Society is finally entering a new age of renewable energy development. For the first time it is truly conceivable to power a vast majority of global energy use with a combination of wind, solar, and other forms of low carbon, renewable power. The rise of electrification and so called ”Power to X”, where renewable energy is used to create other more condensed and potentially storable sustainable fuels, will require a significant increase in the capacity of electrical grid networks worldwide in the coming decades. One of the largest growing sectors in renewable energy is offshore wind power. With farms in operation for over two decades, offshore wind has been predominately deployed in relatively shallow water in the North Sea of Europe. While expanding to global markets is possible with fixed bottom machines, the resource is relatively limited based on the strict seabed requirements. Moving to floating offshore wind platforms, demonstrated in pilot projects like Hywind Scotland, has the potential to vastly expand the potential wind resource and open markets in the Americas and Asia which would otherwise be unreachable....@en

There is continuous effort to try and improve the aerodynamic performance of wind turbine blades. This experimental study focusses on the addition of a passive slat on a thick airfoil typically used in the inboard part of commercial wind turbine blades. Nine different slat configurations are considered, with both a clean and tripped main airfoil. The results are compared with the performance of the airfoil without slat, as well as the airfoil equipped with vortex generators. It is found that, when the airfoil is clean, the increase in lift-to-drag ratio due to the presence of a slat is larger than when vortex generators are used. This is also true for the tripped airfoil but only at small angles of attack. As expected, in all configurations, the presence of the slat delays flow separation and stall. Finally, for a clean airfoil and small angles of attack, the slat decreases the lift-to-drag ratio of the main airfoil only. By contrast, as the angle of attack increases, it seems that the slat changes the flow field around the main airfoil in such a way that its lift-to-drag ratio becomes larger than for the airfoil without slat. These effects are less pronounced when the airfoil is tripped. This work helps to better understand the role of a slat in improving the aerodynamics of blade sections. It can also be used to validate simulation tools in the field.@en

The blade pitch of a Vertical Axis Wind Turbine can have a profound impact on the aerodynamic loading experienced by the turbine. This in turn impacts the structural loads and the performance of the machine. In order to characterize the effects of changing pitch, studies are conducted with fixed pitch offsets from a neutral pitch position in the open jet wind tunnel of TU Delft. Measurements with strain gages bonded to the turbine struts are used to estimate the normal loading of the blades. The measured behavior gives insights into the sensitivity of the turbine loading to the blade pitch angle. Aerodynamic phenomena associated with VAWTs are evident in the data including dynamic stall and blade vortex interaction. Shifting of turbine blade pitch is shown to alter the azimuthally varying normal loading, causing changes in magnitude and direction of rotor thrust. Frequency responses of the turbine and platform mounting structure are presented for rotating and fixed reference frames, respectfully. The effects of stall due to high pitch offsets is shown to excite higher per rev frequencies in both the rotor normal measurements and platform accelerations. The data sets are made available for validation of numerical models.

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Vertical-axis wind turbines have the potential to be installed nearby urban areas, where noise regulations are a constraint. Accurate modelling of the far-field noise with low-order fidelity methods is essential to account for noise early in the design phase. The challenge for the vertical-axis wind turbine is the unsteady azimuthal variation of the flow over the blades, which makes the prediction of the far-field noise complex with low-fidelity methods. In this paper, the state-of-the-art of low-fidelity methods are assessed against scale-resolving high-fidelity numerical simulations of a realistic vertical-axis wind turbine carried out with the lattice-Boltzmann very large eddy simulations method. High-fidelity numerical data are validated against experimental aerodynamics data of the same vertical-axis wind turbine. The low-fidelity method is based on the actuator cylinder model coupled with semi-empirical models for airfoil-self noise and turbulence-interaction noise. Results show a good agreement between the high-fidelity simulations and the low-fidelity model at low frequencies (i.e. between 2 × 10 ¹ Hz and 1 × 10 ² Hz), where turbulence-interaction noise is the dominant noise source. At higher frequencies, the airfoil-self noise dominates and existing methods, based on steady airfoils, do not correctly predict noise. This paper shows that the presented low-fidelity model predicts the aerodynamics and the aeroacoustics of the turbine with an acceptable accuracy for a design stage. However, improvements are needed to better predict the far-field noise for blades in an unsteady field.

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Vortex generators (VGs) have proven their capabilities in wind turbine applications to delay stall in steady flow conditions. However, their behaviour in unsteady conditions is insufficiently understood. This paper presents an experimental study that demonstrates the effect of VGs in unsteady flow, including controlling and suppressing the dynamic stall process. An airfoil, particularly designed for a vertical-axis wind turbine, has been tested in a wind tunnel in steady flow and unsteady flow caused by a sinusoidal pitching motion. The steady and unsteady pressure distributions, lift, drag and moment were measured for a range of cases. The cases vary in motion (mean angle of attack, frequency, amplitude) and VG configuration. VGs have shown to delay or even suppress dynamic stall depending on the VG configuration, with particularly important factors being VG height and VG mounting position. The VGs promote a later dynamic stall onset and reduce the hysteresis loop. As soon as the VG's effectiveness vanishes, the configurations with VGs show a severe loss in normal coefficient, larger than in the case of the clear airfoil. However, the flow reattaches quicker and the airfoil recovers easier from the deep-stall conditions. The experimental results demonstrate that the use of VGs significantly changes the unsteady aerodynamic loads. This experimental database can serve for validation purposes to evaluate whether current modelling strategies in unsteady conditions are sufficient for blades equipped with VGs.

