This study investigates the potential of regenerative wind farming using multirotor systems equipped with paired multirotor-sized wings, termed atmospheric boundary layer control (ABL-control) devices, positioned in the near-wake region of the multirotor. These ABL-control devices generate vortical flow structures that enhance vertical momentum flux from the flow above the wind farm into the wind farm flow, thereby accelerating the wake recovery process. This work presents numerical assessments of a single multirotor system equipped with various ABL-control configurations. The wind flow is modeled using steady-state Reynolds-averaged Navier-Stokes (RANS) computations, with the multirotor and ABL-control devices represented by three-dimensional actuator surface models based on momentum theory. Force coefficient data for the actuator surface models, as well as validation data for the numerical computations, were obtained from a scaled model at TU Delft's Open Jet Facility. The performance of the ABL-control devices was evaluated by analyzing the net momentum entrained from the flow above the wind farm and the total pressure and power available in the wake. The results indicate that, when the ABL-control strategy is employed, vertical momentum flux may become the dominant mechanism for wake recovery. In configurations with two or four ABL-control wings, the total wind power in the wake recovers to 95 % of the free-stream value at positions as early as x/D≈6 downstream of the multirotor system, representing a recovery rate that is approximately an order of magnitude faster than that observed in the baseline wake without ABL-control capabilities. It should be noted, however, that this study employs a simplified numerical setup to provide a proof of concept, and the current findings are not yet directly applicable to real-world scenarios.

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Numerical simulations of wind farms consisting of innovative wind energy harvesting systems are conducted. The novel wind harvesting system is designed to generate strong lift (vertical force) with lifting-devices. It is demonstrated that the tip-vortices generated by these lifting-devices can substantially enhance wake recovery rates by altering the vertical entrainment process. Specifically, the wake recovery of the novel systems is based on vertical advection processes instead of turbulent mixing. Additionally, the novel wind energy harvesting systems are hypothesized to be feasible without requiring significant technological advancements, as they could be implemented as Multi-Rotor Systems with Lifting-devices (MRSLs), where the lifting-devices consist of large airfoil structures. Wind farms with these novel wind harvesting systems, namely MRSLs, are termed regenerative wind farm, inspired by the concept that the upstream MRSLs actively entrain energy for the downstream ones. With the concept of regenerative wind farming, much higher wind farm capacity factors are anticipated. Specifically, the results indicate that the wind farm efficiencies can be nearly doubled by replacing traditional wind turbines with MRSLs under the tested conditions, and this disruptive advancement can potentially lead to a profound reduction in the cost of future renewable energy.@en

Large wind turbines face more intricate atmospheric conditions with turbulent coherent structures sized similarly to the rotor diameter, posing loading challenges. The present study assesses twelve distinct wind fields using the Large Eddy Simulations (LES) and International Electrotechnical Commission (IEC) Kaimal model scaled to their LES counterpart. The hub height wind speed in the different cases was set to 8.5 m/s (below-rated), 11.5 m/s (at-rated), and 14.5 m/s (above-rated). In a previous study, it was found that the unscaled IEC model-based wind field is conservative and scaled IEC model-based wind fields were found to yield different loads than upon use of LES-based wind fields in different atmospheric stability conditions. The present study aims to understand these differences. Utilizing Spectral Proper Orthogonal Decomposition (SPOD), the original wind fields were decomposed and reconstructed to study the influence of large and small coherent structures represented by their distinct frequencies. SPOD analysis was complemented by wind field spectral analysis considering atmospheric surface layer height, integral length scales, and co-coherence estimates. Integral length scales in the scaled IEC Kaimal model were found to be half of those in unstable atmosphere LES wind fields. The aero-elastic impact on the IEA 22 MW reference wind turbine with a 280 m rotor diameter was evaluated. The analysis reveals that large coherent structures, particularly low-frequency (≤0.06 Hz) ones, significantly impact wind turbine loads, contingent upon atmospheric stratification. Compared to the scaled IEC Kaimal model wind field, the maximum tower fore–aft bending moment and the maximum blade root flap-wise bending moment were found to be higher, for example, by 10% and 5% respectively in an unstable atmosphere during below-rated wind turbine operation. In the same scenario, standard deviation of the tower fore–aft bending moment was found to be higher by up to 50% while standard deviation of the blade root flap-wise bending moment was found to be lower by up to 25%. These findings underscore the critical importance of accurately modeling atmospheric turbulence and its coherent structures for more reliable design and operation of large wind turbines.

