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

The accelerating demand for renewable energy has intensified the search for efficient and innovative solutions, particularly within the offshore wind-energy sector. While horizontal-axis wind turbines currently dominate the market, vertical-axis wind turbines (VAWTs) have emerged as a promising alternative — especially for offshore and urban environments where compactness, reduced noise, and improved windfarm power density are crucial. Nevertheless, despite their potential, the large-scale deployment of VAWTs remains constrained by an incomplete understanding of their aerodynamic performance, wake dynamics, and the downstream effects of these wakes within wind farms.
Beyond conventional VAWT designs, novel turbine concepts are being developed to address the limitations of the traditional Darrieus rotor. One such concept, the X-Rotor, seeks to reduce the Levelised Cost of Energy (LCoE) and enhance the competitiveness of VAWTs in the offshore market. However, its geometry inherently induces strong three-dimensional flow features that cannot be adequately captured using the commonly employed, computationally economical, two-dimensional numerical tools..... ... Read more

The development of offshore wind farms has demonstrated significant value in harnessing renewable energy sources. However, the offshore wind energy sector faces increasing challenges, making the realization of new wind farms a greater financial risk. This has reached the point where several high-profile offshore wind projects have been halted. A key contributor to these struggles is the aerodynamic effect of wind farm wake losses. As wind turbine sizes increase, costs rise, but the wind farm concept limits energy yield. This is due to slow wake recovery behind turbines, causing downstream turbines to only be able to harvest energy from low-momentum flow. Based on the literature review conducted, the methods to improve wind farm efficiency by reducing wake losses, such as wind farm layout optimization and rotor yawing or tilting, only partially address the problem.

The concept of regenerative wind farming, proposed by Ferreira, seeks to provide a solution. This involves integrating lifting devices into wind energy harvesting systems that can redirect the wake vertically, enabling the flow from higher atmospheric layers to contribute more to the wake recovery process. Such interactions can dramatically enhance the energy that is replenished into the wind farm, offering a promising approach to overcoming these challenges.

This research aims to evaluate the potential of the novel wind farm concept, regenerative wind farm, through a scaled wind farm experiment. In the scaled wind farm, there are nine wind energy harvesting systems, which are aerodynamically modeled by using porous disks and wings. The study focuses on the far wake and the performance of downstream turbines. The experiment was conducted in the Open Jet Facility (OJF) at TU Delft. The flow field was measured using Particle Tracking Velocimetry (PTV) with Helium-Filled Soap Bubbles (HFSB), and load measurements were accompanied.

Load measurements revealed that thrust values for downstream turbines increased by more than three times when lifting devices were attached to the actuator surfaces. Also, flow field data showed significantly higher wake velocities, as potent vertical flows in the wake regions were induced by the tip-vortices of the wings, which enhanced vertical energy entrainment. This vertical motion actively entrains the flow above the wind farm, allowing high-momentum air to enter the wind farm layer, something that does not occur without the wings. Additionally, the lifting devices reduce turbulence intensity in the rotor projection area at the downstream end of the wind farm, helping to lower the fatigue loading of downstream systems. An investigation of a misaligned row confirmed that the concept remains effective even under such conditions.

This work demonstrates that the regenerative wind farm concept holds great potential to enhance wind farm power output while reducing the required wind farm area. With this concept, wind turbine wake losses are effectively mitigated by entraining high-momentum flow into the wind farm. ... Read more

Offshore wind energy has gained prominence due to its favorable wind resources, turbine scaling potential, and vast installation areas. However, wake losses caused by closely spaced wind turbines remain a significant challenge, reducing overall wind farm efficiency. This thesis explores the application of vertical-axis wind turbines (VAWTs) geometries for maximizing energy density through passive wake recovery techniques. In addition to traditional Darrieus-type VAWTs, the efficacy of passive wake control techniques for novel multi-rotor systems (MRS), namely the X-Rotor and block design concepts, is assessed through experimental studies and proof-of-concept demonstrations.

The thesis begins with a large-scale experimental investigation into the wake dynamics of a high-energy-density VAWT wind farm, providing the first comprehensive dataset of three-dimensional, time-averaged flowfield measurements. A dense grid of nine Htype VAWTs with fixed spacing was analyzed, exploring a passive wake control strategy known as the "vortex generator" mode, where blade pitch is modified to accelerate wake re-energization. Two pitch configurations were tested: positive (pitched-in) and negative (pitched-out). The positive pitch case exhibited significant momentum influx from above and below the rotor, along with lateral wake deflection. In contrast, the negative pitch case induced upwash while injecting high-momentum flow from the sides. Wake recovery was quantified by assessing available power, showing a maximum of 72.4% recovery three diameters downstream in the positive pitch case, 6.4 times higher than the baseline. The negative pitch case reached a 53% recovery four diameters downstream, a 2.1-fold improvement over the baseline. These findings highlight the potential of passive wake control strategies to enhance wind farm energy density.

