As the demand for renewable energy rises, the impact of wake effects on wind farm performance and efficiency has become a primary focus for both industry professionals and academic researchers. However, to effectively mitigate these wake effects and improve wind farm efficiency, there is a critical need for an accurate wake representation. This requires well-established medium-fidelity wake models, which are currently lacking. To achieve this goal, it is essential to develop a wake model capable of accurately generating the axial wind profile within the wake. This model is crucial for precisely assessing the loads on downstream wind turbines and evaluating their energy production potential. Moreover, it provides valuable insights into optimising methods to mitigate wake effects and improve overall wind farm efficiency. For the model to be truly beneficial to the research effort and industry applications, it must demonstrate an increased accuracy with a limited increase in computational cost.

This thesis delves into medium-fidelity wake models to enhance the precision of calculations and predictions concerning wind turbine fatigue and aeroelastic loading. It specifically examines the use of a finite element method with Hermite interpolation basis functions as a numerical approach to solve the Ainslie wake model and achieve these objectives. Additionally, the study employs a downstream marching scheme to solve the partial differential equations and introduces the Newton-Raphson method to address non-linearities within the model. The potential benefit of using a finite element method lies in its potential for improved stability compared to the finite volume method, which shows satisfactory performance but limited stability, and superior performance compared to the spectral method, which has shown to exhibit poor conservative properties and instabilities for higher mode numbers. Furthermore, this thesis aims to bridge the gap between modelling and reality by implementing a pressure Poisson equation. Specifically, the focus has been on implementing the pressure component resulting from the forcing in the pressure Poisson equation.

Implementing Hermite interpolation basis functions posed several challenges. Increasing their order caused significant ill-conditioning and greater sensitivity to mesh quality, reducing their effectiveness for this application. Additionally, higher-order Hermite functions proved ineffective for solving the continuity equation due to the presence of odd basis functions. In contrast, piecewise linear basis functions have shown to be compatible with the continuity equation and were effective in solving the diffusion term. Despite this, stability issues emerged, likely due to the Ladyzhenskaya-Babuka-Brezzi condition not being met.

The implementation of forcing in the finite volume Ainslie wake model demonstrated mass and momentum conservation, and the axial velocity has been shown to align with literature. However, the dependency of the vortex strength on the domain size complicates the determination of a correct converged value for $\Gamma$. Furthermore, comparisons of the velocity field with the IEC 61400 standard implementation without the inclusion of a forcing term revealed that the forcing term implementation more accurately reflects the real flow dynamics, capturing both the blockage effect and the edge force effect.

Overall, this thesis demonstrates that the investigated method does not provide a stable solver for the Ainslie wake model. However, this does not rule out the potential of the finite element method for solving the Ainslie wake model. With adjustments to the basis functions and the application of Taylor-Hood elements, this method is still anticipated to deliver satisfactory results compared to existing methods. Furthermore, this thesis has laid the foundation for the inclusion of a Poisson solver. Specifically, it outlines the integration of pressure gradient resulting from actuator disk forcing and presents various formulations for the wind turbine forcing. However, further research is needed to address the calculation of the pressure induced by the velocity field.

This thesis investigates the potential use of the dynamic wake model FLORIDyn for Annual Energy Production (AEP) estimations in large-scale wind farm applications. Traditional low-fidelity wake models, based on steady-state assumptions, fall short in capturing the transient nature of wake interactions under varying wind conditions. FLORIDyn addresses this by incorporating time-dependent wake effects. This study includes a review of existing models, modifications to FLORIDyn, and calibration procedure using an inverse Bayesian approach with surrogate models. On the calibrated model, a validation study against SCADA data revealed that FLORIDyn's predictions did not outperform steady-state models, mainly due to biases in flow convection and wake accumulation inaccuracies. While FLORIDyn shows promise for small wind farms with large turbine spacings, it requires significant refinement for broader industry application. The thesis concludes with recommendations to address the current deficiencies in both the FLORIDyn model and validation methodology, aiming to enhance AEP estimation accuracy and provide more reliable wind farm performance assessments.

