Wind energy plays an increasingly important role in the transition towards a sustainable and low emission energy system. As wind turbines and wind farms continue to increase in size, which becomes comparable to atmospheric boundary layer height, the accurate prediction of wake effects becomes essential for both power production and structural loading. One phenomenon that strongly influences wake behavior is wind veer, defined as the variation of wind direction with height. Under veered inflow conditions, turbine wakes experience lateral displacement and profile skewness, affecting the performance of downstream turbines.

This thesis investigates the ability of engineering wake models to predict the lateral displacement of wakes under veered inflow conditions using Large Eddy Simulation (LES) data for comparison. The lateral wake displacement has been defined as the lateral position of the maximum velocity deficit over the vertical axis. Existing analytical models developed by Abkar, Qian, Narasimhan and Snaiki are evaluated and compared against the LES, while several improvements are examined, including alternative advection formulations, the use of downstream velocity fields, analytical integration techniques and ellipse fitting methods.

Finally, two different physics-guided neural networks are developed. The first estimates a correction factor for the Abkar model. While the second, predicts the parameters of a rotated Gaussian ellipse. The proposed methods significantly improve the prediction of lateral wake displacement and lead to substantially better agreement with both the training and the test (previously unseen) LES cases. These findings contribute to the development of more accurate engineering wake models for atmospheric flows with strong wind veer.
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Surface dimples are a well-established means of reducing drag on bluff bodies, most famously the golf ball, where tripping the boundary layer into turbulence delays the flow separation and narrows the wake, reportedly halving drag relative to a smooth sphere. Whether the same mechanism benefits a lifting surface is far less settled. An airfoil in attached flow has little separation to delay, so a dimple that helps a sphere can just as easily add drag to a wing. Existing studies on dimpled airfoils reflect this tension, reporting inconsistent benefits that depend strongly on dimple depth, size, and chordwise location, with no overwhelming consensus on where along the chord dimples should be placed. This thesis investigates how the controlled variation of dimple depth and chordwise placement affects the aerodynamic characteristics and flow-separation behaviour of a lifting surface across varying operating conditions, and develops a proof-of-concept prototype capable of mechanically varying dimple depth as a first step toward a fully adaptive surface.

A NACA 63-415 airfoil with chord c = 200 mm was tested with three chordwise dimple coverage families, mid (0.33c to 0.59c), aft (0.59c to 0.85c), and full (0.33c to 0.85c) suction-side coverage, each at three dimple depths, across three Reynolds numbers (1*10^5 , 2*10^5, and 5*10^5). Measurements spanned an angle-of-attack range of−10° to 20°, with 0° to 16° forming the most substantial part of the campaign. The campaign combined wake-rake and wall-pressure measurements, surface-pressure taps, a force balance, particle image velocimetry (PIV), and tuft flow visualisation across two wind-tunnel facilities, the Red Wind Tunnel at DTU and the Small Low-Turbulence Tunnel at TU Delft, supported by RANS and LES simulations.

The results show that dimple depth, chordwise location, and Reynolds number interact strongly, and no single fixed combination performs well across the full envelope tested. At the lowest Reynolds number, mid and full coverage produce substantial glide-ratio gains, up to 132% over the smooth baseline, driven primarily by drag reduction from delayed separation. At the highest Reynolds number, this reverses entirely. Mid coverage becomes uniformly detrimental, while aft coverage becomes the only layout to show any benefit, concentrated near stall. Dimple depth shows a similarly reversing, non-monotonic effect. The depth that gives the largest drag reduction at the lowest Reynolds number is the most harmful choice at the highest, and near stall this non-monotonicity appears even within a single Reynolds number, where a depth of 2.50 mm produces a genuine drag reduction while both a shallower (1.25 mm) and a deeper (5.00 mm) variant do not. PIV, surface-pressure, and wake-rake measurements on the standout mid configuration confirm the underlying mechanism directly. The dimples energise the near-wall boundary layer, suppress separation that is otherwise present on the smooth baseline, and narrow the wake to roughly half the width of its baseline counterpart, extending to a measurable stall-delay effect at a higher Reynolds number. RANS and LES reproduce the broad direction of these trends but not the mechanism itself in detail, consistent with the RANS model’s lack of a transition formulation.