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This paper presents the flow fields and aerodynamic loading of a two bladed H-type vertical axis wind turbine with active variable pitch for load and circulation control. Particle Image Velocimetry is used to capture flow fields at six azimuthal positions of the blades during operation, three upwind and three downwind. Flow phenomena such as dynamic stall and tower shadow are captured in the flow fields. The phase-averaged velocity fields and their time and spatial derivatives are used to calculate the normal and tangential loading at each position for each pitching configuration using the Noca formulation of the flux equations. The results show the effect of load shifting from the upwind to downwind region of the actuator using pitch and the effects of dynamic stall on the blades. The results also provide an unique database for model validation.

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The paper presents an experimental study of applying variable loads on a vertical-axis wind turbine (VAWT). The experiment is conducted in an open-jet wind tunnel on a two-bladed Darrieus VAWT equipped with active individual blade pitch control. Variable loads are achieved by dynamically changing the pitch angle of the individual blades and by keeping the wind speed of the tunnel constant. The blade loads are measured using strain gages and the flow velocity is measured upwind and downwind of the rotor using a hotwire. Dynamic inflow phenomena are clearly visible both in the turbine loads and in the velocity field. A time delay based upon the flow convection in the wake is identified. It results that the induction of the turbine can be controlled by changing the pitch of the blades. The experimental database allows to validate a new dynamic inflow model for VAWT and will be made publicly available for research purposes.

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A digital twin can be described as a digital replica of a physical asset. The use of such models is key to understanding complex loading phenomena experienced during testing of vertical axis wind turbines. Unsteady aerodynamic and structural effects such as dynamic stall and dynamically changing thrust and blade loading are difficult to predict with certainty. This leads to inefficient turbine designs or worse yet premature failures. Many of these phenomena can be better understood through scaled wind tunnel testing. The analysis of these test results is greatly improved by having a well calibrated digital twin model of the turbine. This paper discusses the methodologies used in the development of the model for a H style vertical axis wind turbine. This includes physical measurements of the as built system, updates to the models based upon experimental testing and a final correlation between test and model on a component by component as well as fully assembled system.

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The aerodynamic loading of a vertical axis wind turbine varies with the azimuth position of the blades. The thrust of the VAWT can be computed as a decomposition of the normal force on each of the blades. By varying the blade loading as a function of turbine azimuth, it is possible to vary the direction of the average thrust of the turbine. An experiment is performed using an active pitch controlled H-VAWT turbine in the Open Jet Facility at TU Delft demonstrating the ability to actively vary the rotor aerodynamic loading and as a result the average thrust vector. By applying a sinusoidal pitch actuation with phase offsets, a directional change in the average thrust vector of over 78? was demonstrated.

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Large floating offshore wind turbines are beginning to show promise as a technology with several pilot projects being completed in recent years with more on the near horizon. Due to the complexities of the floating configuration there are substantial costs associated with the platform and mooring systems for these types of deep water machines. The vertical axis wind turbine has been proposed as a potential solution for lowering the overall costs of turbine installations. This is achieved through a lower center of gravity and a greater tolerance to platform motions than an equivalent horizontal axis machine. The cost of the platform system is related to the overturn moment of the turbine in crucial operational states. The largest contribution to this moment is the rotor thrust. In this work, an experimental wind tunnel model has been made to study the loading of a 2-bladed H-type VAWT. The model is capable of individual active pitch control and is equipped with sensors to measure thrust and side loading with respect to the turbine. This paper introduces the experimental wind tunnel model referred to as PitchVAWT, discusses the method of determining rotor thrust and side loads, and presents measured results for a fixed pitch case with varying tip speed ratio. The data presented will be made available for further evaluation and potential validation of turbine numerical codes.@en

Due to advances in numerical modeling and hardware scaling, aspects of Vertical Axis Wind Turbines (VAWTs) can now be studied in greater detail than ever before. Turbine blade pitch has been proposed as a method to control overall turbine loading. A 1.5 meter diameter, 1.5 meter height 2 bladed H-Darrieus VAWT with individual blade pitch control has been designed, built, and tested at the wind tunnel facilities of Delft University of Technology. A computational model of the turbine has been made using an actuator cylinder formulation for multiple tip speed ratios and pitch offset values. The design of this turbine and initial data is presented. A comparison is made between measured normal force loading on the blades and the models predicted performance for multiple blade pitch scenarios.@en

A semi-submersible Tri-Floater has been designed to support a 6 MW vertical axis wind turbine (VAWT) with active blade pitch control. Due to the low centre of gravity and large allowable floater tilt angle, a relatively small floater can be used to support a VAWT. Coupled simulations including hydrodynamics, mooring system, aerodynamics and control system have been performed to analyse the strongly coupled dynamics of floater and wind turbine. Software tools have been developed or upgraded to enable these simulations. Based on typical extreme operational and survival design load cases, it is illustrated that the active blade pitch control system can be successfully used to minimize the governing loads on the floater. Whereas for a VAWT with fixed blades, the parked survival conditions are typically design driving for the floating support structure, this is not the case if blade pitch control is applied. It is concluded that, compared to a horizontal axis wind turbine (HAWT) with the same rated power, a 20 percent lighter floater can be used as support structure for the VAWT with active blade pitch control.@en

Research into the Vertical Axis Wind Turbine (VAWT) has been progressing over the last few years due to the large shift in design constraints for large floating offshore wind turbines by leveraging tools and experience from research beginning in the 1970s and lasting until the HAWT established market dominance in the 1990s. The beginning studies looking into the feasibility of large VAWT turbines on offshore floating platforms have been largely positive and suggest a large cost savings when comparing to equivalent HAWT turbines. These savings assume certain platform cost reductions as well as ease of operations and maintenance due to the possibility of systems being removed, like the active yaw system, and ease of access to significant components such as the gearbox and generator. In order to realize these cost gains, and perhaps identify more, it is necessary to improve the aerodynamic control of the rotor. @en