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Recent studies have revealed the large potential of vertical-axis wind turbines (VAWTs) for high-energy-density wind farms due to their favorable wake recovery characteristics. The present study provides an experimental demonstration and proof-of-concept for the wake recovery mechanism of the novel X-Rotor VAWT. The phase-locked flowfield is measured at several streamwise locations along the X-Rotor's wake using stereoscopic particle image velocimetry (PIV) with fixed-pitch offsets applied to the blades. The streamwise vortex system of the upper half of the X-Rotor is first hypothesized and then experimentally verified. The induced wake deformations of the vortex systems are discussed in comparison with previous studies concerning traditional H-type VAWTs. The results suggest that positive blade pitch is more favorable for accelerated wake recovery due to the dominant tip-vortex generated on the upwind windward quadrant of the cycle. Utilizing theoretical blade load variations along the span explains distinct unsteady flow features in the near wake generated at select quadrants of the rotor rotation, shedding light on the potential of the two pitch schemes.

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This study presents findings from a wind tunnel experiment investigating a model wind turbine equipped with aft-swept blades. Utilising particle image velocimetry, velocity fields were measured at multiple radial stations. These allow the derivation of blade-level aerodynamic parameters, including bound circulation, induction values, inflow angle, angle of attack, and forces normal and tangential to the rotor plane. The measured local lift coefficient aligns well with the lift polar of the design airfoil, validating the experimental approach.

The resulting public dataset provides a comprehensive aerodynamic characterisation of rotating swept blades in controlled conditions. It can serve as a baseline for future experimental research on swept wind turbine blades. Furthermore, it is valuable in validating numerical models of varying fidelity simulating swept wind turbine blades. The provided blade-level aerodynamics are particularly relevant to lower-fidelity models such as blade element momentum theory and lifting-line algorithms. At the same time, the measured flow fields can be compared against higher-fidelity simulation results from computational fluid dynamics.@en

The Horizon 2020 European Commission-funded project - X-ROTOR - proposes a radical rethink of the traditional vertical-axis wind turbine geometry. The X-Rotor vertical axis wind turbine relies on blade-tip mounted rotors, referred to as secondary rotors, for power generation and takeoff. This study examines the aerodynamic effects of secondary rotors on a scaled X-Rotor model's loading in an open-jet wind tunnel. Particle image velocimetry measurements are taken at two cross-stream planes within the volume of rotation of a scaled turbine model at two phase-locked positions. The measurements are compared with cases without secondary rotors present to understand the local impact of the blade-tip mounted devices on the wake and vortex strengths. The results indicate an accelerated turbulent diffusion of the trailing tip-vortex of the X-Rotor, and the subsequent local in-plane velocity gradients induced by the trailing tip-vortex are diminished. These insights and experimental database contribute to the development and validation of numerical models of the X-Rotor with blade-tip mounted rotors.

