Wind energy is a crucial component of the ongoing energy transition. Over the past decade, technologies in this field have advanced significantly. Currently, the industry is following a trend of increasing the size of wind turbines, both onshore and offshore units. However, this trend brings about complexities in design, maintenance, and operation. Wind turbines are highly dynamic structures, and accurate prediction of dynamic loads is crucial for future design and, consequently, installation costs and lifetime expectancy. Dynamic stall is a highly dependent process influenced by various parameters such as Reynolds number and airfoil geometry. It involves significant changes in pressure in the flow field around the airfoil due to the development and shedding of vortex structures from the leading or trailing edge. Another characteristic is flow reversal from the trailing edge, leading to a delay in boundary layer attachment and the formation of significant hysteresis in lift force, pitching moment, and drag force values. Due to the constant rotation of the rotor, this process is cyclic, necessitating accurate modeling for proper assessment of fatigue damage, instability levels, and turbine noise. This study aims to compare the results of dynamic stall models for the FFA-W3-211 airfoil, with a maximum thickness of 21.1% of the chord length. Previously unavailable dynamic data for the considered airfoil is obtained through experiments in a Low-Speed Low-Turbulence Wind Tunnel located at TU Delft. Static and dynamic polars are obtained for various Reynolds numbers, amplitudes, mean angles, and oscillation frequencies. Measurements are conducted using pressure taps located on the airfoil model, and static data is validated against existing data and RFOIL results. The study considers two semi-empirical models. The first is the Beddoes-Leishman model, one of the most popular at the time of writing. Its optimization and comparison with the default version revealed that optimized shape-based coefficients are crucial for obtaining correct results. It is found that the optimized model significantly outperforms the default version in almost all cases. Additionally, it is established that the model lacks the leading edge vortex effect in modeling negative stall. The second model chosen is the first-order Snel model. Optimization of this model also led to significant improvements in the results obtained. A significant improvement in modeling both hysteresis size and reattachment area with increasing Reynolds number is identified. However, a significant drawback of the model in modeling negative stall is revealed. During uppstroke motion, the model predicts trailing edge vortex effects instead of leading edge, which negatively affects the overall model performance. Comparison of the two models identified their strengths and weaknesses. The Beddoes-Leishman model appears more stable for all cases considered, while the Snel model is sensitive to changes in Reynolds number. It is also noted that both models have difficulties in correctly determining dynamic stall onset angle of attack and the slope of the normal force coefficient. Additionally, both models tend to overpredict values in a greater number of cases. When comparing negative stall, the optimized Beddoes-Leishman model performs significantly better than the Snel model. It accurately predicts the overall form and severity of hysteresis as well as the reattachment area. The Snel model requires additional modifications for correct modeling of negative stall.

As the automotive industry increasingly shifts toward electrification, reducing vehicle drag becomes crucial for enhancing battery range and meeting consumer expectations. Additionally, recent European regulations require tire and car manufacturers to provide reliable drag data. A significant factor influencing vehicle drag is the highly turbulent wake generated by rotating tires. Accurate correlation between wind tunnel experiments and numerical simulations is essential, yet discrepancies often arise due to the dynamic behavior and technical challenges in measuring tire deformation, leading to inconsistencies between tire deformations observed in wind tunnels and those modelled in simulations.

This Master’s Thesis investigates the aerodynamic impact of realistic tire deformation parameters—specifically, bulge and contact patch deformations—using Computational Fluid Dynamics (CFD) simulations conducted with PowerFLOW. The deformation parameter tests are carried out in a standalone tire setup, which is validated from previous work by Dassault Systèmes and experimental results. To further assess the impact of these deformations on vehicle drag, the tests were repeated using the DrivAer model, an IP free model provided by the Technical University of Munich. Given the complexity of the flow features in a tire wake, the analysis of the results consisted of two different approaches: a statistical approach based on Principal Component Analysis (PCA), and a vortex behaviour and wake development analysis, including a vortex tracking algorithm based on the Gamma 2-Criterion and single-link hierarchical clustering.

The results identify the most influential deformation parameters based on their impact on the drag coefficient and the wake behaviour changes. Two primary wake development behaviours were identified: wake contraction and wake expansion. An analysis of the transient flow showed how these two trends resulted from changes in the unsteady behaviour introduced by certain deformation parameters. The relevance of these wake development changes was portrayed by the impact of certain deformations on the full vehicle drag as a result of
complex wake interactions.

As wind turbines get bigger and bigger, the simulation of wind turbines becomes more complex. The increase in size brings about a multitude of intricate challenges that must be addressed in the simulation process. These challenges lie in the different aspects of the simulations such as aerodynamics, structural dynamics, power electronics, hydrodynamics, turbine control etc.

The central theme of this thesis is to explore the effect of one of the aspects- structural dynamics, on the wind turbine simulations. Moreover, the thesis also emphasises the development of a structural module and it’s integration in an OpenFOAM-based wind turbine simulation library called TurbinesFoam. TurbinesFoam is an actuator line method-based simulation tool, which enables the study of turbine performance as well as wake dynamics using Computational Fluid Dynamics. The key motivation is also to contribute towards the accurate simulation of wind turbines by performing a successful integration as said above.

To fulfil the goals, a structural module was developed in Matlab to simulate the
structural dynamics of wind turbines. The module is developed from the theory
of spinning elements assuming the blade and tower as Euler-Bernoulli beam. The developed module code is called while running the CFD simulations in OpenFOAM to add deflection and rotation of the blades and towers, using a MatLab pipe class.The accuracy of the developed code was measured using BModes and OpenFAST. The results exhibited satisfactory agreement between the outputs.

