Idling instabilities of large wind turbines in extreme winds is a relatively well known problem for manufacturers, at least in terms of the results in calculations. However, many are still sceptical of how realistic such instabilities are in actuality due to limitations in the current state of the art aeroelastic tools. The overall aim of this thesis is twofold. Firstly, through the use of analytical solutions for a parked blade developed by adapting existing formulations for the aerodynamic damping forces as well as numerical means in the form of the aeroelastic tool Focus 6, Phatas, versions 9083 and 10041 (kindly provided by WMC), this thesis aims to determine how parameters relating to the local angle of attack of a wind turbine blade contribute towards inducing idling instabilities in large wind turbines and to subsequently identify the underlying mechanisms that drive such instabilities to occur in simulations. Subsequently, from the results gathered from the analyses, the second goal is to then determine if the current state of the art aeroelastic tools are able to reasonably capture such instabilities. The reference wind turbine used in this thesis was the NREL 5mW turbine. The method used to obtain the results in the analytical solutions were first validated against existing literature and generally showed good agreement in the trends. The analytical solutions showed the aerodynamic damping values of a blade section at various nacelle yaw, rotor azimuth as well as blade pitch angles. The results from the analytical solutions showed that under certain conditions, idling instabilities were present. The results of the numerical simulations when first run with quasi steady aerodynamics and a constant wind profile showed good agreement with the analytical results (and also existing literature) and also suggested that the instabilities were driven by the edgewise modes of the tubine. Assessment of the outputs after the inclusion of dynamic stall (where simulations were largely limited to angles of attack between -45° and 45° as this was the range where the dynamic stall model was applied in its full extent) as well as turbulence to further simulations showed that Focus 6, Phatas still calculated instabilities at settings close to the analytical solutions’ predictions. As a final conclusion, this thesis provided a theoretical basis to argue for the case of idling instabilities being a possible issue. The numerical simulations largely support the findings of the analytical solutions where an assessment of the numerical results gave no indications to suggest otherwise.

Aeroelastic codes are fundamental for the design of wind turbines and the prediction of instabilities. Such codes rely on engineering models which limit their accuracy. With wind turbine blades becoming more slender to lower the costs, it is important to reduce these uncertainties to maintain a safe and stable turbine design. The dynamic stall model is one of these engineering models which simplifies the physics involved and so an improvement in accuracy might be possible. Various dynamic stall models have been published over the last 50 years. However, dynamic stall has proven to be a complex phenomenon to model accurately over a wide range of conditions and is an ongoing topic of research. Here four semi-empirical dynamic stall models, the Øye, Risø, Snel and ONERA models, are compared first in 2D against wind tunnel data to understand their accuracy and limitations. The experimental data for this comparison uses the NACA0015, NACA0030 and NACA4415 airfoils over a variety of cases relevant to wind turbines. Hereafter the models are compared within an aeroelastic code as a part of complete horizontal axis wind turbine simulations for an extreme load case (IEC design load case 1.4), standstill instabilities (IEC design load case 6.2) and the flutter speed to understand the effect of the different dynamic stall models. For the standstill cases the aerodynamic damping provided by each model is compared by reducing the structural damping of the blades to find the point where the blades become unstable. Next the Øye and Risø models are tuned to the wind tunnel data. The results from the 2D comparison are inconclusive with each model showing different strengths and weaknesses. In general the attached flow physics in the Risø and ONERA models improves the fit for the 15% thick airfoils. The ONERA model captures the lift peak the best, although it usually has the largest least squares error to the data due to the drop in lift after this peak being too early and too sharp. Once in the aeroelastic code the ONERA model started to show unphysical behaviour by reducing the deflection in the extreme load case and adding negative damping to the standstill cases. Snel’s model, on the other hand, adds so much damping that the blades remains stable even when the blades have only a small amount of structural damping, which is likely not physical. The Øye and Risø models show similar damping levels and reduce the required structural damping to prevent the standstill instabilities by at least a factor two with respect to no dynamic stall model being used. For the classical flutter analysis the Risø model shows an increase in flutter speed as expected from the implementation of Theodorsen’s theory, while the ONERA model decreases the flutter limit. Overall the Risø model is seen as the best for in an aeroelastic code due to showing better behaviour in the full turbine cases than the ONERA and Snel models. Furthermore, it is superior to the Øye model due to correctly modeling the attached flow physics and improving the drag and moment coefficients.