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Wind farm control can play a key role in reducing the negative impact of wakes on wind turbine power production. The helix approach is a recent innovation in the field of wind farm control, which employs individual blade pitch control to induce a helical velocity profile in a wind turbine wake. This forced meandering of the wake has turned out to be very effective for the recovery of the wake, increasing the power output of downstream turbines by a significant amount. This paper presents a wind tunnel study with two scaled wind turbine models of which the upstream turbine is operated with the helix approach. We used tomographic particle image velocimetry to study the dynamic behavior of the wake under the influence of the helix excitation. The measured flow fields confirm the wake recovery capabilities of the helix approach compared with normal operation. Additional emphasis is put on the effect of the helix approach on the breakdown of blade tip vortices, a process that plays an important role in re-energizing the wake. Measurements indicate that the breakdown of tip vortices and the resulting destabilization of the wake are enhanced significantly with the helix approach. Finally, turbine measurements show that the helix approach was able to increase the combined power for this particular two-turbine setup by as much as 15%.

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Vertical-axis wind turbines (VAWTs), particularly in offshore wind farms, are gaining attention for their capacity to potentially enhance wake recovery and increase the power density of wind farms. Previous research on VAWT wake control strategies have demonstrated that the pitch offset is favorable for VAWT wake recovery. In the present study, an investigation on the wake recovery and its mechanisms for an H-Rotor and a novel X-Rotor VAWTs with fixed blade pitch offsets is conducted through qualitative and quantitative methods. The actuator line method is utilized in this study. Results indicate that the two rotors produce distinct vortex systems that drive the wake recovery process—which is augmented with pitch offsets. Through quantitative studies, the contribution of wake recovery due to advection increases dramatically with pitch offsets in the near wake. With pitch offsets, the inline available power increases up to 2.3 times for the rotors when compared to when there is no pitch offset. The mean kinetic energy flux occurs mostly above and below the rotors as well as the windward side, suggesting the mechanism of power replenishment for these rotors with pitch offsets. These results encourage further research into the effectiveness of wake recovery in the wind-farm level with the ground and atmospheric boundary layer influences.

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Vertical axis wind turbines (VAWTs) have been identified as a technology that, in association with wake steering, can increase power density of wind farms. In this study, we validate a free wake method for VAWT wake prediction, which leads to satisfactory results. We then use this method to simulate wake steering by means of fixed pitched blades and struts. We demonstrate that combining pitched wakes and struts can lead to very advantageous wake behavior, but only when the interactions between the tip vortices are taken into account. The possibility to inject more high momentum flow into the wake while moving the vortex system away from the next turbine could make pitched blades and struts a powerful tool for future wind farms.

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With distributed propulsion and electric vertical take-off and landing aircraft on the rise, fast and accurate methods to simulate propeller slipstreams and their interaction with aircraft components are needed. In this work, we compare results obtained with a filament-based free wake panel method to experimental and previously validated numerical data. In particular, we study a propeller-wing configuration at zero angle of attack and the aerodynamics of the blade-resolved slipstream interaction with the wing. We use a prescribed wake on the wing and a free wake on the propeller, which greatly accelerate the computations. Results indicate that, while forces are overpredicted due to the inviscid nature of the panel method, the free wake is able to capture the slipstream deformation and shearing with remarkable success. We find that a filament-based free wake panel method can be a useful tool for propeller-wing interaction in preliminary aircraft design.

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Operating a conventional propeller at negative thrust results in the operation of positively cambered blade sections at negative angles of attack, leading to flow separation. Consequently, accurately simulating the aerodynamics of propellers operating at negative thrust poses a greater challenge than at positive thrust. This study offers a comprehensive assessment of the aerodynamic modeling capabilities of numerical methods, spanning low to high fidelity, for computing propeller performance across both positive and negative thrust regimes. Low-fidelity methods, namely, blade-element momentum and lifting line theories, effectively predict propeller performance trends at positive thrust. However, they fail to capture trends at negative thrust beyond the maximum power output point due to the neglect of three-dimensional flow effects. Both steady and unsteady Reynolds-averaged Navier–Stokes (RANS) simulations with y ⁺ < 1 perform well across both positive and negative thrust conditions, with errors below 2%for both thrust and power magnitudes near the maximum power output point. Lattice-Boltzmann very-large-eddy simulations (LB-VLESs) with y ⁺ ≤ 10 exhibit excellent agreement with experimental data with less than 1% error near the maximum power output point but with significant computational costs. Conversely, LB-VLESs with y ⁺ ≥ 15 offer a more economical approach to capture general trends with the computational cost of the same order as unsteady RANS. However, wall models introduce errors in modeling flow separation, leading to a 16% overestimation of power magnitude near the maximum power output point. The results highlight the necessity of using tools with increased fidelity levels when considering propeller operation at negative thrust compared to the conventional positive thrust regime.