These findings show that no single, fixed dimple geometry is optimal across a realistic operating envelope, motivating a surface that can adjust its own geometry in response to the flow condition. A proof-of-concept prototype was designed and built around a pneumatically actuated, depth-adjustable membrane, and structured-light scanning related each actuation setting to a physical dimple depth. Wind-tunnel testing at Re = 1*10^5 showed the prototype reproducing the drag behaviour of the static configurations in trend, with a peak drag reduction of approximately 22% relative to its own unactuated state, and reproducing the same suction-side wake narrowing at matched depth. The correspondence is qualitative rather than exact, and no lift benefit is observed, but a single airfoil is shown to select between aerodynamic states that individually resemble distinct static geometries, establishing the feasibility of an adaptive dimpled surface. ... Read more

Wind turbine fatigue assessments commonly represent load histories as independent 10-minute records, although slow atmospheric and operational changes can generate cycles that cross these boundaries. The resulting fatigue error and its dependence on structural response and material sensitivity remain insufficiently established.

This thesis determined how independent 10-minute processing affects fatigue estimates compared with longer continuous histories and whether the missing cycles can be recovered efficiently. Six months of load and operational measurements from a 7 MW offshore turbine were analysed for blade root flapwise and edgewise bending, and for two tower-bottom bending directions. Continuous rainflow counting, conventional independent-block processing and residue-aware low-frequency fatigue dynamics (LFFD) processing were compared across observation periods and Wöhler exponents. BHawC aeroelastic simulations compared 600 and 3600 s histories, continuous and segmented operating-state sequences, and concatenated short simulations.

LFFD reproduced the continuous reference while enabling analysis of the full six-month record. Independent 10-minute processing underestimated the flapwise damage equivalent load (DEL) by approximately 16% to 20%, and the tower bottom DELs by 3% to 20%, depending on the Wöhler exponent, while the edgewise difference remained below 0.2%. Daily residue aggregation recovered approximately 95%–96% of the full-record flapwise DEL and weekly aggregation about 99%. Cycles connecting different operating regimes contributed approximately 61% of the residue-aware flapwise fatigue contribution.

Extending stationary simulations from 600 to 3600 s increased the combined DEL by 3.62% for flapwise and 1.76% for fore-aft tower bending, whereas continuously processing an idling–startup–production–shutdown sequence increased them by 19.19% and 9.74%, respectively. Concatenated short simulations did not consistently reproduce directly simulated one-hour histories.

The results show that 10-minute storage remains practical, but processing those blocks independently can produce non-conservative estimates for aerodynamically driven responses. Residue-aware processing and targeted continuous transition simulations therefore provide more reliable fatigue estimates.
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The continued upscaling of offshore wind turbines toward the 20 MW class introduces significant aeroelastic and atmospheric challenges not fully captured by conventional structural models and simplified inflow assumptions. As rotor blades grow increasingly long and slender, torsional flexibility and bend-twist coupling become non-negligible factors in determining the aerodynamic and structural response of the rotor. This thesis investigates the influence of torsional degrees of freedom and atmospheric boundary layer stability on the aerodynamic performance and structural loading of the IEA 22 MW reference wind turbine, using OpenFAST with LES-generated turbulent inflow across below-rated, rated, and above-rated operating conditions. ElastoDyn overestimates rotor thrust by 7.0% at below-rated conditions compared to BeamDyn, driven by its rigid torsion assumption neglecting nose-down blade twist. The atmospheric stability analysis reveals that stable stratification produces the highest flapwise damage equivalent load at rated conditions despite having the lowest load variability, while unstable conditions produce approximately 10% higher torsional fatigue loading at above-rated conditions. These findings demonstrate that both structural modeling fidelity and atmospheric stability class have significant consequences for load and fatigue assessment of large offshore wind turbines. ... Read more

As the need to reduce the carbon footprint of the energy sector grows, wind energy has emerged as a leading renewable source for sustainable power generation. In particular, the offshore sector has gained significant attention due to its higher wind speeds as well as its reduced noise impact. TU Delft's Faculty of Aerospace Engineering has identified accelerating the energy transition towards renewable energy sources as one of its main goals. To achieve this, offshore wind turbines must be tested under realistic Atmospheric Boundary Layer (ABL) conditions.

Wind-tunnel ABL simulations typically use spires, barriers and roughness elements as passive devices. However, this configuration requires a long downstream length for the boundary layer to develop. Therefore, an alternative approach developed by Cowdrey (1967) is used, which employs a non-uniform rod grid to produce a prescribed power-law mean velocity profile. Moreover, unlike spire-based configurations, this method allows for variations in the turbulence levels of the resulting boundary layer through the grid design parameters.

