Floating wind is a proven concept to harvest wind energy in deeper water. Most technology of floating wind is transferred from the oil and gas industry. However, a commercial wind farm can consist of 50 wind turbines, and an efficient transport method is needed. Wet-towing is an efficient transport method, but it introduces fluid-structure interaction and should be carefully investigated. For wet-towing operations, the expected drag load is of interest, so a vessel with the right capacity can be chosen. Vortex shedding can occur since most semi-submersible platforms consist of several cylinders. This can cause vortex-induced motion and cause undesired motions such as sway motion.

While previous studies have examined fluid-structure interaction of semi-submersible platforms, two key gaps remain. First, most are conducted at model scale, where the lower Reynolds number leads to different flow regimes and may not reflect full-scale behavior. Second, many vortex-induced motion studies use single-phase simulations with a double-body assumption, neglecting the free surface and its influence on wake dynamics. These gaps highlight the need for full-scale, multiphase simulations to capture realistic wet-towing hydrodynamics.

This study carries out full-scale multiphase simulations to investigate three key aspects. It examines the influence of towing configuration, specifically heading and draft, on the hydrodynamic performance of the platform. It evaluates the impact of the free surface on vortex-induced motion of semi-submersible platforms. Finally, it compares the accuracy and efficiency of drag prediction and vortex behavior across three modeling approaches: full-scale multiphase, full-scale single-phase, and model-scale multiphase simulations.

The results indicate that 180 degree heading is preferable to 0 degree, as it produces a more stable wake, lower drag, and avoids the constant offset lift forces observed in the 0 degree case. A shallower draft of 10 m is also favored, as both the free end and free surface effects help suppress coherent vortex shedding, reducing the risk of vortex-induced motion and lowering drag. Comparing models, full-scale single-phase simulations exhibit vortex shedding, while multiphase simulations develop a steady wake. This discrepancy arises from the unrealistic symmetry constraint at the free surface in single-phase cases. Finally, while model-scale simulations scaled by Froude’s method can offer a conservative estimate for drag, they lack the fidelity needed to capture accurate vortex behavior.

These findings highlight the importance of using appropriate CFD models when evaluating towing strategies, and provide a foundation for optimizing semi-submersible transport operations in future floating wind projects.

With increasing computational power, Large-Eddy Simulations (LES) are becoming more prevalent for high-fidelity wind farm analysis. Accurately representing wind turbines in these simulations requires knowledge of blade load distributions, often obtained from Blade Element Momentum (BEM) theory. However, BEM requires numerous geometric and aerodynamic parameters, which are frequently unavailable in industrial settings due to confidentiality. Consequently, industry-scale LES are often limited to uniform Actuator Disk (AD) models, which lack accuracy and cannot capture load distributions.

This thesis implements an Analytical Body Force Model (ABFM) that estimates blade load distributions from LES data using limited turbine information. Both one-way and two-way coupling strategies are explored. In the one-way approach, the ABFM supplements the AD model to compute blade forces without influencing the LES. In the two-way approach, the ABFM forces are actively fed back into the LES in a feedback loop, replacing the uniform AD model and improving simulation fidelity.

Wind energy is central to many international climate goals which promote the transition to carbon-free energy sources. Most commercial wind turbines at present are horizontal axis wind turbines (HAWTs). However, vertical axis wind turbines (VAWTs) can fill two gaps in wind energy: offshore floating wind energy and urban wind energy, in part due to their insensitivity to the incoming wind direction, low center of gravity, high power density, and low noise production. Historically, VAWTs have suffered from fatigue issues, low power efficiency, and difficulty in self-starting. Active variable blade pitch control can address these issues. It can be used to maximize the power efficiency, minimize blade load fluctuations to alleviate fatigue, and minimize torque fluctuations. Numerous studies on VAWT blade pitching have been done in the past, both numerical and experimental, and both with fixed and variable blade pitch. Numerical studies use models with varying accuracy. If pitch optimization is conducted, the most common objective is maximizing the power coefficient, C_p. However, blade pitch control can also be used to improve the loading characteristics on a VAWT.