Within the context of the new wave of eVTOL aircraft development, Cyclotech GmbH has been working on the development of cyclorotors, a type of propeller consisting of rotating blades along a common longitudinal axis, where the pitch angle of the blades is actively controlled through what is known as a pitch curve. This thesis proposes a methodology to solve an inverse design of this pitch curve, which results in a certain target lift distribution. A framework using Bayesian inference is proposed, altogether with CFD simulations for the aerodynamic solution of the cyclorotor. To reduce the number of CFD evaluations and improve the framework efficiency, gradient descent methods are used altogether with semi-transient adjoint CFD gradients. Furthermore, low-fidelity information is added to the framework through the definition of a prior model to help speed up the inference. Using this methodology, three different test cases, from lower to higher complexity, are defined and successfully solved.

While the assumption of incompressible flow has been the prevailing standard for numerical simulations of wind turbine aerodynamics, the limit of this assumption is approached as the industry progresses towards increasingly higher operational tip speeds. This thesis explores the implications of compressibility and transonic flow phenomena, particularly in relation to a new rotor design that integrates the concept of rotor tilt.

The study is split into two phases. The first phase involves two-dimensional Computational Fluid Dynamics (CFD) simulations, employing an Unsteady Reynolds Averaged Navier-Stokes (URANS) framework, specifically for the thick NACA 63(4)-421 airfoil. Attention is directed at the effects of high subsonic and transonic Mach numbers, and the complex flow phenomena emerging from shock wave-boundary layer interactions (SBLI). The second phase extends the findings from the first phase by simulating a 3MW concept rotor, utilizing a lifting line method. Here, particular emphasis lies on runaway conditions; the operating point of an unloaded rotor where rotational velocity maximizes. Since airfoil drag significantly increases through the onset of “shock-stall”, it might provide a passive mechanism for overspeed protection.

The URANS study provides the first exploration of transonic buffet for a thick non-symmetrical airfoil; the phenomenon characterized by periodic shock movement, leading to large-scale load oscillations. The findings indicate that these fluctuations are significant and could pose a threat to the structural integrity of a full-scale rotor. Additionally, regarding the subcritical compressible regime, the study found that the von Karman-Tsien correction is the most appropriate method for the prediction of airfoil loads prior to reaching the critical boundary.

In the second phase, the rotor is simulated using the averaged values of the compressible aerodynamic coefficients, obtained in the first phase. Two flow scenarios are considered: Storm conditions typical for the North Sea, and subsequently, higher wind speeds often encountered during tropical typhoons in the Western Pacific. The results show that for the first scenario, shock-stall does not prevent the rotor from overspeeding. Instead, using rotor tilt is suggested as a more effective strategy. For the second scenario, a distinctive maximum rotational velocity has been found for an incoming wind speed of $57.5$ m/s, leading to the conclusion that shock-stall could effectively prevent the rotor from overspeeding.

Since the averaged values of the aerodynamic coefficients are used to assess the adequateness of shock-stall as an overspeed protection mechanism, the load fluctuations attributed to transonic buffet are initially neglected. A subsequent preliminary assessment of these fluctuations revealed that operating in the high transonic regime potentially poses severe risks regarding rotor safety.

This thesis presents a comprehensive study on Floating Offshore Wind Turbine’s (FOWT’s) modeling and simulation, focusing on the IEA 15MW wind turbine coupled to the VolturnUS-S semi-submersible platform. The primary goal was to develop the building blocks of a high-fidelity Computational Fluid Dynamics (CFD) coupled model using OpenFOAM to simulate large-scale FOWT behavior, verify, and compare each component individually against the mid-fidelity tool OpenFAST. The project builds on the frameworks of previous studies by Pere Frontera Pericàs [81] and Scarlatti [98], expanding them to accommodate the complexities of a larger turbine and more intricate environmental conditions.