The X-Rotor introduces an innovative design featuring an X-shaped VAWT, referred to as the "primary rotor," and blade-tip-mounted HAWTs, known as the "secondary rotors." This design employs an "aerodynamic gearbox" mechanism, where the primary rotor extracts mechanical power while the secondary rotors drive electrical generators at the blade tips. This thesis presents the first experimental wake measurements of the X-Rotor, revealing that its wake remains concentrated within its projected frontal area, shaped by the coned blades. The shed vorticity follows an elliptical pattern, inducing crossflow components along the height. This dataset provides a baseline for evaluating the secondary rotors’ impact on wake evolution near the bottom blade tips and demonstrating the aerodynamic gearbox mechanism. ... Read more

Future regional and short to medium-range aircraft are expected to use propellers for sustainable aviation, possibly with (hybrid-)electric propulsion systems. This will enable innovative integration of aero-propulsive systems, potentially increasing the overall aerodynamic efficiency of the airframe and propellers, resulting in a lighter airframe and increased passenger comfort. The presence of electric motors and propellers in such configurations, in addition, offers a unique opportunity to leverage propellers as airbrakes by operating them at negative thrust. This approach offers multiple potential advantages, including shorter landing runs, enhanced landing maneuverability, energy harvesting during braking operations, faster aircraft turnaround times, and reduced community noise exposure.

Despite these potential advantages, the aerodynamic and aeroacoustic characteristics of propellers operating in negative thrust mode remain largely unexplored. This dissertation addresses this knowledge gap through computational analysis, comparing the performance of isolated propeller configurations in negative thrust mode with the well-understood positive thrust mode. The results reveal that the distinct aeroacoustic characteristics of propellers operating in negative thrust conditions, compared to conventional positive thrust conditions, offer a promising avenue for reducing community noise, not only through the possibility of steeper descents but also through changes in the noise emissions from the propeller itself.
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The rapid advancement of science and technology is driven by the goal of improving life on Earth while maintaining environmental responsibility. This progress is evident across numerous fields. In wind energy, the continuous development of new wind turbine designs and innovative wind farm configurations is crucial for accelerating the energy transition. The aerospace industry constantly introduces advanced airplane and helicopter designs to improve performance and efficiency. Urban planning increasingly relies on detailed analysis to optimize airflow within cities, addressing environmental and health concerns. These are just a few examples where innovation is driven by the need to enhance performance and sustainability.

A common requirement across these diverse fields is the need for an in-depth understanding of aerodynamics. Accurate aerodynamic analysis is essential for enhancing design, efficiency, and effectiveness. However, the fast-paced nature of modern advancements limits reliance on experimental methods alone, as these can be time-consuming, costly, and impractical for all possible configurations. Consequently, combining experimental and computational studies becomes crucial.

This is where Computational Fluid Dynamics (CFD) comes into play. The development of efficient and accurate CFD tools has become essential in scientists' and engineers' hands to explore aerodynamics quickly and understand the physics of the flows. This fact has driven the present research. The primary goal of this dissertation is the development of a computational tool that is both accurate and efficient for exploring external aerodynamics simulations.

The main approaches in CFD today are the Eulerian and Lagrangian approaches, each comprising a family of methods. Eulerian methods, like the Finite Volume Method (FVM) and Finite Element Method (FEM), have been extensively used in exploring external aerodynamics, with their greatest advantage being accuracy in capturing the boundary layers. However, due to the diffusive nature of these methods, artificial diffusion is introduced into the flow, damping the vortex structures, which are crucial in many applications driven by strong body-vortex interactions. Moreover, the study of multibody objects often requires special treatments, especially for mesh generation, making the simulations extremely costly.

On the other hand, Lagrangian methods, like the Vortex Particle Method (VPM), are excellent for studying flows with a high presence of vortices, as they can preserve the vortex structures without damping them. Additionally, the particles participating in the flow are self-adaptive, satisfy the far-field boundary conditions automatically, and allow for easy implementation of multiple bodies into the simulation. However, resolving the boundary layer is very challenging and often very costly due to the inability to use anisotropic elements, making them less ideal for predicting aerodynamic forces.