This thesis investigated the capability of Cowdrey's method to reproduce an offshore ABL in a short open-jet test section. Particle Image Velocimetry and Hot-Wire Anemometry were used to measure the mean velocity profile, turbulence intensity, integral length scales and power spectral density. Cowdrey grids were evaluated over a test matrix with four rod diameters, two cross-sectional shapes, two turbulence design settings, two target boundary layer thicknesses at three free-stream velocities.

Within the tested velocity range, the use of Cowdrey grids increased the boundary layer height by up to 26 times compared with the naturally developing layer within the same fetch length. The fitted power-law exponents for most configurations were slightly larger than the design value but remained close to the target value overall. Moreover, most grids exhibited a good agreement with the logarithmic law in the inner layer and within the available downstream distance, attained the characteristic aerodynamic roughness length of an offshore Atmospheric Boundary Layer. The measured turbulence intensities exhibited elevated near-wall levels relative to semi-empirical relations but qualitatively followed the predicted offshore ABL trends. The power spectral density conformed to the von Kármán model across all sampled heights, with noticeable departures only near the top of the outer layer at the furthest downstream station. Lastly, the longitudinal integral length scales tended to be consistently lower than predicted by semi-empirical relations.

The study provided valuable insights into the applicability of Cowdrey’s method in reproducing a scaled offshore ABL. It laid the foundations for further research regarding ABL testing in TU Delft's experimental facilities. Further improvements should focus on bringing the fitted power law closer to the target value and refining the turbulence characteristics to minimize deviations from accepted standards. ... Read more

In a world with an urgent demand for sustainable energy, the wind energy industry plays a key role in accelerating this transition. Over the past decades, wind turbines have evolved from expensive, relatively inefficient machines into increasingly cost-effective and highly efficient technologies, supported and promoted by many countries worldwide. However, this rapid progress has come at a cost: larger and more efficient turbines require substantial investments in raw materials, research and development, component standardization, and supply chain optimization.

In recent years, the curtailment of wind power in Europe has increased, largely due to insufficient grid capacity and limited energy storage. As a result, wind turbines are more frequently operated in parked conditions, with their rotors brought to a standstill. Under these circumstances, one of the key challenges in scaling up turbine size is the risk of vortex-induced vibrations (VIV) in the blades. In parked conditions, the blades are often pitched to very high angles of attack (close to 90°) to cut out of the wind. If the vortex shedding frequency approaches the blade’s natural frequency, a lock-in phenomenon may occur, leading to strong vibrations. This vibration in the long term can contribute to the overall fatigue load of the wind turbine and reduce the structural life.

Although increasing attention has been given to VIV in wind turbine blades, significant gaps remain in understanding the fundamental flow physics that govern these vibrations, specifically the unsteady aerodynamics of airfoils at high angles of attack. This dissertation therefore investigates the unsteady aerodynamics of both static and oscillating airfoils under such conditions, with the aim of building a detailed physical understanding of VIV from an aerodynamic perspective.

The research was carried out through a series of wind tunnel measurements. First, a campaign on a static airfoil examined unsteady aerodynamics across a wide range of angles of attack (up to 310°). Aerodynamic forces, vortex shedding patterns, and shedding frequencies were compared between forward flow (leading edge upwind) and reverse flow (trailing edge upwind) conditions. Although reverse flow is uncommon in normal operation, it can occur during parked or installation phases; the insights gained in this research therefore form a critical foundation for subsequent studies on oscillating airfoils.

The main focus of the dissertation is the unsteady aerodynamics of oscillating airfoils, studied using the forced motion method to mimic VIV. Three motion types, namely surging, plunging, and pitching, were investigated. Particle Image Velocimetry (PIV) was employed to capture the flow fields, while surface pressure measurements provided aerodynamic forces. By correlating vortex dynamics with force responses, the study reveals how the mean angle of attack and motion parameters (such as frequency and amplitude) influence the overall unsteady aerodynamics of the airfoil and how lock-in is triggered under different motion kinematics. Comparisons between forward and reverse flow conditions further enrich the findings, where the reverse flow dynamic stall was thoroughly discussed—from vortex dynamics and aerodynamic forces to a newly proposed dynamic stall vortex and trailing edge vortex onset determination method.