In this study, individual active variable blade pitch control optimization is performed with multiple objectives, recognizing that there are multiple uses of pitch control for VAWTs. The goal of this study is to develop an optimal pitch function(s) that maximizes C_p and minimizes the fluctuations in rotor normal force and torque. The pitch angle is implemented as a third-order sinusoidal function. This is a numerical study, which uses the two-dimensional actuator cylinder (2D AC) model. The unified non-dominated sorting genetic algorithm III (U-NSGA-III) is used to perform pitch function optimization. The multiple objectives are 1) maximize C_p, 2) minimize rotor normal load fluctuations, and 3) minimize rotor torque fluctuations. The objectives are built up in successive optimization cases resulting in three optimization problems, a single-, two-, and three-objective optimization problem, respectively. Solutions are found which achieve all three objectives at the same time. Adding the second and third objectives changes the optimal solutions, and the multiple objectives are important to consider because only maximizing C_p leads to an increase in unfavorable normal load and torque fluctuations.

When only maximizing C_p is considered, the power coefficient can be increased by 17.3% over the base case (with zero pitch). The power coefficient can be increased by 10.3% over the base case without increasing the normal load fluctuations or the torque fluctuations. The normal load fluctuations can be reduced by 22.1% compared to the base case while maintaining the power coefficient and torque fluctuations. The torque fluctuations can be reduced by 13.9% compared to the base case while maintaining the power coefficient and normal load fluctuations. The optimal pitch functions which maximize C_p and/or minimize the torque fluctuations require high variations in the pitch angle throughout the rotor revolution. Meanwhile, optimal pitch functions which minimize the normal load fluctuations require fewer oscillations (they resemble more first-order sinusoids). For the majority, the optimal pitch functions pitch the blade outward in the upwind half and inward in the downwind half of the revolution, though adjusting the pitch angle around the transitions between the upwind and downwind halves can increase the power coefficient. Optimal pitch functions which minimize the normal load fluctuations reduce the angle of attack and loads throughout most of the revolution and shift the loading on the rotor toward the downwind half as compared to the upwind half. Minimizing the torque fluctuations requires pitch functions with relatively large amplitudes. The pitch functions spread out the blade loading and power generation across the revolution by reducing the loads and power generation in the middle of the upwind and downwind regions and increasing them around the transitions between the upwind and downwind regions. In some cases, the optimal pitch functions are transferable to other operating conditions and turbine designs. They can lead to an increase in C_p for a certain range of tip speed ratios, however, the optimal pitch functions depend on the tip speed ratio. The optimal pitch functions and their effectiveness are similar when the turbine has three compared to two blades. Implementing an optimal pitch function found in this study can increase the power output of VAWTs while not compromising their structural integrity and/or alleviate fatiguing load fluctuations while maintaining the power output. Optimal blade pitching can lead to a significant improvement in the operating performance of VAWTs.

With a symmetric rotor, tip vortex helices develop regularly before interacting, following the leapfrogging instability. The latter can occur earlier when the helices are placed at an initial radial offset, which is realized by considering blades of different lengths. This study investigates the spatio-temporal development of near-wake behavior for rotors with a significant blade length difference. Large eddy simulations and actuator line method on a modified NREL 5MW turbine under various inflow conditions were conducted to evaluate the impact of blade length differences ranging from 5 to 30%. The development of tip vortex helices, the onset of leapfrogging, vortex merging, and ultimately their three-dimensional breakdown were monitored. The analysis is corroborated by using a simplified (two-dimensional vortex-filament model).
The results show that the leapfrogging process begins immediately downstream of the vortex release when blades of different lengths are considered. The instability growth rate obtained from the 2D vortex model agrees with the LES data. Although the rotor asymmetry accelerates the leapfrogging and, in some conditions, the vortex merging process alone, it proves insufficient to cause a large-scale breakdown of the helix system and enhance wake recovery. Under laminar and 5% turbulence inflow conditions, the rotor asymmetry has minimal effect on momentum deficit recovery. In other conditions, no appreciable variations of wake recovery can be observed.

Wind turbine technology has advanced significantly as a result of the growing need for renewable energy to fight climate change and lessen reliance on fossil fuels. Dual-rotor wind turbines are one of these developments that shows promise for increasing energy extraction efficiency by absorbing wake energy that remains from the upwind rotor. The impact of rotor spacing, tip-speed ratio, rotating direction, and pitch angle on turbine efficiency are the main topics of this study, which examines the aerodynamic performance of coaxial dual-rotor wind turbines.