Lagrangian solvers have become very popular in the last two to three decades, primarily due to the significant advancements in computer hardware, especially GPUs, which enable very fast calculations. This has encouraged engineers to explore ways to leverage this advantage in CFD, leading to the development of hybrid solvers that couple Eulerian and Lagrangian solvers. In this coupled approach, Eulerian solvers can be applied near the solid body to accurately and efficiently resolve the boundary layer region, while Lagrangian solvers preserve the vortex structures further from the body.

Building on this approach, this dissertation introduces a hybrid Eulerian-Lagrangian solver, named VPMFoam, developed to combine the strengths of both methods while minimizing their limitations. This is achieved by integrating OpenFOAM, a widely used open-source CFD software, with a Lagrangian VPM. The primary goal is to create an accurate tool focused on external aerodynamics that can efficiently handle cases with strong body-vortex interactions and multibody scenarios while maintaining a lower computational cost compared to pure Eulerian solvers. OpenFOAM was chosen primarily for its open-source flexibility and its extensive user base in academia and industry, offering broad access to this tool.

The solver's development focuses on a 2D version, with validation conducted through a step-by-step approach, starting with simple cases that exclude solid bodies. Once the successful coupling of the two solvers is verified, validation proceeds with cases involving solid bodies, such as flow around a cylinder. In these cases, the solver accurately predicts fluid flow and aerodynamic coefficients, showing strong agreement with established Eulerian solvers and effectively preserving the vorticity field in the wake. Further validations address dynamic mesh motions and multibody applications.

Following validation, the solver is applied to more realistic scenarios, including the static and dynamic stall of an airfoil and the simulation of hybrid Vertical Axis Wind Turbines using actuator models. A performance analysis of the code’s efficiency is also presented, highlighting the solver’s capability to conduct fast and accurate simulations.

By effectively combining the strengths of both Eulerian and Lagrangian approaches, VPMFoam addresses key challenges in aerodynamic analysis, particularly in cases with strong body-vortex interactions, and lays a strong foundation for further applications, including potential 3D extensions. This makes VPMFoam a valuable asset for applications requiring both high fidelity and computational efficiency. ... Read more

This thesis aims to study the effects of enhanced flow entrainment in a VAWT wind farm. This is achieved by advection of low momentum wake into the free-stream flow via lateral forces, and these forces are introduced by pitching the rotor blades.
For this study, scaled down VAWT models have been designed and setup in a wind farm layout. This wind farm model has been tested in the Open Jet Facility (OJF) of the Delft University of Technology to gather insight into the farm's flow behaviour. The wake is acquired using Tomographic Particle Tracking Velocimetry (PTV) measurement technique, where the flow is measured by acquiring images of Helium Filled Soap Bubbles (HFSB), suspended in the flow-stream. The performance of individual turbines is characterized by taking the load measurements, and comparing the thrust coefficients.
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Wind turbines play a crucial role in the worldwide effort to embrace sustainable energy, utilizing sophisticated aerodynamic principles to efficiently capture wind energy. A thorough comprehension of deep dynamic stall, a phenomenon that greatly impacts wind turbine performance, is essential for maximizing efficiency, maintaining structural integrity, and propelling the development of wind energy. This study presents an experimental exploration of deep dynamic stall phenomena through wind tunnel experiments carried out on a NACA643418 airfoil at TU Delft.

The study commences with the development of a comprehensive test matrix drawing from existing literature, with a focus on angles of attack of 40, 50, and 90 degrees. Through precise experimentation, the research team meticulously measures and corrects for wind tunnel effects, uncovering crucial trends in lift and drag coefficients. Significantly, the study identifies laminar separation bubbles and trailing edge separation as the main stall mode before the deep stall regime.

The analysis of static and dynamic pressure data offers valuable insights into the aerodynamic characteristics of the airfoil in deep stall conditions. Notably, significant variations in aerodynamic performance between the upstroke and downstroke are evident, particularly at high angles of attack surpassing 25 degrees. The data underscores the intricate interplay of pitching frequency, amplitude, and airflow separation, highlighting the pivotal influence of shedding frequency on dynamic stall phenomena.

Moreover, measurements of dynamic pressure reveal the intricate relationship between the frequency and amplitude of pitching and the patterns of vortex shedding. The research shows that vortex shedding is most significant at angles of attack close to 90 degrees, even during dynamic pitching. These discoveries emphasize the significance of comprehending deep dynamic stall to enhance the design and performance of wind turbines in various operational conditions.