The model was implemented using OpenFOAM’s waves2Foam and Moody libraries for wave and mooring modeling, respectively, while aerodynamic simulations employed the turbinesFoam package with an actuator line approach. A spatial convergence study optimized mesh parameters for computational efficiency and accuracy, ensuring reliable and stable simulation results for motions up to 24 meters in surge. Comparisons between OpenFOAM and OpenFAST using a P-Q analysis highlighted significant differences, particularly in modeling damping. OpenFAST was shown to underestimate both linear and quadratic hydrodynamic damping due to its simplified representation of fluid dynamics. Discrepancies in the equilibrium positions were found between the decay tests using CFD; their origin still needs more investigation.

In aerodynamic simulations, steady-state and prescribed motion tests revealed critical differences in thrust and power predictions between OpenFOAM and OpenFAST. OpenFAST overpredicted the reduction in axial wind speed, leading to a reduced power output for steady turbine simulations. The CFD model provided a more accurate representation of rotor wake effects and dynamic responses, particularly in high-frequency surge conditions, where OpenFAST overestimated power fluctuations. These findings underscore the importance of using high-fidelity models like OpenFOAM to improve the accuracy of performance predictions in floating wind turbines.

The study concludes by offering recommendations for future research, including model validation using experimental data and the implementation of overset mesh techniques to improve simulation stability for large motions. The work establishes a reliable CFD framework for simulating this large FOWT, offering insights into enhancing mid-fidelity models and guiding future developments in floating wind turbine design and analysis. It also builds a fully coupled CFD model in OpenFOAM to investigate its behavior.

Operating in real-world conditions, modern large capacity wind turbines often experience off-design situations, enduring dynamic loads characterized by complex unsteady aerodynamics. Key among the challenges in predicting these dynamic loads is understanding the effects of wind shear and turbulence, both individually and in their complex interplay. This research aims to shed light on these phenomena, with an emphasis on their impacts on wind turbine fatigue loads and power production.

The research first provides an in-depth analysis of the influence of atmospheric stability on wind shear profile, aiming to extend the wind shear profile beyond the range of LiDAR measurements. Recognizing the limitations of existing power law and logarithmic law extrapolation methods, the study validates the use of multiple stability correction functions for accurate wind speed extrapolation. Subsequently, the research delves into the intricate effects of wind shear and turbulence on fatigue loads at the blade root of wind turbines, leveraging aeroelastic simulations. This research addresses the challenge of assessing wind turbine suitability for sites where one or several wind climate parameters surpass their design class values. It investigates the potential of the Response Surface Methodology (RSM) to estimate site-specific fatigue loads, a process that conventionally requires extensive aeroelastic simulations. This research also extends the scope to include the assessment of site-specific wind turbine power curves, validating the use of the Rotor Equivalent Wind Speed (REWS) and turbulence renormalization methods. Both methods show promise in estimating site-specific wind turbine power curves using a power curve measured under varying wind conditions.

In essence, this study emphasizes the significant impact of wind shear and turbulence on the performance and longevity of wind turbines. By shedding the light on potential improvements, this study hopes to contribute towards accurate power output and fatigue load assessments.