The study computes the effect of axial induction while neglecting tangential induction. It also highlights the distinct advantages of both co-rotating (CO-RWT) and counter-rotating (CR-RWT) systems, examining the aerodynamic differences between them. Actuator Disc Model (ADM) simulations, Blade Element Momentum (BEM) theories, and analytical techniques are used to determine velocity deficits and assess rotor interactions. The results show that CR-RWT setups provide better power. The findings demonstrate that CR-RWT configurations achieve high power coefficients due to improved wake recovery and reduced turbulence, while CO-RWT designs prioritize increased torque and power density.

The analysis reveals that parameters such as rotor spacing and TSR significantly influence the aerodynamic interactions between both the rotors. Optimal spacing minimizes wake interference, while precise control of TSR and pitch angles improves overall turbine performance. By systematically investigating these factors, this work identifies the conditions under which dual-rotor wind turbines can maximize energy output and efficiency. The study provides valuable insights into optimal design parameters for dual-rotor configuration, contributing to the development of cost-effective, high-efficiency wind turbines for future energy needs.

Wake losses in wind farms, caused by closely spaced turbines, reduce downstream power production and overall efficiency. These losses can be mitigated through wind farm flow control strategies, specifically wake steering, which involves yaw control to misalign turbines with incoming wind, and redirecting wakes to enhance overall power output. This thesis aims to develop a real-time, model-based controller that addresses operational and environmental challenges in large-scale wind farms. The controller adapts to off-design conditions such as changing atmospheric factors and offline turbines, making it more effective than traditional controllers based on pre-optimized lookup tables. The research modifies engineering wake models to account for heterogeneous wind conditions, improves the computational efficiency of the Serial-Refine optimization strategy for real-time use, and incorporates tuning of wake model parameters to ensure adaptability. Additionally, the impact of offline turbines on optimal yaw misalignment angles is examined. Results indicate that adapting wake models using sensor data improves power predictions in non-uniform wind environments, while the distributed optimization framework for real-time yaw adjustments performs comparably to centralized methods. Incorporating offline turbines into the control strategy further boosts power output, leading to increased revenue for the wind farm. This research advances wind farm flow control by developing control policies that are tailored to dynamic, real-world site conditions.

Numerous dynamic stall models exist to improve aerodynamic modelling. As such, they play an integral part in aeroelastic analyses. In numerous publications, their qualitative damping effect on stall-induced instabilities of wind turbine blades compared to using quasi-steady aerodynamics has been proven. However, often different dynamic stall models are used without consideration of quantitative differences between aeroelastic predictions. Thus, the goal of this thesis is to quantify the damping effect of dynamic stall models on stall-induced instabilities. A model used in HAWC2, openFAST, one developed by IAG, and one unpublished model are compared amongst each other and in comparison to predictions with quasi-steady aerodynamics. A typical blade section at 75 % span of the DTU 10 MW reference wind turbine is modelled. The blade the section belongs to is facing vertically up at a pitch angle of 90° in parked conditions. The inflow ranges from 5 m/s to 50 m/s with yaw-misalignment angles from -25° to 25°. In comparison to using quasi-steady aerodynamics, the dynamic stall model from openFAST reduces the amplitude of edgewise limit cycle oscillations from maxima of 22.5 m to 7.2 m. The model from HAWC2 further reduces the maxima to 5.4 m. No edgewise limit cycle oscillations occur using the IAG or unpublished model. Instabilities other than limit cycle oscillations are not predicted. Analyses of the time series of dynamic stall parameters, dynamic lift, drag, and moment power, as well as the unsteady force and moment coefficient loops and aeroelastic damping ratios, did not trivially explain the models' different predictions.

