Wind turbines operate in the earth's atmospheric boundary layer - an environment where the climate is turbulent and unsteady. In addition, wind turbines usually operate in clusters (wind farms) and are thus subjected to the unsteady wakes generated by upstream turbines. Because of the unsteady inflow, wind turbines suffer from structural fatigue damages. There is therefore motivation to better analyse and model the unsteady loading in order to design better turbines and reduce the cost of energy. To study the unsteady aerodynamics phenomena, an actuator disc model under unsteady loading was simulated using experimental and numerical methods. Experimentally, a porous disc rotor was constructed and tested in the Open Jet Facility, TU Delft. The design involved a novel method to dynamically control the rotor's thrust coefficient through its porosity. Numerically, a dynamically loaded actuator disc was implemented in the RANS equations and solved by the commercial CFD solver Star-CCM+. Results from the experiment, CFD simulations, as well as a free wake vortex ring model were used to benchmark the performance of dynamic inflow engineering models. Results show that the wake of a wind turbine under steady loading is also turbulent and meandering. The Strouhal number of the wake fluctuation closely agrees to that from a previous experiment of a 2-bladed wind turbine (Medici & Alfredsson, 2006). This suggests that the wakes of blade rotors are comparable to that from disc rotors. The tower shadow effect also bears a major influence on the inner wake profile which causes further velocity deficit and higher turbulent intensity. For rotors with higher CT , the velocity recovery process occurs relatively earlier. This is because of the enhanced momentum entrainment arising from a steeper velocity gradient at the shear layer. For the disc rotor under unsteady loading, the wake is affected by unsteady aerodynamics or the dynamic inflow effect. The transient velocity profile experiences an overshoot due to the passage of the 'old' and 'new' shed vortices, which are generated at the wake edge as a consequence of the uniform load profile on the rotor. The subsequent decay of the velocity in the wake or inflow at the rotor plane to the 'new' equilibrium state is a result of the rate of convection of the 'new' shed vortices to the far field. From this research, numerical results showed lower inflow velocity decay rates (velocity response due to the change in loading on the rotor) than that predicted by engineering models. While this is so, it was found that the decay rate is highly influenced by the ambient flow's turbulence. At high ambient turbulence, the 'new' equilibrium state is achieved in a shorter time due to the enhanced momentum mixing process. It is hypothesised that this is the primary reason behind the higher decay rates predicted by empirical-based engineering models.

Big wind farms with big wind turbines are more cost effective and produce greener electricity compared to smaller ones. This is one of the reasons for the growth of wind turbine size during the last 3 decades. However, as wind turbines become bigger, their blades become longer, thicker and heavier. This results in larger unsteady loads on blades which is an important limitation for their life time and their size growth. Flow control has emerged as the promising solution both for improving the aerodynamic efficiency and controlling the unsteady loads on wind turbine blades. DBD plasma actuator is an active flow control mechanism that has shown high potentials for unsteady load control with the capability to change lift coefficient to a significant amount with a very fast response time. The current research first intends to identify the variations of angle of attack and lift coefficient on wind turbine blades as a result of gravitational loads, mass and aerodynamic imbalances, turbulence, wind shear, yawed inflow and tower shadow and investigate their corresponding frequencies and the fatigue damage from the blade root bending moments. Then, (single and multi) DBD plasma actuator with different configurations will be used on three different trailing-edge shapes (round, half-round and sharp) of modified version of ’NACA64-2-A015’ airfoil to control the aerodynamic loads via circulation control. This is managed either by manipulation of Kutta condition or acting as a virtual Gurney flap. Furthermore, it is intended to investigate the correlation between the frequency of actuation, frequency of vortex shedding and the amount of lift enhancement.