Leading edge roughness is an important challenge for the wind energy industry as blades are permanently exposed to the damaging impacts of the environment. The contamination due to impact of insects, ice accretion and dust; as well as the erosion of the blade due to rain, debris and abrasive airborne particles increase roughness upon the surface. Leading edge roughness leads to the deterioration of aerodynamic performance, including early transition from laminar to turbulent flow, increased drag, and reduced lift. These performance losses directly translate into a decreased annual energy production (AEP) and increased levelized costs of energy (LCoE). The aim of this study is to enhance the understanding of airfoil aerodynamics with LER, specifically focusing on the usage and impact of tripping tapes as a standard representation of LER in wind tunnel campaignsb by deviating proven zigzag tapes in non-standard setups. The research objectives include investigating critical roughness height, analyzing the impact of different tripping tape setups, studying drag generation in the boundary layer, and exploring the effects of real erosion.
Contrary to expected reference values, the critical height was found to be traceable with the Roughness Reynolds number for zigzag tape usage as Rek = 125, opposing to expected critical Reynolds number Rek = 200 in current literature. Most interesting application parameter after
critical condition, often related to height, is found as tape location. Drag and lift performance penalties behave as linear function of tape location for pre-stall drag bucket and linear lift region. In stall, the performance penalty is heavily related to locations closer to the leading edge with increased importance of tape height. Additionally, real roughness of reference studies has been more closely resembled by tripping tapes closest to the leading edge in terms of magnitude and as function of pitching angles when compared to industry standards. The boundary layer measurements have indicated that panel code simulation data has errors connected to transition location rather than boundary layer development, which reflects in polar data and transition location.

Overall, this study emphasized that ZZT is more versatile than the industry standard setup, highlighting the importance of considering different application variations for accurate assessments of LER impact, while indicating possible panel code improvements related to transition location
as observed in performance polars and boundary layer thickness.

With an increased demand for renewable energy wind farms, their efficiency receives growing attention from both the industry and academics. Mitigating wake interaction effects in wind farms is one of the ways wind farms could increase their efficiency. Control techniques to increase wake reenergizing are currently under development, however, are being held back by a lack of available medium-fidelity wake modeling methods. To sufficiently support developments in the wind industry, the wake model should yield an axial wind profile that can be used for load calculations for the downstream wind turbine and assess its energy production. Moreover, for the model to be useful in the research effort the method must either show an increase in accuracy or a decrease in time-cost. Finally, the model is desired to include the effects of wake control techniques in later iterations outside the scope of this research. This thesis investigates medium-fidelity models for wind turbine wake control techniques, with an exploratory study into applying a semi-infinite spectral method with an adjusted Laguerre basis to wind turbine wake modeling. The wake model used in this research is an altered version of the Ainslie wake model. To model the wake model a spectral method with Laguerre functions for the basis is employed in the radial direction and a marching scheme with a piecewise linear basis for the basis in the downstream direction. The potential benefits of spectral methods are lower rank systems and spectral convergence. The adjusted Laguerre functions used for this research naturally fit the boundary conditions for the wake model. At the start of the domain, a Dirichlet boundary condition is applied with fitted Laguerre functions to the result of a blade element simulation. The results did not account for wake expansion and diffused the wake much sooner than physical. The implementation of the explicit formulation for the triple product integral hindered higher-order simulations, disabling research into simulations for domains much higher than 11 radial modes. The errors for the different conservated quantities are significantly higher than they should be. From the verification efforts, a relation can be observed between the number of radial modes in the domain with respect to the number of radial modes used for the inflow, which suggests that the model should use much higher numbers of radial modes than this implementation allowed. i

The global emission of harmful greenhouse gasses has to be reduced. Commercial shipping contributes to these emissions by using fossil fuel as the only energy source to provide propulsion and power auxiliary systems. In order to reduce fuel consumption, much research is focused on increasing the efficiency of the propulsion system and reducing the required power consumption. In recent years an increasing amount of research focuses on utilizing wind energy as an alternative energy source. This is a renewable source of energy which is plentiful at sea. This thesis focuses on the novel concept of using a Vertical Axis Wind Turbine (VAWT) as a means of ship propulsion and power generation. Existing Wind Assisted Ship Propulsion (WASP) systems can only be utilized in conditions when the ship is in transit and when the apparent wind conditions are favorable. The VAWT concept has the ability to generate power in wind conditions where conventional sailing systems are not functional, including when the ship is in port. This system can also be used as a thrusting device when the wind conditions are favorable for sailing systems. Therefore this system can be used more frequently which is beneficial for the overall power savings. The use of a VAWT for WASP in combined operation as a thruster and a power generator does currently not exist. Therefore the design of the rotor has to be investigated and optimized in order to evaluate effective designs for different conditions and to compare this device to existing sailing concepts. The aerodynamic performance of the VAWT will be evaluated using numeric simulation models. This model will be coupled to an optimization algorithm in order to find the optimal design parameters for different wind conditions and modes of operation. This project will result in a better understanding of the working principles of VAWT used for WASP and the potential energy saving of optimized designs for the operational modes as a generator, thruster and combined-operation. There is a large opportunity for capturing wind energy at sea. This project has the potential to add a new method of ship propulsion to reduce shipping emissions and reducing the cost of ship operation. When one or multiple of these systems can be applied on a large share of the worldwide shipping fleet, this research can contribute to the reduction of commercial shipping emissions.