Large scale wind farms consisting with floating offshore wind turbines (FOWTs) will be a solution for offshore wind energy industry to access more wind resources. However, wake structures and wake interactions of FOWTs subject to motions are still not yet been fully understand, especially when they are under turbulent inflow conditions with realistic turbulent intensities. These will be critical for designing floating offshore wind farms. Note that the majority of previous research are conducted with single rotor using models having relatively low fidelities and/or focusing on laminar inflow conditions. To advance the knowledge about wake and wake interactions of FOWTs, numerical studies about rotors of FOWT with prescribed harmonic surging motions are conducted in this thesis project with high fidelity CFD models, namely large eddy simulation (LES) coupled with actuator line model (ALM). Cases with single rotor without controller, dual rotors in tandem without controller, and dual rotors in tandem with controller are simulated with various of settings, including different surging settings and different inflow turbulence intensities.

For the cases with single rotor without controller, it is found that the differences of wake structures between fixed and surging rotors are pronounced when under laminar inflow conditions, where the periodic structures related to the harmonic surging motions can be detected straightforwardly; while the differences are much less significant when under inflow conditions with realistic turbulence intensities, and the periodic structures are clearly revealed only after phase-locked averaging. Moreover, surging cases with laminar inflow conditions have wake recovery rates which are significantly higher than the fixed case with laminar inflow; however, with turbulent inflow, wake recovery rates for surging cases are only slightly higher than the fixed case.

For the cases with dual rotors without controller, it is found that the wake interaction modes between the two rotors are significantly affected by the surging settings for the laminar cases, while the turbulent cases are insensitive. However, the power performances of the downstream rotors will be increased slightly with surging upstream rotors for the turbulent cases.

For the cases with dual rotors with controller, it is found that the implemented simple controller cannot improve the performances of the rotors as designed due to the large rotational inertia, and thus the modes of wake interactions are not altered a lot. However, the downstream rotors' operational parameters were successfully changed to more desirable values by the controller, demonstrating the controller's potential for numerical analysis of wake interactions between wind turbine rotors.

The findings of this effort demonstrates that the wake structures due to the surging motions of FOWT rotors will be smeared out by the ambient turbulence; and to achieve better power performances, more advanced controlling strategies may have to be implemented for FOWTs subject to surging motions.

The wind turbine wake is a downstream region of kinetic energy and velocity deficit, higher turbulence and has a complex helical vortex structure. The wind turbine wake’s stability depends mainly on the ambient turbulence and the wind turbine tip- speed ratio. Researchers have developed several wake control techniques to mitigate this issue, such as static axial induction control by torque or pitch control, dynamic axial induction control by pitch control and actuating flaps or other actively controlled add- on devices. Some gaps are present in these methods, in terms of applicability of these techniques in terms of full scale validation and practicality to have the devices as add on to existing blades.

To address these research gaps, this study focuses on designing and evaluating segmented Gurney flaps (in line with the ECN (now TNO Wind Energy) patent by Edwin Bot and Arne Van Garrel) for wind turbine blades to enhance the wake recovery by inducing turbulence in the wake to excite the tip vortices. 4 Gurney flaps were attached in the tip region of each blade of a GE 3.8MW research wind turbine. Field tests were conducted in this study for the wind turbine wake (using a scanning LiDAR to scan a sector up to 5.5D downstream at different altitudes; with a scan time of roughly 3 minutes) and performance analysis (using 10 minute averaged measurement data). Free vortex wake simulations were conducted to validate the faster wake breakdown by the change in lift distribution upon addition of segmented Gurney flaps. Simulations using dynamic blade element momentum theory with IEC NTM inflow conditions from cut in to cut out wind speed were conducted to assess the structural impacts on the retrofitted wind turbine.

The field tests’ wake analysis was quantified with different wind speed, turbulence intensity, wind shear, wind direction conditions. Post processing of LiDAR data involved filtering, creating bin averaged data set, using Gaussian process regression and retrieval of the required wind component. The results show a consistent increase in wake recovery, generally at all downstream distances. The retrofitted configuration results are associated with a higher standard error because of a shorter testing period (due to increased noise levels) than the baseline configuration. The wake simulations indicate an earlier wake break down position, by roughly 2D.

The simulations indicate AEP increase for the retrofitted wind turbine to be roughly +0.2% (+50MWh), at the expense of increased blade and tower structural loads. The wind speed weighted damage equivalent blade flapwise bending moment was found to increase by roughly +2% to +4%; with peaks observed in partial load region of the wind turbine, associated with the operating angle of attack in this region.