The phase-averaged PIV images serve as a valuable complement to the pressure data, offering a visual confirmation of how flow dynamics impact aerodynamic performance in deep stall conditions. These images unveil clear disparities in airflow behaviour during upstroke and downstroke motions, shedding further light on the aerodynamic obstacles encountered by wind turbine blades during dynamic operation. ... Read more

In response to the urgent need for renewable energy amidst the escalating impacts of global warming, this study delves into the forefront of wind energy research, particularly focusing on the innovative Multi-Rotor System (MRS). The MRS concept offers a promising departure from the prevailing trend of scaling up traditional turbines, instead proposing a configuration comprising multiple smaller rotors mounted on a single support frame. This approach, identified for its inherent upscaling advantages, presents an opportunity to reduce costs and weight relative to conventional Horizontal Axis Wind Turbines (HAWTs). While the concept is not entirely novel, its application with Vertical Axis Wind Turbines (VAWTs) is. Recent studies have demonstrated the significant potential of the VAWT-based MRS, showcasing comparable power performance to large HAWTs while enhancing Operations and Maintenance (O&M) aspects. Furthermore, the adaptable support frames of the MRS facilitate the integration of wake control devices, such as external lift-generating wings, which introduce cross-flow loading to rapidly re-energize the wake. However, despite these advancements, understanding the wake dynamics within MRS systems, particularly concerning their implications for wind farm applications, remains limited.

The current work aims to investigate the intricate near-wake dynamics of the VAWT-based MRS and study the effects of external lift-generating wings on the deflection and recovery of the wake. To do so, a scaled wind tunnel model of such a VAWT-based system has been designed together with a set of removable high-lift wings. These, in turn, have been tested in the Open Jet Facility of the Delft University of Technology to gather insight into the behaviour of the near-wake. Tomographic Particle Image Velocimetry using Helium Filled Soap Bubbles has been deployed in combination with load measurements to gather data.

Load measurements revealed that the thrust coefficient behaviour of the MRS closely resembles that of single-rotor VAWTs, with an additional thrust induced when the external lift-generating devices are present, attributed to accelerated flow on the suction side of the wings. Furthermore, the PTV measurements have provided a detailed visualization of the near-wake, showcasing symmetric wake structures and lateral deflection induced by the presence of individual rotors. The introduction of external lift-generating wings significantly altered wake behaviour, inducing lateral contraction and promoting streamwise momentum recovery through enhanced vertical advection. Furthermore, analysis of velocity deficit recovery highlighted substantial improvements in power recovery behind the MRS with external wings.

The findings presented in this work underscore the potential of the VAWT-based MRS, particularly when such a system is equipped with lift-generating devices. The presence of such devices effectively manipulates the near-wake of the turbine, enhancing wind farm efficiency, and thereby advancing innovative wind energy solutions. ... Read more