This thesis concerns the optimal design of wind turbine blades for bend twist coupling effects. Bend-twist coupling effects can prove to be an effective way of reducing fatigue damage and root bending moments for a wind turbine blade. However, how can bend-twist coupling be incorporated into a wind turbine blade considering the many structural requirements? This thesis investigates the optimal design for bend-twist coupling in wind turbine blades. Gradient based numerical optimization algorithms are applied to obtain composite material layups favorable for bend twist coupling. The thesis work builds a framework that allows for the selection of fiber angles at several sections along the blade, while considering structural constraints. This involves the use of a cross section analysis software and a finite element beam formulation for the evaluation of responses, while an aeroelastic software is used for load mitigation validation. The structural constraints include tip twists and deflection, eigenfrequencies and material failure criteria of which the analytic sensitivities are calculated, implemented in the framework and validated against central finite difference gradient values. A simple cantilever sandwich beam is considered first as a trial for the framework and four problem formulations are defined to test both the framework and the considered optimization algorithms. A full scale wind turbine blade for a 10MW wind turbine is then subject for the optimization. A converged design for the full scale blade is obtained and subject to aeroelastic simulations in a steady state and turbulent inflow. From this, the load mitigation effects are observed and discussed. With the work done in this thesis project, a step closer to an effective framework for aeroelastic tailoring is achieved.

Material bend-twist coupling has been widely studied in the research community as a passive control mechanism for a wind turbine. However, there is a lack of research in incorporating it in the rotor design process, with little research being conducted for its application to small wind turbines. The present study focuses on this issue by including bend-twist coupling in the design of a 500W wind turbine by using a combination of parametric studies and a multidisciplinary constrained optimisation approach. By doing so, it aims to establish the effectiveness of bend-twist coupling as a tool for passive load alleviation in small wind turbines. The effectiveness is tested through obtaining a significant decrease in the flapwise blade root bending moment accompanied by only a marginal decrease in the AEP, when compared with the baseline uncoupled turbine. The reference blade is designed under limitations imposed by the rules of the Small Wind Turbine Design Contest. Bend-twist coupling is introduced in the blades with a fixed aerodynamic design. The rotor performance is analysed inHAWCStab2. The internal structure of the blade is created with the intention of producing flexible blades with a single composite material used throughout the blade. Carbon-epoxy and glass-epoxy FRPs are considered as the material to be chosen in the unidirectional laminae. The cross-sectional stiffness analysis is conducted using BECAS for a range of fibre layup angles in both carbon-fibre and glass-fibre blades. Carbon outperformed glass for all fibre angles with regard to the amount of coupling seen in the crosssections. Apart from flapwise bend-twist coupling, other secondary torsion couplings are also present. A load response study is carried out for varying positive fibre layup angles in both carbon-fibre and glass-fibre blades, under steady wind conditions for the operational wind speed range. An increase in the flapwise blade tip displacement and a reduction in the flapwise bending loads with increasing fibrelayup angles is observed for both glass-fibre and carbon-fibre blades. The HAWTOpt2 aero-structural design tool using OpenMDAO as its core is utilised to implement the optimisation. The spanwise fibre layup angle and laminate thickness distribution are the only design variables that were allowed to be manipulated by the optimiser. The objective function is comprised of two different individual objective functions weighed according to the situation. The optimisation cases failed due to inaccurate gradients of the objective function and constraints. Due to time constraints the manufacturing of the blade and, static load and wind tunnel testing were not carried out.