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This article presents a comparison study of different aerodynamic models for an X-shaped vertical-axis wind turbine and offers insight into the 3D aerodynamics of this rotor at fixed pitch offsets. The study compares six different numerical models: a double-multiple streamtube (DMS) model, a 2D actuator cylinder (2DAC) model, an inviscid free vortex wake model (from CACTUS), a free vortex wake model with turbulent vorticity (from QBlade), a blade-resolved unsteady Reynolds-averaged Navier–Stokes (URANS) model, and a lattice Boltzmann method (from PowerFLOW). All models, except URANS and PowerFLOW use the same blade element characteristics other than the number of blade elements. This comparison covers the present rotor configuration for several tip-speed ratios and fixed blade pitch offsets without unsteady corrections, except for the URANS and PowerFLOW which cover a single case. The results show that DMS and 2DAC models are inaccurate – especially at highly loaded conditions, are unable to predict the downwind blade vortex interaction, and do not capture the vertical/axial induction this rotor exhibits. The vortex models are consistent with each other, and the differences when compared against the URANS and PowerFLOW mostly arise due to the unsteady and flow curvature effects. Furthermore, the influence of vertical induction is very prominent for this rotor, and this effect becomes more significant with fixed pitch offsets where the flow at the blade root is considerably altered.@en

In contemporary wind farm design, the primary focus has traditionally been on reducing wake interference to optimize energy capture from horizontal wind flows. However, with the scaling up of wind farms, their interaction with the Atmospheric Boundary Layer (ABL) evolves, making vertical entrainment the main mechanism for the exchange of momentum and energy. This study introduces a methodical approach to augment the efficiency of large-scale offshore wind farms by actively controlling this vertical entrainment of momentum within the ABL. The strategy involves the precise engineering of advection fluxes to alter wind flow dynamics, utilizing turbines as effective vortex generators, toward a process of "regenerative wind farming."This setup aims to create a vorticity and vertical flux system akin to those observed in highly unstable ABLs. Expanding upon previous studies that focused on single Vertical Axis Wind Turbines (VAWTs), our research explores the implementation of multi-rotor systems equipped with lift-generating wings. These systems are designed to exert forces perpendicular to the prevailing wind direction, thus creating trailing vortices and directing the flow orthogonally for improved vertical advection. This research is part of a comprehensive investigative framework that combines experiments and multifidelity simulations. The current study extends those findings to wind farm simulations, aiming to assess the impact of ABL control on a full wind farm scale. The first part of the work validates an established analytical wind farm performance model against real wind farm data for thirty-one wind farms in the North Sea and Baltic Sea. The results confirm the predicted trend of decreased performance with increased wind farm size and density. The model is used to calculate the performance of a wind farm for varying regimes of vertical entrainment due to the creation of large-scale circulatory systems. The results are compared against 3D vortex simulations of the full wind farm in "regenerative wind farming"mode. Our results demonstrate a notable improvement in wind speeds at the turbine hub height and the potential to double the feasible density of wind farms without compromising efficiency compared to traditional setups. These findings suggest a promising pathway towards a more sustainable and profitable future in wind energy, achieved through the strategic manipulation of ABL momentum, regenerating the energy in the wind farm.