Vertical axis wind turbines (VAWTs) are popular design solutions for omnidirectional wind and limited installation area, particularly in urban areas, due to their advantages of low noise, low energy cost, and lower sensitivity to turbulence. They are often found in either of two distinct designs: the lift-driven Darrieus, with high aerodynamic power, and the drag-lift-driven Savonius, which excels in its start-up performance. The Darrieus- Savonius combined vertical axis wind turbine (hybrid VAWT) emerges from combining the advantages of Darrieus and Savonius. However, the hybrid VAWT has complex fluid dynamics, with multiple scales and interactions. Therefore, a comprehensive analysis and modeling of hybrid VAWTs can benefit from a multi-fidelity approach.
The choice of numerical methods, in either Eulerian or Lagrangian reference frames, holds paramount importance in simulating VAWTs, each method offering distinct ad- vantages and limitations. Through high-fidelity Eulerian unsteady Reynolds averaged Navier Stokes (URANS) simulations, insights into airflow patterns, and turbulence phe- nomena across varied operating conditions are gained. Conversely, Lagrangian models such as the vortex particle method (VPM) can enable efficient analyses of vortical struc- tures and wake interactions. While URANS simulation offers high-fidelity representa- tions of complex flow phenomena and allows for precise optimization of turbine design parameters, it demands significant computational resources and expertise. In contrast, VPM excels in capturing flow features efficiently but may struggle to accurately repre- sent boundary layer effects and near-wall flows. Consequently, the integration of both the Eulerian URANS and the Lagrangian VPM (hybrid method) is crucial for achieving comprehensive, reliable, and cost-effective simulations of VAWT performance. In this thesis, the hybrid VAWT is investigated using multi-fidelity numerical tools to estimate rotor/blade aerodynamics, computational efficiency and accuracy. Eulerian (U)RANS simulations in OpenFOAM and Lagrangian VPM are first applied to different types of VAWTs, followed by the application of the hybrid method in the hybrid VAWT case.
The goal of this thesis is also to investigate the flow features and power performance of the hybrid VAWT with multi-fidelity methods. In the context of the Eulerian reference frame, this thesis advances the knowledge of hybrid VAWT aerodynamics in several as- pects. The Darrieus and Savonius parts in a hybrid VAWT are modeled as uniform force fields to exclude the effects of structural and operational parameters on the power losses of the wind turbines. The results show that the hybrid configuration cannot show a sig- nificant power increase, and it is only beneficial for the startup performance. The vor- tex dynamics behind the hybrid VAWT are analyzed in different attachment angles and tip speed ratios. The blade-vortex interaction is characterized and correlated with the torque generation of the Darrieus blade. Results show that the Darrieus blade torque in- crease is dependent on the interaction with the shed vortex from the advanced Savonius blade.
In the context of Lagrangian reference frame, both Savonius and hybrid VAWT con- ix
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figurations are employed to assess the computational efficiency and accuracy of the vor- tex particle method. In the case of Savonius rotor simulations using the vortex method, the Savonius is defined as a rotor with two trailing edges because it has no clear lead- ing/trailing edge like an airfoil, named double-trailing-edge-wake-modeling vortex par- ticle method (DTVPM). Results show that a maximum power coefficient is achieved at a tip speed ratio of approximately 0.8, consistent with experimental findings. Further- more, the process of trailing-edge vortex generation and detachment is effectively cap- tured. A comparative analysis between Eulerian URANS simulations and Lagrangian DTVPM reveals that DTVPM offers a more efficient simulation of Savonius rotors with- out the need for empirical parameters. Notably, DTVPM demonstrates remarkable com- putational speed, with simulations being approximately 20 to 104 times faster than par- allel URANS simulations over five revolutions. This significant reduction in computa- tional time underscores the potential of DTVPM to enhance existing engineering mod- els for wind energy applications. In the case of hybrid VAWT simulations, this thesis extends the application of VPM to hybrid VAWTs and introduces a viscous correction to improve simulation accuracy. By incorporating the airfoil polar, the proposed La- grangian DTVPM effectively predicts comparable force variations to Eulerian URANS simulations. Importantly, the computational efficiency of URANS and DTVPM for hy- brid VAWTs is compared, revealing that serial DTVPM simulations are approximately 20 times faster than parallel URANS simulations over ten revolutions. This notable increase in computational speed highlights the potential of DTVPM to provide efficient and ac- curate simulations for hybrid VAWTs, facilitating further advancements in wind energy technology.
In the context of the hybrid Eulerian-Lagrangian reference frame, this study aims to enhance accuracy and efficiency in analyzing complex flow phenomena. Through con- ducting various scenarios of hybrid VAWT using the hybrid method, this study concludes with the demonstration of the hybrid solver’s capability in simulating hybrid VAWT aero- dynamics. The final goal of this thesis is to ascertain an efficient model for hybrid VAWT and determine the limits of individual Eulerian and Lagrangian methods while consid- ering specific flow features of VAWTs. Overall, this study contributes to comprehensive insights into the correlation of blade-vortex interaction and torque variation. The mod- eling challenge in the hybrid VAWT simulation is studied and a hybrid model is suggested to understand the performance and flow features of hybrid VAWTs. ... Read more

In the transition from fossil fuels to renewable energy sources, the advancement and deployment of wind turbine technology plays a crucial role. A notable trend in wind turbine design is the ever-growing rotor size, which entails that wind turbine blades have become very slender and flexible structures.

This increasing flexibility offers an opportunity for tailoring the aeroelastic behaviour of wind turbine blades. One such aeroelastic tailoring technique is blade sweep, defined as a displacement of the blade axis in the rotor plane. Blade sweep couples bending and torsion deformations and can thus be used to passively alleviate loads on the blade.

For the design, optimisation, and certification of wind turbine blades, blade element momentum theory (BEM) remains the aerodynamic simulation method most relied upon. However, BEM-based numerical tools inherently assume a straight blade geometry and, hence, cannot accurately model the additional flow complexities introduced by blade sweep.

This dissertation starts by presenting a newly developed BEM correction model for swept blades. The focus is on accurately modelling the azimuthal displacement of trailed vorticity and the curved bound vortex self-induction while maintaining BEM's streamtube-independent approach and rapid calculation speed. The developed model shows good agreement with mid-fidelity modelling (lifting line simulations), which intrinsically can model the two aforementioned effects of blade sweep.