The NREL 5MW turbine was used to study the effects. The results revealed that,
at slightly above the rated condition, the turbine showed 1% decrease in power
production due to elasticity when compared with a rigid turbine. The thrust and
torque coefficients showed similar trends of reduction in value. Moreover, as the inflow wind velocity increased, the differences in performance got broader, due to increased deflection value. The wake region of the elastic turbine showed a mixed region of both increased and decreased wind velocity when compared with the rigid turbine.

Floating Offshore Wind Energy represents an enormous potential for the future of wind energy and will play a pivotal role in the global energy transition. However, important technical challenges still need to be overcome by the industry regarding the standardization of processes like transportation, floating substructure concept, and supply chain optimization. To successfully achieve these milestones, FOWTs must be accurately modeled to minimize uncertainties to ultimately attract investors and capital in the industry. This thesis has the objective of developing and validating a high-fidelity model for the hydrodynamics of FOWTs with the ultimate goal of creating reliable results to be used as a benchmark for faster mid-fidelity software mostly used in the offshore industry.

The model expanded throughout this thesis is developed in OpenFOAM C++ framework. The whole numerical model is built upon three main pillars: Waves2Foam used for the generation of relaxation zones in the numerical wave tank for wave-current generation and absorption; MoorDyn/Moody numerical tools employed for the mooring lines spatial discretization for the station keeping of the floating platform; InterFoam applied as the main OpenFOAM solver for the multi-phase solution which, in combination with a mesh morphing technique and a rigid body solver, is capable of deforming the mesh to accommodate the 6DOF motions of the platform.

A sequential approach has been undertaken. The first step involves the simulation of second-order Stokes wave propagation in an infinite 2D wave tank, followed by a sensitivity study with an associated uncertainty quantification. The next step involves conducting 3D simulations of a floating box, comparing motion and forces under different mooring models: dynamic FEM, dynamic lumped mass, and quasi-static catenary. In the last step, the results and lessons learned from the previous studies are combined for coupled hydrodynamic/aerodynamic simulations of the OC4 DeepCwind semi-submersible floater with the 5MW NREL offshore wind turbine. Forces, displacements, and tensions are extrapolated and compared with experimental and numerical results, validating the model's accuracy.

Simulated across diverse wind and wave conditions, the OpenFOAM model demonstrated remarkable robustness and excellent ability to achieve convergence even under challenging environmental conditions, while maintaining a high level of result accuracy. The framework effectively reproduced the hydrodynamic behavior of a Floating Offshore Wind Turbine (FOWT), establishing itself as a dependable and reliable tool for conducting future high-fidelity FOWT simulations.

The quasi-steady model (QSM) was developed to calculate the power output of a soft-wing airborne wind energy (AWE) system operated in pumping cycles. The comparison with experimental data shows that the original formulation of the model underestimates the power output. The model does not take inertial forces into account and highly simplifies the reel-out flight path. In this work, these assumptions are critically reviewed and their impact on the simulation results assessed.

The effect of inertial forces is analytically described for a helical flight path with fixed turning radius around the downwind axis. The derived analytical solution for this flight path verifies the formulation of inertial forces in spherical coordinates proposed in earlier studies. The proposed equations are modified and added to the QSM. Both, the analytically developed set of equations and the QSM with added inertia are compared for the helical flight case. It is observed that at larger cone angles where the tether length is rather short, but the turning radius is large, the inertial effect increases the tether force and the power output. With small cone angles this effect reverses. Based on the example case, the impact of the inertial effects on the output power computed with the QSM is limited to 1-2% for lightweight soft kites.

In the original formulation of the QSM the figure-of-eight maneuvers are not resolved. Instead, the tether is reeled out at representative constant values for the elevation, azimuth and course angles. QSM output based on real flight path coordinates is used as a benchmark to calculate the accuracy of reel-out path simplifications. The power output error of resolving reel-out flight with constant angles is on average 4.1% when compared to the reference simulations. An alternative flight path representation is developed, fitting a known figure-of-eight, lemniscate, to measured asymmetric reel-out flight paths. By flying the fitted figure in the QSM, the output power error caused by this path representation is estimated to be only 1.7% in comparison to the reference simulations. It is recommended to use this fitting method based on a class function/shape function transformation technique (CST), to parametrize the reel-out flight path, despite the added complexity.

The study has shown that the accuracy improvement by including the inertial effects in the QSM is marginal if the model is limited to lightweight soft wing simulation. As a follow-up study the inertial effects should be tested with bigger kites and heavier rigid-wing systems. The adjusted CST fitting method together with the inertial equations enable these maneuvering flight simulations in the QSM.

This research presents the modelling of struts of a vertical axis wind turbine and its effects on the wake and performance. The 3D non-linear lifting line solver CACTUS is extended to include the struts in the rotor as lifting bodies. A case study is performed for different strut configurations in the rotor, with the focus set on the inclination angle of the struts. The results are analyzed and compared to the reference turbine excluding struts. The inclined strut's interference on the main rotor blade reduces blade loadings on the downwind pass. Part of the torque generation by the blades is therefore transferred to the struts, also incurring a small loss. It is furthermore found that the inclusion of the inclined struts distorts the flow field in the wake. The wake tends to expand and deflect more downstream then without the struts.