The development of offshore wind energy, especially the steps towards deep water and/or higher density wind farms, revives the prospects of vertical axis wind turbines (VAWTs). Because VAWTs may reduce the cost of floating structures, there is a potential to lower energy costs. However, VAWTs are often assumed to be less efficient and less reliable due to a lack of understanding of their complex aerodynamics. This research is motivated by the fact that the performance of isolated turbines is no longer the most important factor, but rather performance at the wind farm level. The objective is to comprehend the possible performance of VAWTs in a wind farm. This dissertation advances the knowledge of wind farm aerodynamics of VAWTs mainly in four aspects: a) It demonstrates the relationship between the rotor loading and wake deflection/deformation, indicating directions for simplified modelling of VAWT wake control; b) It identifies vital characteristics of a VAWT wake, confirming the positive effects of wake deflection on wake recovery and interaction; c) It presents high fidelity experimental data on the wakes and wake interactions of VAWTs placed upwind and downwind, and validates some cutting-edge models with the data; d) it demonstrates the potential of increased power performance of VAWT arrays by controlling the VAWT flow fields. In pursuit of these advances, the dissertation identifies and tackles a series of research topics. The first is on the simplified wake models. The state-of-the-art VAWT wake models are mostly transposed from that for HAWTs, based on the planar actuator disc model. However, the effects of the actuator discs’ shape, specifically the aspect ratio of rectangular ones (corresponding to VAWTs with various height-to-width ratios), on the wake recovery are not considered. We propose the effective mixing diameter D∗ to normalise the shape effects on the wake velocity recovery based on momentum conservation. D∗ is validated through particle image velocimetry (PIV) experiments and Reynolds averaged Navier-Stokes (RANS) simulations, and it outperforms the existing scaling lengths in the literature. The dissertation further questions the validity of planar actuators as surrogates of VAWTs. It compares the three-dimensional wakes of an actuator disc and a lab-scale VAWT using robotic volumetric PIV. The comparison reveals substantial differences in the vortex systems, pointing out the limitations of planar actuators in reproducing VAWT wakes, especially when the wakes are deflected. The results indicate that surrogates for VAWTs should be three-dimensional, coinciding with the swept areas of blades. Based on the three-dimensional actuator cylinder model and a simplified formulation of the vorticity transport equation, we demonstrate the underlying physics of the generation of the streamwise vortex system, highlighting the effect of different load distributions on the wake convection and mixing. We propose four idealised force distributions resulting in different vortex systems and wake topologies; The proposed model is validated qualitatively with stereoscopic PIV measurements on a lab-scale VAWT. We quantify the faster wake recovery consequent from the wake deflection using the experimental data. Furthermore, the wake interaction of two VAWTs placed upwind and downwind is investigated experimentally via PIV and load measurements. The upwind VAWT with positively pitched blades deflects the wake significantly, improving the inflow condition of the downwind VAWT, and thus increasing the overall extraction of the streamwise momentum. With the high-quality experimental data, we validate the state-of-the-art analytical wake models and simulations for VAWTs and identify their validity ranges. Two analytical wake models (the Jensen model and the Bastankhah-Porte-Agel model), five wake superposition models (four algebraic models and one momentum-conservation based model) and an unsteady Reynolds averaged Navier-Stokes (URANS) simulation with VAWTs represented by the actuator line model are compared in both isolated and interaction scenarios. Based on the validated URANS simulation, we explore the wake deflection effects on the enhancement of wind power extraction for two up-scaled VAWTs placed in tandem. The blades of these large H-type VAWTs operate in a high Reynolds number (chordbased, Rec ≈ 1×107), which ensures a high stall angle; The tip speed ratio is set to a relatively high value (4.5) to avoid severe dynamic stalls. And thus, the simulated VAWTs are optimised for engineering operations and perform better than the lab-scale model introduced earlier. Combinations where each turbine operates in three different fixed pitch angles (-10◦, 0◦, 10◦) resulting in different wake deflections are compared. With wake deflections, the overall power coefficient is increased by up to 45%for a tested configuration, which also depends on the inter-turbine distances. Most interestingly, when the turbine blades are pitched in the same direction, the vorticity system in the wake is enhanced and thus yields a flying formation effect for a VAWT array. Furthermore, wakes of three inline VAWTs are scrutinised, focusing on the wake interactions, floor effects and momentum recovery. For all the cases the three VAWTs’ blades are pitched in the same direction following the so-called flying formation scheme. The vertical flux of momentum is notably enhanced by the VAWT array with positive blade pitches even with the floor present, which is vital to the overall increment of power extraction in a wind farm operating in the atmospheric boundary layer. The overall power extraction is increased by 35% compared to the array with zero blade pitches; More importantly, the downwind VAWTs increase their performances by 113%-154%. The latter indicates the tremendous potential of large wind farms consisting of VAWTs employing blade pitching. @en

Wakes and wake interactions in wind turbine arrays diminish energy output and raise the risk of structural fatigue; hence, comprehending the features of rotor–wake interactions is of practical relevance. Previous studies suggest that vertical axis wind turbines (VAWTs) can facilitate a quicker wake recovery. This study experimentally investigates the rotor–wake and wake–wake interaction of VAWTs; different pitch angles of the blades of the upwind VAWT are considered to assess the interactions for different wake deflections. With stereoscopic particle image velocimetry, the wake interactions of two VAWTs are analysed in nine distinct wake deflection and rotor location configurations. The time-average velocity fields at several planes upwind and downwind from the rotors are measured. Additionally, time-average loads on the VAWTs are measured via force balances. The results validate the rapid wake recovery and the efficacy of wake deflection, which increases the available power in the second rotor.