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This study investigates the near-wake aerodynamics of actuator disks (multirotor devices) paired with lift-generating devices (rotor-sized wings, dubbed ABL-control devices). These rotor-sized wings generate vortical structures that enhance the vertical momentum flux from above the atmospheric boundary layer (ABL) into the wind farm, aiding wake recovery. Using three-dimensional actuator surface models based on Momentum theory, the study employs steady-state Reynolds-averaged Navier-Stokes computations in OpenFOAM to address the current proof-of-concept model. The numerical results of this paper are validated with a comparison against the experimental results of a scaled multirotor device in a wind tunnel. The performance of the ABL-controlling devices is evaluated through the wind farm's total pressure and vertical momentum flux. Results indicate that ABL-control significantly accelerates wake recovery, with designs featuring two or four ABL-control devices achieving 95% total pressure recovery at x/D ≈ 5, one order of magnitude shorter than the baseline setup without ABL-control.

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With the growing trend towards larger wind turbine rotor diameters, the impact of wind shear on rotor performance and loads becomes increasingly significant. Atmospheric stability strongly influences wind shear, leading to higher wind shear under stable atmospheric conditions. In this study, the aeroelastic performance of the IEA 22 MW rotor is assessed under inflow conditions generated by different methods. Inflow conditions were generated using turbulence models specified in the IEC Standards and also by Large Eddy simulations. Standalone OpenFAST simulations were conducted with the respective inflow conditions. It was found that at rated and above-rated wind speeds, the time-averaged wind turbine design loads were higher in stable atmospheric conditions, in comparison to the IEC NTM inflow conditions, while the opposite held for below-rated wind speeds. Specifically, the time-averaged root flapwise bending moment and rotor thrust were found to be higher by up to 7% in stable atmospheres. However, maximum design and fatigue loads were considerably higher in the IEC NTM case due to elevated turbulence levels. Compared to the IEC NTM case, the damage equivalent root flapwise bending moment was found to be 30% to 70% lower in the different scenarios.

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This study assesses the wake recovery mechanism between an H-type Darrieus and an X-type vertical-axis wind turbine, named H-Rotor and X-Rotor respectively for different blade pitch offset configurations. The analysis is conducted in OpenFOAM using the actuator line method to model the turbines to present qualitative (velocity and vorticity contours) and quantitative (available power) studies for three different fixed blade pitch offsets. The results demonstrate that the H-Rotor recovers the wake much faster than the X-Rotor at positive pitch offsets. Overall, applying fixed pitch offsets to VAWT blades helps recover the wake faster than having no blade pitch offset.

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The airfoil DU91-W2-150 was investigated in the Low Speed Low Turbulence Tunnel at the Delft University of Technology to study unsteady aerodynamics. This experimental study tested the airfoil under a wide range of angles of attack (AoA) from 0 ◦ to 3 1 0 ◦ at three Reynolds numbers ( Re) from 2 × 1 0 5 to 8 × 1 0 5 . Pressure on the airfoil surface was measured and Particle Image Velocimetry (PIV) measurements were conducted to capture the flow field in the wake. By examining the force coefficient and comparing the wake contours, it shows that an upwind concave surface provides a higher load compared to a convex surface upwind case, highlighting the critical role of surface shape in aerodynamics. When comparing separation at specific locations along the chord for all three Res, it is observed that as Re increases, separation tends to occur at lower angles of attack, both for positive stall and negative stall. The examination of the aerodynamic force variation indicates that, during reverse flow, fluctuations are more pronounced compared to forward flow. This is owing to separation occurring at the aerodynamic leading edge (geometric trailing edge) in reverse flow. In terms of vortex shedding frequency, the study found a nearly constant normalized Strouhal number ( St) of 0.16 across various Res and AoAs in fully separated regions, indicating a consistent pattern under these conditions. However, a slight increase in St, between 0.16 and 0.20, was observed for AoAs exceeding 180 degrees, possibly due to the convex curvature of the airfoil in the upwind direction. In conclusion, this research not only corroborates previous findings for small AoAs, but also adds new data on the aerodynamic behavior of the DU91-W2-150 airfoil under large AoAs, offering various perspectives on the effects of surface curvature, Re, and flow conditions on key aerodynamic parameters.@en