Due to its immense potential to harvest larger amounts of energy from the wind at a higher capacity factor, offshore wind energy is the key to reach the goal of a more sustainable future. The wind energy research community has been focusing all its efforts to bring the new concept of floating offshore wind turbines (FOWT) into the market. By installing the turbine on top of a floating platform the restrictions of offshore turbines regarding the sea depth are lifted and numerous new sites now become available for the wind energy market.
Due to the motions that FOWT experience during their operation several questions arise regarding their aerodynamics, structural integrity and control efficiency. Since aerodynamic models are utilized in almost every field involved in the design of a wind farm, its particularly important to make sure that the current industrial aerodynamic codes, like BEM, are able to capture the unsteady aerodynamics of this complex system. High-fidelity CFD codes and experiments is the only way to truly assess the performance of BEM.
Surge motion is recognized by all authors as one of the most important motions imposed to FOWT. Thus, in the present thesis the impact of surge motion in the aerodynamics of FOWT was investigated by employing a high-fidelity blade-resolved CFD model in OpenFoam. After the extensive validation of the CFD model with the high-fidelity experiment of the UNAFLOW project, two test cases were examined. One with low surge frequency/amplitude which was employed to test whether momentum theory can accurately predict the induction of FOWT and one with an extreme surge frequency/amplitude which was used to test whether propeller and vortex ring states occur during the operation of FOWT - rendering BEM invalid.
In the first test case, Ferreira-Micallef induction computation model was utilized to extract the induction field from the CFD field data. After extensively comparing the induction computed by Ferreira-Micallef model and momentum theory it became apparent that due to the 2D nature of both models it was not possible to draw any significant conclusions about the accuracy of momentum theory. Specifically, due to the existence of 3D radial flow during the operation of FOWT Ferreira-Micallef model is not reliable and can not be used as a basis to compute the error of momentum theory. Having said that, in the mid-span region of the blade and when the FOWT is operating in the mild surge conditions, 3D flow was not present and a comparison of induction fields between momentum theory and Ferreira-Micallef model yielded very similar results. In the second extreme surge test case, two observations were made. First, even though thrust was acquiring negative values during the surge motion of the rotor, the stream-tube did not seem to experience any propeller state since it was expanding at all times. Second, in all the 3D iso-surface q-criterion scenes that were produced there was no large vortex ring visible in the wake, thus there was no indication that vortex ring state occurs for a FOWT surging under these harsh surge conditions.
The present thesis contributes in the better understanding of FOWT aerodynamics and provides clarity on the ambiguous research topic of whether propeller/vortex ring states occur during their operation. Furthermore, the high-fidelity blade-resolved CFD model which was developed can be used as a starting point for several future research efforts.

In recent years, there has been renewed interest in propeller research due to environmental concerns. Propellers are known for their low-speed efficiency and compatibility with electric motors. The utilization of regenerative energy during deceleration has been demonstrated to be beneficial for automobiles and has the potential to be applied to propeller-driven aircraft. This technology allows for the recharging of batteries and could increase the overall efficiency of the propulsion system by utilizing the regenerated energy in other flight maneuvers. The negative thrust mode of propellers has several additional advantages, including weight reduction, improved safety, and increased maneuverability. However, when operated in reverse thrust conditions, the performance of conventional propellers is suboptimal because they are designed for forward thrust. This suboptimal operation can lead to boundary layer separation on the blades, increasing broadband noise and potentially making it more dominant over tonal noise. To assess the relative importance between an isolated propeller's tonal and broadband noise sources under positive and negative thrust conditions, an experimental study was conducted using a scaled propeller setup in a low-turbulence wind tunnel (LTT). Aeroacoustic measurements were taken using a microphone array with 63 microphones placed in the floor and wall of the test section. The advance ratio was varied across different wind speeds at Reynolds numbers ranging from Re_c^0.7R = 0.8·10⁵ to Re_c^0.7R = 1.9·10⁵. Signal processing methods were used to analyze the data and quantify tonal and broadband noise sources, including fast Fourier transformation (FFT), phase averaging, beamforming, and blade element momentum (BEM) analysis. The results showed that the broadband noise component of the propeller significantly increased in negative thrust mode compared to positive thrust mode due to fully separated boundary layers. The tonal propeller components were often masked by interference from wind tunnel and motor noise, suggesting the need for further research to validate propellers in regenerative mode.

