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.

In order to be more competitive with conventional, polluting energy resources, the levelised cost of energy (LCoE) of wind turbines has to decrease significantly [1]. A method to achieve this goal is to reduce the amount of parts in a wind turbine and more specifically avoid the use of expensive and failure-prone pitch systems. Using blades which passively deform under high aerodynamic loading and effectively reduce the peak, design-driving loads on the turbine structure might offer an interesting alternative to the pitch system. However, the conventional material for construction of blades, glass-fibre reinforced plastics,
does not allow for such large deformations during normal operating conditions. Prior to this project, a study to incorporate more flexible materials has been carried out in the design of the current XANT-21 wind turbine in order to increase the flexibility of the blades. However, introducing a high degree of flexibility in the wind turbine blades might increase the risk of aeroelastic instabilities. These aeroelastic instabilities often result, in combination with a non-linearity in the aerodynamics or structure, in limit cycle oscillations (LCOs). The need to identify the key structural parameters which play a role in the initiation of the aeroelastic instabilities has led to the objective of this thesis: Investigate the influence of the critical structural parameters on the onset velocity and the behaviour of high-amplitude limit cycle oscillations of a wind turbine airfoil by means of numerical simulations.