To investigate the effect of force distributions of each turbine component on the power performance of the Darrieus–Savonius combined vertical axis wind turbine (hybrid VAWT), the hybrid VAWT is modeled as idealized turbine under various force distributions. The goal of idealization is to simplify the intricate interactions between the Savonius and Darrieus components. The simulation actuator surfaces with uniform force distributions lead to a cost-effective way to identify the optimal force distribution of each turbine component. The numerical model was validated against momentum theory. The results demonstrated that the numerical and theoretical results yield similar predictions in the low-thrust cases but show differences in the high-thrust cases. The maximum power coefficient (Formula presented.) of an idealized hybrid VAWT with given thrust coefficient (Formula presented.) is lower than that of a single actuator. This is a consequence of the nonoptimal loading on the actuator. The results indicate that an idealized hybrid VAWT does not show a significant power increase compared with an optimal single Darrieus rotor. Therefore, the presence of a Savonius rotor inside a Darrieus rotor leads to a lower power output in any circumstance. The hybrid configuration is primarily advantageous for the start-up performance of the combined rotor, which is not explored in this study.

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As the demand for renewable energy increases, wind turbine rotors will become larger with slender blades. Vortex Generators (VGs) are used for passive flow control to avoid flow separation and reduce unsteady loading on the thick root section of slender blades due to their simplicity, inexpensiveness, and the ability to retrofit them to blades. Aerodynamic load calculations for VGs involve long experimental campaigns or resource intensive CFD calculations. Due to their inherent time-intensive nature aeroelastic optimisation and design tools prefer to use simplified but accurate analysis tools for aerodynamic load calculations of clean airfoils. One such class of tools are viscous-inviscid interaction solvers that use Integral Boundary Layer (IBL) methods for viscous calculations in the boundary layer coupled with an inviscid solver for the rest of the domain. The most popular example of this tool is XFOIL. An engineering model for VGs in IBL methods has previously been developed and implemented in XFOIL. In this research, the model parameters are tuned for implementation in another tool RFOIL based on XFOIL. RFOIL has been developed for accurate and robust analysis of wind turbine airfoils with improvements for thick airfoils and rotational corrections. The aerodynamic performance predicted the VG model in both XFOIL and RFOIL is then validated with an extensive database of airfoil data consisting of airfoils between 21% to 60% thickness, as well as Reynolds numbers between 1 million to 14 million, equipped with and without VGs. Finally, the computation time for the VG model is compared with that for a clean airfoil analysis in the same viscous-inviscid interaction solvers RFOIL and XFOIL. The investigation provides an overview of the usability of the engineering model in airfoil design methodologies in the wind turbine industry.

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Hybrid computational solvers that integrate Eulerian and Lagrangian methods are emerging as powerful tools in computational fluid dynamics, particularly for external aerodynamics. These solvers rely on the strengths of both approaches: Eulerian methods efficiently handle boundary layers, while Lagrangian methods excel in reducing numerical diffusion in flow convection. Building on our prior development of a two-dimensional hybrid solver that combines OpenFOAM with vortex particle method, this paper extends its application to the complex phenomena of airfoil stall at low Reynolds numbers. Specifically, we examine both static and dynamic stall conditions of a National Advisory Committee for Aeronautics (NACA) airfoil series 0012 (NACA0012) across a wide range of attack angles and oscillation frequencies, comparing our results with established data. The findings demonstrate the accuracy of hybrid Eulerian–Lagrangian solvers in replicating known stall behaviors, underscoring their potential for advanced aerodynamic studies. This work not only confirms the capability of hybrid solvers in accurately modeling challenging flows but also paves the way for their increased involvement in the field of external aerodynamics.@en