The vertical axis wind turbine (VAWT) concept in its current, fixed-pitch iteration is hindered by lower aerodynamic efficiency compared to existing horizontal axis wind turbines. Implementing a form of circulation control on the VAWT by actively pitching the blades throughout the turbine’s rotation could potentially enhance power capture efficiency and reduce loads variability. In this thesis, circulation control is investigated in 3D, since the 3D flow field differs significantly from the 2D equivalent.After a review of available 2D and 3D aerodynamic models for the VAWT, a suitable candidate emerges in the form of HAWC2 NW, which is a lifting-line based extension to popular aeroelastic modelling suite HAWC2 designed to model trailed vorticity in the near wake of the blade. HAWC2 NW is used to conduct fast 3D aerodynamic simulations of a highly simplified vertical axis wind turbine geometry to demonstrate the influence of 3D effects on the flow field surrounding the VAWT. The slenderness of the turbine geometry, expressed using the turbine aspect ratio H/D, turns out to be a major factor in determining the performance losses due to 3D effects.Subsequently, HAWC2 NW is used to analyze optimized pitch sequences with various objectives, such as maximum power or a certain normal loading distribution. A relationship is found between the ratio of upwind and downwind normal loading, the turbine aspect ratio H/D and the 3D power coefficient CP,3D. The most important conclusion is that when considering the design of a vertical axis wind turbine with a turbine aspect ratio H/D < 5, losses due to 3D effects can be minimized by implementing a circulation control strategy in such a way that the normal loading is uniformly distributed between the upwind and downwind half-cycles.

Given the prime importance of wind tunnel setup when it comes to aero-elastic study, the main intent of this project is to devise a mechanism for supporting system of 2D aero-elasticity experiment in the Red Wind Tunnel Facility at DTU that allows three-degree-of-freedom motion of the rigid model. This project work is mainly inspired by the fact that deep stall aero-elastic Characteristics of wind turbines, which govern the design of stall-regulated wind turbines at high wind speed, have been barely investigated in literatures so far. So in redesigning the test section of the wind tunnel facility the following sub-tasks are included: (1) Literature survey on test section design/construction for 2D aero-elastics testing of aerofoils. (2) Panel code simulation of wing/tunnel interaction. (3) Adapted design of new test section to the Red wind tunnel facility. (4) To some extent building and testing the new test section. The implementation and actual design may be too time demanding to be fulfilled within the given time slot, however a specific design for the Red tunnel facility is documented through flow analysis and drawings at the end of the project. Among which the first and third item have been addressed and the fourth item partially as well, but the second item is still to be done with the flow solver Q3UIC which was developed by Néstor in DTU. Since the aerodynamic code of this tool is only for pitching motion, extending this tool for this thesis work happened to be out of scope.

Cost of energy is one of the main reasons why wind power generation hasn’t been able to compete with fossil fuels. However, in the past decade wind energy field has made quite a big leap in making this way of energy generation cheaper and more competitive. As a result, the price of onshore wind energy has already reached the price of fossil fuel power generation. The progress has been made with offshore wind turbine technology as well, however ever increasing energy demand has forced industry to push the limits even further. Attempts are being made to build wind farms further offshore where wind resource is much higher and where more space is available. This on the other side requires new technological development and finding new ways to decrease the cost of energy. This thesis is a part of a larger project (DOT - Delft Offshore Turbine) which represents one such attempt to make the offshore wind energy much more competitive by combining wind turbine and hydraulic turbine technology. DOT company works on the development of the fluid power transmission in offshore wind turbines, using seawater as medium. The idea is that every wind turbine drives a hydraulic pump. Each turbine thus creates a flow of water under high pressure which is converted to electricity using a Pelton turbine generator. The main objective of this thesis is to identify and try to overcome challenges in design of Pelton turbines for high water pressures, variable flow rates and seawater conditions. In order to accomplish this objective, theoretical, experimental and numerical approach was used. Theoretical approach was used to develop a tool for initial Pelton turbine design. Small program was developed in Python programming language which enabled faster and easier turbine dimensioning for any operating conditions based on standard Pelton turbine design procedure. Next, test rig was designed and built as part of the experimental investigation. Test was divided in two phases. Phase I was meant to show the influence of the flow rate, pressure and pressure fluctuations on the development and quality of the water jet. Pressure is measured at the inlet of the injector and flow visualization of the jet was conducted with the high speed camera. Experimental set-up for Phase II was concluded while performing experimentation has been left out for a future research project. As last step, CFD simulation of the designed model was conducted using Ansys CFX commercial solver and results were presented and discussed. Although, all three approaches are conducted independently each of them contributed to answering the research questions and giving a better insight in problem definition. Furthermore it helped identifying the differences between the standard Pelton turbine design and the DOT concept. The research brings us one step closer to making DOT concept a viable sustainable energy solution for the future.