Overall, the comprehensive experimental dataset and resulting conclusions advance the fundamental understanding of unsteady airfoil aerodynamics at large angles of attack. These findings not only clarify the underlying mechanisms causing VIV from the perspective of vortex dynamics and aerodynamic forces, but also provide a valuable basis for future aeroelastic VIV studies and the development of engineering models. ... Read more

Over recent decades, the wind energy sector has undergone rapid expansion, driven by escalating global energy demand and the widespread transition toward sustainable power generation. This development has been accompanied by a substantial increase in wind turbine rotor size. However, scaling up turbine dimensions introduces significant engineering challenges, particularly in achieving high aerodynamic performance while maintaining cost effective structures. The resulting trend toward longer and more slender blades has amplified both static and dynamic loading, making structural integrity a central concern in contemporary turbine design. To address these challenges, modern rotor concepts incorporate thicker inboard and mid-span airfoils, including flatback configurations, which improve structural efficiency and aerodynamic performance while reducing overall blade mass. At the same time, enlarging blade radii leads to greater elastic deformations, which further increase the dynamic loads experienced by the rotor.

Ensuring reliable turbine operation requires accurate load assessment, which depends heavily on aeroelastic and dynamic load simulations across a broad range of operating conditions. As blades become larger and more flexible, they are subjected to stronger inflow turbulence, more pronounced unsteady variations in angle of attack, and an increased likelihood of transient stall phenomena. These factors make accurate unsteady aerodynamic modeling essential. In this context, dynamic stall models are frequently employed to predict unsteady aerodynamic forces. Historically, however, most dynamic stall models were developed for thin airfoils in helicopter applications, and despite subsequent adaptations for wind turbine use, their suitability for thick airfoils remains limited. The aerodynamic response of thick airfoils, particularly those typical of large modern blades, still lacks comprehensive validation, resulting in uncertainties in load predictions. Moreover, thick mid‑span airfoils experience large unsteady angles of attack and are more susceptible to early stall, necessitating active or passive flow‑control strategies. Flatback airfoils and vortex generators have emerged as promising solutions, offering increased lift, delayed flow separation, and improved stall characteristics. Nevertheless, their combined impact on unsteady aerodynamics is not yet fully understood, revealing a critical gap in current aeroelastic modeling capabilities.

This knowledge gap compromises the reliability of current aeroelastic models and poses risks for the design, optimization, and structural assessment of large modern turbines. Consequently, this dissertation is motivated by the need to advance the understanding and modeling of dynamic stall phenomena in thick wind turbine airfoils, with particular emphasis on the combined effects of vortex generators under unsteady flow conditions. By addressing these gaps, the work aims to contribute improved physical insight and enhanced predictive capability for the aerodynamic behavior of next generation wind turbine blades.

Therefore, several wind tunnel campaigns were conducted to investigate the unsteady aerodynamic characteristics of two $35\%$ thick airfoil configurations, comprising a conventional airfoil and a flatback airfoil, tested in different setups in Low-speed, low-Turbulence wind Tunnel (LTT) of the Delft University of Technology.

The results show that flatback airfoils exhibit inherently more stable hysteresis loops, as evidenced by reduced force fluctuations, larger lift overshoots, and delayed separation compared to sharp trailing edge airfoils, primarily due to their more favorable suction side pressure gradients under the studied operating conditions. Dynamic stall on these thick airfoils is governed not by leading edge vortex formation, as seen in thin airfoils, but by a gradual trailing edge separation mechanism that produces moderated load transients. Reduced frequency strongly influences dynamic stall, i.e, higher frequencies delay separation, enhance lift overshoot, and stabilize flow evolution for both airfoils. The addition of vortex generators substantially improves flow attachment during the upstroke phase, increases lift overshoot, and suppresses force fluctuations, particularly when placed near the leading edge. Their effectiveness, however, depends sensitively on chordwise placement, the underlying airfoil geometry, and the oscillation regime. While flatback airfoils generally benefit from improved stall delay, they experience larger force excursions during fully separated flow, indicating both the potential and the risks of employing blunt trailing edge designs. The evaluation of the Beddoes–Leishman and Risø dynamic stall models demonstrates that both models are not capable to predict separated flow behavior on thick airfoils, especially regarding lift overshoot, lift drop, post-stall loads, drag, and pitching moment. Lack of modeling accuracy lead to systematic discrepancies in model predictions, even when tuning time constants. These findings highlight the need for new or revised separation and reattachment modules tailored specifically for thick airfoils for reliable load prediction in the inboard and mid-span regions of large modern wind turbine blades. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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.
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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. ... Read more

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. ... Read more

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. ... Read more

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.
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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.
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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. ... Read more

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. ... Read more