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.

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.

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.

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.