Multirate aero-elastische simulaties van grote windturbines met het actuatorlijnmodel

The continuous upscaling of modern wind turbines has introduced significant challenges to the wind energy community, driven by structural limitations and new operating regimes. To increase the energy yield per unit of rotor swept area, wind turbines have increased in size, yet structural weight remains a key limitation on design. Consequently, the next-generation models exhibit low mass-to-stiffness ratios, resulting in increased structural flexibility and strongeraeroelastic coupling. Moreover, these megastructures have now entered higher altitudes in the atmosphere, where wind shear, veer, turbulence intensity and stability effects become increasingly significant. The combined influence of flexibility and complex inflow conditions has therefore introduced new modelling challenges for accurately predicting unsteady aerodynamic loading, fatigue and overall turbine performance. Accurately resolving these coupled dynamicsrequires high-fidelity frameworks capable of capturing the unsteady multi-scale flow and aeroelastic phenomena while also maintaining computational efficiency. This challenge motivates the development of improved time-integration coupling strategies for next-generation aeroelastic simulation tools. In a first step, we develop a novel coupling architecture for aeroelastic frameworks, by tightly coupling a large-eddy simulation type flow solver with a multibody structural model, where the turbine aerodynamics are represented through an actuator line model. To do that, a third-order-in-time multirate Generalized Additive Runge-Kutta scheme is employed to couple the flow and turbine dynamics within the in-house large-eddy simulation solver SP-Wind. This formulation enables for the decomposition of the system into multiple timescales that represent the inherent temporal evolution of each subsystem, thereby allowing each component to be advanced at different rates. By assigning independent temporal resolutions to the flow and turbine solvers, the multirate framework reduces the computational cost without compromising accuracy. Two alternative coupling techniques are explored by varying the assignment of slow and fast components in the multirate method, with the flow solver consistently treated as the slow component since it consumes the majority of the computational effort. In the first coupling strategy, referred to as fast-aero, we assign the turbine aero- and structural dynamics to the fast component. In the second variation, defined as the slow-aero approach, the aerodynamics are assigned to the slow component while structural dynamics remain on the fast partition. Next, the developed novel multirate framework is evaluated through large-eddy simulations of a single NREL 5 MW turbine operating inside uniform inflow conditions. In this study, we examine the influence of the multirate parameters, such as the slow and fast time-steps on the scheme's performance. We find that the method maintains a third-order temporal accuracy for both flow and turbine components, while different behaviours were observed between the two coupling configurations. In general, the fast-aero yields smaller numerical errors, whereas the slow-aero configuration demonstrates better computational efficiency, with performance gains approaching a factor of thirty compared to single-rate schemes. The framework is further assessed under unsteady conditions using theDTU 10 MW turbine in a pressure-driven boundary layer. The results confirm third-order accuracy in both flow and structural variables. Turbulence is found to intensify load fluctuations and excite natural frequencies, demonstrating the framework's capabilities and robustness in realistic operating conditions. However, some limitations are observed that are related to the accuracy of the actuator line model and its inability to correctly account for the tip and root vortices, leading to a systematic power overprediction. The final part of this work, focuses on the influence of hydrostatic blockage on the performance of modern offshore wind turbines, such as the IEA 15 MW model. Building upon the previously developed multirate aeroelastic framework,the model is enhanced with the filtered lifting line correction to improve the actuator line accuracy. The final multirate framework is evaluated first under uniform inflow conditions with varying levels of physical fidelity. The analysis shows that both structural flexibility and the filtered lifting line correction improve the power and load predictions, as they mitigate the underestimation of induced velocities and the overprediction of aerodynamic loads in the absence of these effects. The thesis concludes with an investigation on the effect of boundary-layer height in shallow, conventionally neutral boundary layers on the power performance and structural response of large wind turbines. A preliminary sensitivity analysis on the computational domain size is performed with a coarse grid resolution, where it is shown that narrow domains intensify blockage effects and sideways re-entry phenomena, which in turn can reduce turbine efficiency and modify the wake structure. We then select a domain configuration as a balance between these findings and computational resources, which we employ to conduct fine resolution studies with varying boundary-layer depths. The results demonstrate that in shallower cases, the hydrostatic blockage is intensified and a slow wake recovery is observed downstream the turbine. This leads to a significant effect on the power output, where we observe a negative correlation between the power curve and the boundary-layer depth, with the power dropping by 15% in the shallowest case. As the boundary layer deepens the blockage effect diminishes and enhanced vertical mixing facilitates faster wake recovery. Structural results further indicate that flapwise deflections and load variability are affected by the presence of a capping inversion, while overall fatigue loading remains mostly insensitive to boundary-layer height.