Wind turbines experience a wide variety of load cases, both when in operation and when parked. While a turbine in operating conditions often experiences larger loads than in parked conditions, uniquely large inflow angles may occur in parked conditions resulting in less well-understood aeroelastic conditions. Vibrations under these conditions are referred to as stall and vortex-induced vibrations (SVIV). While there has been some success in simulating these conditions, there has been a lack of experimental studies in the literature.

To address this gap, this study analyses the experimental data obtained from a 3.8 MW research wind turbine. This turbine was equipped with strain gauges at the root of 2 blades and in the tower, as well as pressure sensors on one blade and accelerometers in the nacelle and tower. Several experiments were performed with a focus on simulating various conditions where SVIV may occur. This included pitch traverses of a single blade, yaw traverses, and a traverse of the azimuth angle at a yaw angle of 90 degrees to determine the effect of the inclination angle.

Severe stall-induced vibrations were identified at the yaw angle of 110 degrees, when a single blade pointing up in the sky was pitched 180 degrees, and the other two blades were in vane positions at 85 degrees pitch. The severity of these vibrations strongly varied with the wind speed, at an average wind speed of 19.5 m/s the test needed to be stopped early for the safety of the wind turbine, while at 16.6 m/s, the vibrations appeared to reach a limit that was still considered to be safe. The first tower mode experienced the most severe increase in magnitude during these conditions of SIV. In other conditions, no severe case of SIV was identified. However, under several conditions, smaller increases in vibration magnitude were identified. These are likely the result of slightly reduced aerodynamic damping.

No vortex shedding was identified in the data obtained from these experiments. This means that the tested turbine/blade design either did not experience vortex shedding or that it simply could not be measured by the installed instruments. Having pressure sensors installed in multiple locations along the blade would likely help in identifying vortex shedding for future experiments.

Simulations from the experimental conditions were performed using the aeroelastic software PHATAS together with the aero module from ECN (now part of TNO). This simulation used beam-based structural modeling with either a BEM or free vortex wake aerodynamic model. These simulations confirmed the need for realistic turbulent inflow conditions and the need for a good dynamic stall model. However, the structural model failed to simulate all the natural frequencies with the expected accuracy. This should be improved upon for future use of this structural model in parked conditions.

Understanding the fatigue load history of wind turbines is critical for taking decisions regarding the lifetime of a project. However, direct measurement of fatigue loads at each turbine in a wind farm is unfeasible. For this reason, surrogate models offer a useful alternative. In this thesis, a methodology for creating surrogate models for emulating fatigue loads of offshore wind turbines is presented. The methodology is unique in that it accounts for the variability of site-specific conditions that may be present between wind turbines of the same class. First, a method for creating simplified structural models which depends only on a few degrees of freedom is derived. After this, a database of simulation data is assembled by varying the geometric, dynamic, and environmental degrees of freedom within ranges which capture a high degree of variability of possible site-specific conditions. This database is then used to train surrogate models using neural networks.

In order to reduce the levelised cost of energy, the rotors of wind turbines are increasing in size. To increase the energy yield, wind turbine rotors need to have an innovative tip design; such as winglets. Winglets are used widely in aircraft design; however, they remain mostly absent in state-of-the-art wind turbine design. The low-fidelity wind turbine design models used by industry, such as BEM, are insufficient to capture the full 3D flow physics in such an innovative design. Therefore, high-fidelity methods, such as vortex methods, are becoming more and more important in such a design phase in wind energy research. Single-objective optimisation has been applied in earlier works to maximise power production. Winglets have shown the potential of increasing power production while simultaneously increasing the design-driving loads (DDLs), for example, the thrust or flapwise bending moment. This work focuses on optimisation using machine learning of a winglet that increases power production without increasing DDLs.