To validate the BEM correction model for swept blades beyond the comparison with lifting line simulations, two wind tunnel campaigns are conducted, one with straight blades, being thrust-scaled versions of the IEA 15 MW reference wind turbine blades, and one with swept blades. While the former is intended to provide a baseline for the accuracy of numerical modelling, the latter then provides means to assess the impact of blade sweep and how this is captured in low-fidelity numerical simulations. The validation is conducted based on blade-aerodynamic quantities derived from flow fields measured using particle image velocimetry (PIV). It is demonstrated that the application of the BEM correction model improves the match with the experimental data compared to simulations without the correction model being applied.

Furthermore, this dissertation covers three diverse research efforts conducted within the framework of the TIADE project, a field experiment on a full-scale wind turbine. Firstly, a robust approach to optimise the spacing of pressure sensors for aerodynamic measurements on wind turbine airfoils is presented. The approach considers the expected turbine operating conditions and improves the lift prediction accuracy compared to a simpler, cosine sensor spacing over a wide range of angles of attack. Given that two fundamentally different optimisation routines arrive at close-to-identical solutions, it can be concluded that an optimal solution exists for placing pressure sensors around an airfoil to conduct aerodynamic measurements.

Secondly, pressure measurements obtained on the TIADE research wind turbine over multiple months are employed to validate aeroelastic simulations. The validation is performed based on both ten-minute average data and time-resolved data and using both the integrated sectional forces and the underlying pressure distributions. Generally, a reasonably good agreement between simulated and measured data is found. This indicates that BEM-based aeroelastic algorithms are still valid tools to simulate modern, multi-megawatt wind turbines and their slender and flexible blades.

Finally, a design study of a blade with swept tip for the TIADE field experiment and thus under realistic geometric and load restrictions is conducted. Simulations suggest that flapwise fatigue and extreme blade root loads can be reduced. The same holds for the fore-aft and yawing moments at the turbine tower base. Simultaneously, the turbine performance in terms of power output remains unaffected. These results highlight the potential benefits of blade sweep as an alternative tip geometry for modular blades or as a conscious design choice for future generations of blades.

In conclusion, this dissertation contributes to a more accurate understanding and numerical modelling of swept blade aerodynamics. By moving from fundamental analyses all the way to more applied investigations of swept blade tips for a field experiment, the presented research helps pave the way towards swept blades being a valid option in future wind turbine designs. ... Read more

The increasing global electricity consumption requires a corresponding increase in energy generation. Wind turbines generate electricity without depleting natural resources or emitting greenhouse gases, offering a straightforward solution to the energy dilemma. Wind power, because of recent technology breakthroughs, is a financially viable, clean, and quickly expanding source of energy. Traditional horizontal axis wind turbines (HAWTs) have been favored over the years due to a lack of research and development on vertical axis wind turbines (VAWTs). However, as wind turbines are deployed further offshore, conventional HAWTs begin to make less sense for floating platforms, as their high center of gravity can cause major tilting concerns.

VAWTs can be considered instead, as the rotor design can provide numerous possibilities to make them more commercially competitive than HAWTs. There is currently a lack of understanding of how specific design parameters influence the rotor configuration of modern VAWTs. This thesis attempts to identify design drivers for the VAWT design while accounting for their aerodynamic behavior and aeroelastic stability.

The VAWT is modeled in an aeroelastic analysis tool named HAWC2. First, the cross-sectional parameters of the blades and struts are determined using the BECAS software, which can be used as an input file for HAWC2. The original model is based on the reference model of Schelbergen, which is then verified. After that, several design factors are found and investigated to see how they affect the performance and behavior of the wind turbine. These design parameters include elements, such as the thickness of the laminate, the placement of the struts, and an additional diagonal strut. The study carries out a parametric analysis, to understand what can benefit an optimal VAWT design.

The verification phase shows that the reference model has significantly lower power output, most likely due to implementing a dynamic stall model in the aeroelastic analysis. The significance of proper modeling of the dynamic stall effects is highlighted. As with the reference model, a range of laminate thicknesses is provided, therefore, multiple models with different laminate thicknesses are examined. The assessment reveals that opting for thinner laminates can lead to reduced mass and expenses without significant power output compromise but with less structural integrity. Moreover, the optimal placement of the struts for increasing power output while preserving structural integrity can be identified through strut placement analysis. Besides, an additional diagonal strut shows enhanced structural stability but could be more expensive. The upscaling of the model is done by increasing the aspect ratio (with the blade length). The power generation is increased, but it is crucial to pay attention to the structural integrity and aeroelastic stability, as the deflection of the blades increases significantly. The study also looks into the Huisman VAWT design, which is compared to the company's own findings. The design is simplified but still shows good power output and blade behavior.