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Wake steering of vertical axis wind turbines (VAWTs) is investigated experimentally and numerically via stereoscopic particle image velocimetry and Reynolds averaged Navier-Stokes simulations. Three different blade pitch angles (-10°, 0°, 10°) of straight H-type VAWTs are adopted to deflect and deform the wake. The experimental results confirm the efficacy of blade pitching on the wake steering, and validate the simulation for both moderate and significant wake deflections. The simulation is then extended to full-scale VAWTs, exploring the wake deflection effects on the power performance of VAWT arrays. The effects of inter-turbine distances and pitching configurations are considered. With the upwind VAWT deflecting the wake, the overall power coefficient is increased by 41%.

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Wake losses are a critical consideration in wind farm design. The ability to steer and deform wakes can result in increased wind farm power density and reduced energy costs and can be used to optimize wind farm designs. This study investigates the wake deflection of a vertical axis wind turbine (VAWT) experimentally, emphasizing the effect of different load distributions on the wake convection and mixing. A trailing vortex system responsible for the wake topology is hypothesized based on a simplified vorticity equation that describes the relationship between load distribution and its vortex generation; the proposed vorticity system and the resulting wake topology are experimentally validated in the wind tunnel via stereoscopic particle image velocimetry measurements of the flow field at several wake cross-sections. Variations in load distribution are accomplished by a set of fixed blade pitches. The experimental results not only validate the predicted vorticity system but also highlight the critical role of the streamwise vorticity component in the deflection and deformation of the wake, thus affecting the momentum and energy recoveries. The evaluation of the various loading cases demonstrates the significant effect of the wake deflection on the wind power available to a downwind turbine, even when the distance between the two turbines is only three diameters.

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Dynamic induction control (DIC) has proven to be an effective method of increasing the power output for a wind farm in both simulation studies and wind tunnel experiments. By pitching the blades of a wind turbine periodically, the recovery of the low-velocity wake is accelerated, thereby increasing the energy available to downstream turbines. The wake itself of a turbine operating with DIC has not yet been studied experimentally. This paper presents a wind tunnel experiment where the wake of a wind turbine under periodic excitation is investigated. Using three-dimensional particle image velocimetry, the velocity field behind the turbine was reconstructed. Analysis of the velocity fields indicated that the available power in the wake increases when using DIC. This increase was partially due to a lower average thrust force experienced by the turbine with DIC. However, a large difference was seen between measurement results and actuator disk theory, indicating enhanced recovery of the wake is contributing to the increased energy. Instantaneous measurements visualizing the development of blade tip vortices also showed how the location of vortex breakdown, which is directly related to re-energizing the wake, shifts over time with DIC. We believe this shifting location is contributing to the enhanced wake recovery of DIC, providing more energy to downstream wind turbines.@en

The wake recovery from planar porous actuators that surrogate the effect of wind turbines is investigated, focusing on rectangular shapes for vertical axis wind turbines (VAWTs). We proposed an effective mixing diameter D∗ to scale the streamwise momentum recovery for actuators of arbitrary shape. The length-scale D∗ is given by the ratio between frontal area and disc perimeter characterising the wake-freestream interface, whereby the momentum loss and the turbulent exchange of momentum take place. Wind tunnel experiments of planar actuators from porous plates are presented. The three-dimensional development of the wake is surveyed up to six widths/diameters downstream of the actuators making use of robotic particle image velocimetry with helium-filled soap bubbles as flow tracers. The recovery rate analysis is performed using D∗ for wake normalisation. The scaled wake data agrees well among actuators in different shapes. And it is significantly improved for rectangular actuators, comparing with existing scaling lengths. The flow behaviour is confirmed with numerical simulations of VAWT wakes with different aspect ratios, indicating the validity of this scaling concept for wind turbine wake modelling.

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Wind tunnel experiments on a scaled vertical axis wind turbine (VAWT) and square porous plate with a porosity of 64% are conducted in the W-tunnel of TU-Delft. The VAWT thrusts in axial and lateral directions are measured with an in-house load cell system based on moment conservation. Wake of the VAWT in tip speed ratio of 1.5 and 2.5 and the porous plate is measured with the robotic particle image velocimetry technique, which enables a three-dimensional velocity measurement in a combined volume encompassing from 1 diameter upstream to 3 diameters downstream. Counter-rotating vortex pairs in VAWT wake and the wake shape deformation and deflection are discussed, which are related to the lateral thrust. A square porous plate inducing a similar axial thrust is compared, which has the same shape as the cross-section of the VAWT. Wake of the right porous plate with a yaw angle of 15? is investigated, which produces similar deflection as the VAWT.

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