A parameterised design for a winglet on a wind turbine is created. Different design constraints, such as DDLs or a diameter constraint, are explored to determine under which constraints and conditions a winglet can have an added value to the wind turbine blade design. Multi-objective Bayesian optimisation is used to maximise the rotor's power production and minimise DDLs. Surrogate models, created using machine learning techniques such as Gaussian Processes and Bayesian Neural Networks, are used in combination with an acquisition function, to determine what designs should be evaluated by the lifting line model AWSM. This has the goal to obtain designs that lie on the Pareto front of two or more objectives. The recent Bayesian Neural Networks were able to find the Pareto front most effectively in this work. Furthermore, the results show that different DDL constraints led to different winglet designs, with noticeable differences between upwind and downwind winglet designs obtained by the optimiser. Both a downwind and upwind winglet were found to be able to increase power without increasing the thrust, root flapwise bending moment and flapwise bending moment at 80% of the rotor radius.

The rapid development of the wind industry over the past few years has pushed turbine manufacturers to meet the growing energy demands by designing and producing large scale wind turbines.This also means development of larger monopile foundations for the foundation designers in the case of offshore wind turbines.\ Generally, the turbine tower and monopile are modeled together and the loads from the rotor-nacelle assembly are provided by turbine manufacturers.\ The offshore industry is now showing more interest in extracting the loads from the top mass by developing their own tools in order to reduce the dependency on the manufacturers. In order to aid in this process, the present master thesis aims to develop a linear model based on the concept of Dynamic Substructuring which employs a set of equations to compute the interface forces using the kinematics.\ Furthermore, the developed prediction model is used to analyze the loads occurring at the interface between the rotor-nacelle assembly and the tower for different wind speeds and wind conditions.Consequentially, the model was found to produce acceptable loads at higher wind speeds for selected degrees of freedom at the interface while failing to do the same for other degrees of freedom.The results in time domain were converted to the frequency domain to analyse the resonance.The influence of resonance on the interface degrees of freedom was found to be higher at wind speed below the rated condtion.\ These findings can be used as a basis to conduct further investigations into the application of numerical integration concepts to aeroelastic structures.

Wind turbines operate in the earth's atmospheric boundary layer - an environment where the climate is turbulent and unsteady. In addition, wind turbines usually operate in clusters (wind farms) and are thus subjected to the unsteady wakes generated by upstream turbines. Because of the unsteady inflow, wind turbines suffer from structural fatigue damages. There is therefore motivation to better analyse and model the unsteady loading in order to design better turbines and reduce the cost of energy. To study the unsteady aerodynamics phenomena, an actuator disc model under unsteady loading was simulated using experimental and numerical methods. Experimentally, a porous disc rotor was constructed and tested in the Open Jet Facility, TU Delft. The design involved a novel method to dynamically control the rotor's thrust coefficient through its porosity. Numerically, a dynamically loaded actuator disc was implemented in the RANS equations and solved by the commercial CFD solver Star-CCM+. Results from the experiment, CFD simulations, as well as a free wake vortex ring model were used to benchmark the performance of dynamic inflow engineering models. Results show that the wake of a wind turbine under steady loading is also turbulent and meandering. The Strouhal number of the wake fluctuation closely agrees to that from a previous experiment of a 2-bladed wind turbine (Medici & Alfredsson, 2006). This suggests that the wakes of blade rotors are comparable to that from disc rotors. The tower shadow effect also bears a major influence on the inner wake profile which causes further velocity deficit and higher turbulent intensity. For rotors with higher CT , the velocity recovery process occurs relatively earlier. This is because of the enhanced momentum entrainment arising from a steeper velocity gradient at the shear layer. For the disc rotor under unsteady loading, the wake is affected by unsteady aerodynamics or the dynamic inflow effect. The transient velocity profile experiences an overshoot due to the passage of the 'old' and 'new' shed vortices, which are generated at the wake edge as a consequence of the uniform load profile on the rotor. The subsequent decay of the velocity in the wake or inflow at the rotor plane to the 'new' equilibrium state is a result of the rate of convection of the 'new' shed vortices to the far field. From this research, numerical results showed lower inflow velocity decay rates (velocity response due to the change in loading on the rotor) than that predicted by engineering models. While this is so, it was found that the decay rate is highly influenced by the ambient flow's turbulence. At high ambient turbulence, the 'new' equilibrium state is achieved in a shorter time due to the enhanced momentum mixing process. It is hypothesised that this is the primary reason behind the higher decay rates predicted by empirical-based engineering models.