In short, this study examines the design drivers of the VAWT design, paving the way for advancing VAWTs in practical applications and future research. ... Read more

The thesis presents an analysis of unsteady vortex shedding and vortex induced vibrations (VIV) on wind turbine blades using a combination of Stereoscopic Particle Image Velocimetry (SPIV) and Computational Fluid Dynamics (CFD). A spanwise uniform blade with a symmetric NACA0021 airfoil section, 0.075 m chord, and 0.4 m span was tested experimentally for a static, plunging, and surging blade oscillating at 90 deg angle of attack, 5 Hz, and 2.5 Hz frequency, 1 chord oscillation amplitude. Equivalent CFD simulation cases were run to compare the results and validate the CFD setup, which was used to analyse additional cases of interest. The extent of the three-dimensional flow behaviour due to the tip vortex was described, along with determining the lock-in region for the two motions using additional simulations. The results indicated that the spanwise convection of the tip vortex and the transition to a ‘2D’ flow were only dependent on the blade aspect ratio and independent of the freestream wind speed, the type of motion, and the oscillation frequency. The lock-in region for the plunging and surging blades was presented, noting that all the surging cases were locked in, most likely due to the large oscillation amplitude. However, the surging cases had a positive aerodynamic damping and hence, VIV would not be excited for a freely vibrating blade. It is suggested as future work to test a flexible blade and study the onset and development of VIV. ... 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 final report from Delft University of Technology outlines the TurbEye project, focusing on the development of an autonomous drone designed for the inspection and maintenance of offshore wind turbines. The project was conducted by a team of students with the objective of creating a drone capable of performing maintenance checks through various Non-Destructive Testing (NDT) methods.

The project targeted the offshore wind energy market, which is growing rapidly compared to the more saturated onshore market. Offshore wind farms are increasingly significant due to their larger turbine sizes and greater capacity for energy production. These larger turbines require more efficient and automated inspection methods, which is where the TurbEye drone comes into play.

The TurbEye drone is designed to be fully autonomous, using a hydrogen propulsion system to achieve a longer range and endurance. The drone can cover a range of 280 kilometers and fly for up to 3.5 hours on a single hydrogen tank, inspecting up to six wind turbines per trip. This extended range and autonomy are crucial for reducing the high costs associated with manual inspections and human intervention, which currently dominate the industry.

The market analysis revealed that the offshore wind energy sector is expanding, with significant investments and a projected increase in capacity. The analysis also highlighted the need for innovative inspection solutions to manage the growing number and size of turbines. The TurbEye team identified a market gap for hydrogen-propelled autonomous drones, which offer advantages in terms of efficiency and cost-effectiveness over current manual and semi-automated inspection methods.

The drone’s system comprises several subsystems: structures, propulsion, control, and inspection. The propulsion system includes a hydrogen tank, fuel cell, backup battery, and propellers, selected for their safety and efficiency. The structural design uses lightweight and strong materials, ensuring the drone's robustness. The control system integrates advanced sensors and algorithms for autonomous navigation and inspection, while the inspection system employs a combination of visual cameras, passive thermography, and geometry inspection tools to detect surface and sub-surface damage.

Operationally, the drone follows a five-stage process during missions, from pre-flight checks and sensor calibration to data retrieval and refueling post-inspection. This systematic approach ensures the drone operates efficiently and safely. The planning of inspection routes is optimized using the Vehicle Routing Problem (VRP) algorithm, tested on the Hornsea 2 wind farm, the world’s largest. This optimization minimizes the number of trips and fuel consumption, enhancing operational efficiency.

AI algorithms are implemented to analyze the inspection data, detecting damage and differentiating between dirt and actual damage. These algorithms were trained on publicly available datasets but require further refinement due to limited training data.

The financial analysis estimates the production cost of a single drone at around 60,000 euros, driven mainly by the cost of the fuel cell, 3D scanner, and cameras. The financial projections suggest a total revenue of 9.77 million euros over five years, with a return on investment of 30.8%.

The project also emphasizes sustainability, aiming to minimize environmental impact through the use of green hydrogen and recycled materials, and aligning with the United Nations' Engineering for Sustainable Development framework.