Wind energy has emerged as a promising alternative to fossil fuel energy sources over the last two decades partly due to considerable reductions in the cost of power production. A common trend towards cost reduction has been to build larger and lighter wind turbines. These produce more power per unit and use less material. Another strategy for cost reduction could be to reduce the Operations & Maintenance (O&M) costs of wind turbines by using optimal O&M strategies. O&M costs account for approximately one-fourth of the overall energy cost for the full lifetime of a wind turbine. They assume even greater significance in the wake of growing interest in offshore wind farms since offshore wind turbines operate in harsher environments compared to their onshore counterparts and are more difficult to access. Condition monitoring has been proposed as a novel preventive maintenance strategy wherein sensors are employed to collect data related to the functioning of an operational wind turbine. This data can be processed to get meaningful information about component health of the wind turbine. An aeroelastic model can be used to simulate the fault-free response of the wind turbine for given operating conditions. By comparing real-time information from the condition monitoring system and aeroelastic model, it may be possible to develop routines which can detect developing faults in the wind turbine components. This forms the basis of a model-based condition monitoring system (MOD-CMS). For purposes of model-based condition monitoring it is required to have an aeroelastic model which is computationally fast to be capable of running real-time aeroelastic load simulations, and is highly accurate in order to detect faults. Furthermore, it is also desirable to have a linear aeroelastic model since this can also be used for controller and state observer design. A state observer can help estimate states of the wind turbine which are not easily measurable. This thesis reviews state-of-the-art aeroelastic tools to get a better understanding of their limitations. A reduced-order aeroelastic model is developed using the aeroelastic module in STAS WPP (State Space Analysis of Wind Power Plants) which is an open-source aero-hydro-servo-elastic tool in the Matlab/Octave environment. The linear reduced order aeroelastic model is verified by running test cases in the frequency domain for the NREL 5MW Baseline wind turbine. The accuracy of the linearized model is demonstrated by performing a stability analysis study for the NREL 5MW Baseline wind turbine.  Furthermore, different coupling and numerical integration schemes are studied to develop a time marching simulation tool. Two main approaches are proposed. The first involves time marching of the monolithic, strongly coupled non-linear aeroelastic model using a multistep predictor-corrector integration scheme. In the second approach, a partitioned, loosely coupled version of the linear aeroelastic model is implicitly integrated over time. In the present work a tool based on the first approach of integrating the non-linear aeroelastic model is developed and subsequently verified by running simple power production test cases.   In conclusion, this thesis discusses and implements a framework of strategies which can be implemented to reduce the order of a high-order aeroelastic model. It suggests coupling and numerical integration schemes to utilize this reduced-order model in a time marching simulation tool.

This thesis presents an individual tip control (IPC) system based on blade tip deflection measurements. The controller is based on novel sensor inputs which measure flapwise tip deflection distance at a high sampling rate. IPC plays a key role in reducing fatigue loads in wind turbine components. These fatigue loads are caused by differential loads such as wind shear, yaw misalignment and turbulence. The presented controller is implemented in HAWC2 and high fidelity load measurements are produced using the DTU10MW Reference Wind Turbine. Lifetime equivalent load reductions were seen in both rotating and fixed frame components under extreme turbulence, inverse shear conditions and in normal operating conditions. A novel implementation of IPC is also presented where the blade tips are guided along a fixed trajectory to maximise blade-tower clearance. The motivation of this implementation is to reduce the chance of blade-tower interactions for large diameter turbine rotors. The theoretical background used in this study is presented first along with details of controller discretisation methods. Details of the iterative control design process is presented, and the simulated fatigue loads are compared for a number of control architectures. Finally, the implementation of the tip trajectory tracking control is presented along with an analysis of the pitch rate limits and the effect of IPC on electrical power output.