simulated with the Focus6 software making use of BEM based model for aerodynamic load calculations. A selection of most relevant load cases is to be done on the basis of results for the load cases which violate the BEM model. Finally, the selected load cases are further simulated with a vortex wake model coupled to Focus6, and a recommended practice for more accurate and fast results is devised.