Future steps involve further improvements in control systems, AI model reliability, detailed CAD designs, and dynamic simulations of the propulsion system. Extensive testing of subsystems and the complete system will follow before moving into production and operational phases. The ultimate goal is to attract customers and perform efficient and cost-effective wind turbine inspections, thereby reducing downtime and maintenance costs for offshore wind farms. ... Read more

Wind turbines experience a variety of different operating conditions during their operation. As a result, sections of their blades experience a range of angles of attack. Additionally, several commonly used modelling methods use the sectional loading of the blade directly. To adapt this sectional loading for higher angles of attack dynamic stall models are often used. However, these models are generally based in empirical results from pitching airfoils at moderate angles of attack. In this context moderate angles of attack are considered to be below 30 degrees. As a result, their applicability to stationary airfoils at much larger angles of attack is not known. Placing an airfoil at such angles causes it to enter deep stall conditions. During deep stall, vortex shedding becomes a dominant effect on the loading of the airfoil. This behaviour can greatly affect the loading on the turbine and thus should be modelled accurately.

Based on these observations, the presented work aims to investigate the applicability of current dynamic stall models to deep stall conditions. Additionally, an adapted dynamic stall models is created. This model was designed to improve the prediction of the vortex shedding frequencies for stationary airfoils in deep stall. In this process, three different experiments were used for comparison to experimental results.

Three previously developed dynamic stall models were considered, which all showed different responses in terms of vortex shedding. These responses ranged from being completely aperiodic to undamped oscillations. However, none of the models was able to accurately predict the vortex shedding frequencies of the considered experiments. Therefore, the current dynamic stall models were found not to be applicable to the deep stall regime.

Therefore, an adapted model was constructed and calibrated based on the experimental results. This model greatly improves the prediction of vortex shedding frequencies in all considered experiments. Additionally, the accuracy of this model when applied to pitching airfoils at moderate angles of attack was examined. When applied to those conditions the model performed only slightly worse compared to a reference dynamic stall model. Hence, the presented models shows promising results for extending current dynamic stall models to the deep stall conditions.

Finally, it should be noted that the general validity of the adapted model remains to be investigated further. For this purpose, more experiments are to be considered. However, the approach outlined in this research has been shown to provide good results for creating a dynamic stall model which includes vortex shedding in deep stall conditions. Therefore, the presented method can be used to develop a validated dynamic stall model for such conditions provided more experimental results are available. ... Read more

The hunt for new methods to harness wind energy is at an all-time high as wind energy quickly overtakes other renewable energy sources. Deep waters are promising alternatives with higher average wind speed (thus, higher yields) and also due to lesser availability of land on shore. Even though the design of the blades and rotor of a FOWT is largely similar to that of onshore counterparts, they are mounted on floating platforms, unlike the fixed platforms on onshore wind turbines. Due to the excitation from wind and waves, these foundations are made to oscillate in all six degrees of freedom. Pitch and surge motions have the most significant effects out of all the possible types of motions. Since surging motion comes closest to the pitching motion, this study attempts to compare the aerodynamics of these two motions by simulating a pitching wind turbine using an actuator disk CFD model. This study can also be considered a continuation of the actuator disk study of the surging motion.

The primary objective of this thesis is to understand the differences between the aerodynamics of a surging and a pitching wind turbine using an actuator disk model. The actuator disk is made to pitch sinusoidally with the prescribed pitching frequency and amplitude. In order to identify the effects of motion and thrust prescription separately, three different types of actuator disk models were created. Using the first model, which simulates a pitching actuator disk with constant thrust, the effects of pitch motion are captured. The second model which simulates a still actuator disk with dynamic loading gives the effects of change in thrust on the rotor. The last model, which is a combination of the first two with a pitching actuator disk under dynamic loading, attempts to simulate a more realistic pitching actuator.

On comparing the surge and pitch motions of the actuator disk, the stark difference observed is the meandering in wake due to flow shear and asymmetric tip vortex shedding. At high thrust and low-frequency cases, turbulent wake
states are also visible in the wake. At higher frequencies, they are nearly as strong as the tip vortices and introduce non-linear effects on the induction field. They also tend to introduce varied dynamic responses based on the radial location. It was also observed that the motion introduces a phase delay that varies with the radial position on the rotor.

Thus, this project provides a comprehensive view of the effects of platform pitching motion on a FOWT and it differences with respect to the surging dynamics. This project also confirms that an actuator disc model is capable of reproducing the effects of pitching in a FOWT. Using the insights provided in this thesis, a better dynamic inflow can be developed for use in industries ... Read more

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.
... Read more