Floating offshore wind turbines (FOWTs) have the potential to harness wind resources in deepwater, which is so far prohibitive for conventional approaches. This, however, comes at a cost: the platform’s extra degrees of freedom (DoFs) introduce complex aerodynamic and hydrodynamic behaviours. Therefore, FOWTs must be accurately modeled to reduce load uncertainties that ultimately prejudice their economic viability.

This project implements a framework for the coupled, high-fidelity simulation of FOWTs in OpenFOAM. The tool is built upon two existing libraries: turbinesFoam—for rotor modeling based on the actuator line method— and waves2Foam—for wave-field generation and absorption based on the relaxation zone method. The multi-phase simulation uses the interFoam solver in combination with a morphing mesh technique and rigid-body model to represent the platform. The mooring restraints are computed with a quasi-steady, catenary model from waves2Foam. The turbinesFoam library, targeted at bottom-fixed turbines, is modified so that it can accommodate arbitrary motions along the rigid-body DoFs. The platform-turbine FSI coupling follows a serial sub-iterating strategy based on the PIMPLE scheme.

The presented framework proved capable of modeling the aerodynamic performance of turbines under prescribed motion and produced plausible results for a semi-submersible FOWT under combined wind and wave conditions. Once carefully validated, this tool will have the potential to serve as a reliable technique for the advanced modeling of FOWTs.

Under real sea conditions, floating vertical-axis tidal turbines experience motions in six degrees of freedom which influence the relative velocity perceived at the turbine's blades, with a direct effect on the loading and performance of the rotor. Understanding the fluid-structure interaction of a vertical-axis tidal turbine under the floating carrier’s motions provides insights about critical aspects for the design of floating hydrokinetic systems.
In this project, the TU Delft in-house U2DiVA code is indicated as a conservative and time-effective tool to assess the hydrodynamic response of a vertical-axis tidal turbine under surge, sway and yaw motions compared to the computational fluid dynamics approach commonly used in the scientific literature. The effect of surge motion on the fluid-structure interaction of two existing vertical-axis tidal turbines is investigated using U2DiVA. The analysis focuses on the details of the time evolution of the flow field perceived at the blades and on the modified cyclic loading and power extraction when given surging conditions are imposed.

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.

The aim of this thesis is to evaluate the influence between the two counter-rotating VAWT rotors with the Double Actuator Cylinder (DAC) model which will be validated against the panel code. The DAC model is developed from the original Actuator Cylinder (AC) model which includes the Modified-Linear (Mod-Lin) correction. The original AC model for modelling single rotor is investigated, modified and validated against the panel code to be sufficient which is the premise for a sufficient DAC model. The original AC model is modified to become a dynamic AC model including Cheng’s modification, flow curvature effect and unsteady aerodynamics. The unsteady aerodynamics is included by coupling the modified Risø dynamic stall model with the AC model. An investigation with Migliore’s conformal mapping method as well as panel code revealed that the modified Risø dynamic stall model implicitly included the flow curvature effects. The derived dynamic AC model is validated to show good agreements with the panel code. It is also revealed that further improvements could be made to the Mod-Lin correction method. The DAC model is finally derived, and the performance on predicting the influence between the two rotors is then investigated.