The accuracy of the Beddoes–Leishman and Risø dynamic stall models is evaluated against experiments on thick wind turbine airfoils with a relative thickness of 35% and trailing edge thicknesses of 10% and 2%, both with and without vortex generators. The dynamic lift, drag, and pitching moment coefficients simulation results are compared with the measurements, obtained in the TU Delft LTT wind tunnel at a Reynolds number of Re=1×10⁶ and dynamic reduced frequency of 0.064. The study revealed that while the aforementioned models successfully predicted the direction of the dynamic cycles, they inaccurately captured the dynamic stall behavior of thick flatback and non-flatback airfoils in all configurations, particularly in separated flows. There was no significant difference observed in the performance of the two models. The reasons for modeling failure are thoroughly examined from both fundamental and mathematical perspectives, and suggestions for improvements are provided. The findings raise concerns regarding the accuracy and reliability of the dynamic load assessment and aeroelasticity analysis for modern large wind turbines, using current dynamic stall models and underscore the necessity for enhancing the existing models.

During wind turbine installation or idling, the blades often operate at large angles of attack, where vortex-induced vibration (VIV) can occur. This study experimentally investigates the aerodynamic characteristics of a plunging NACA0021 airfoil at a fixed angle of attack of 90∘ and amplitude of one chord length, focusing on vortex dynamics, lock-in effect, and unsteady force generation. Phase-locked particle image velocimetry (PIV) was conducted at two reduced frequencies of 0.19 and 0.38. At the lower reduced frequency, asymmetric vortex shedding prevents synchronization between shedding and plunge motion frequencies, whereas at the higher reduced frequency, lock-in occurs with periodic shedding of separated leading- and trailing-edge vortices. Compared with previously studied surging motion under identical conditions, plunging requires a higher frequency to achieve lock-in and produces weaker wakes that break down more quickly downstream. Additionally, the aerodynamic load is extracted from the PIV flow field. For the plunging motion, the aerodynamic loads are dominated by pressure forces, with a maximum streamwise coefficient of approximately four times the static value at 90∘ angle of attack. This contrasts with the surging motion, where higher force variations are observed, and both pressure and mean momentum convection play comparable roles in the overall force. These results indicate that lock-in behavior depends strongly on both motion frequency and kinematics, where the effective angle of attack variation and the resulting vortex dynamics also determine whether synchronization can occur.

After one century from the original patent of J.M. Darrieus on a lift-driven “turbine having its rotating shaft transverse to the flow of the current”, vertical-axis wind turbines (VAWTs) are stuck in a dichotomy between being the most fertile field of research for aerodynamicists and a technology that still fails in turning into an industrial reality, with many companies going in and out of the market. After the research on VAWTs restarted in the early 2000s, significant progress was made in understanding their complex aerodynamics and modeling them with simulations of different fidelity, leading in turn to the definition of design guidelines able to reduce the performance gap with respect to horizontal-axis wind turbines (HAWTs) to less than ten percentage points. This did not persuade the consolidated HAWT industry to leave the most efficient and reliable horizontal-axis archetype, especially after the rush towards diffused onshore energy production by VAWT started in the 2010s failed. However, recent discoveries are putting again Darrieus VAWTs in the spotlight as a possible game changer in offshore wind energy, where their faster wake recovery could bring to farms with unprecedented energy density in a market where competition for marine space is strong. Different to other reviews made to date, the present study not only presents the evolution of VAWTs over the last decades, but also critically analyses the lessons learnt made along the way and highlights the recent discoveries that are opening after a century new prospects for VAWT technology.

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 310° at three Reynolds numbers ((Formula presented.)) from (Formula presented.) to (Formula presented.). 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 (Formula presented.) values, it is observed that as (Formula presented.) increases, separation tends to occur at lower AoA, 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 ((Formula presented.)) of 0.16 across various (Formula presented.) and AoA values in fully separated regions, indicating a consistent pattern under these conditions. However, a slight increase in (Formula presented.), between 0.16 and 0.20, was observed for AoA values exceeding 180°, possibly due to the convex curvature of the airfoil in the upwind direction. In conclusion, this research not only corroborates previous findings for small AoA values but also adds new data on the aerodynamic behavior of the DU91-W2-150 airfoil under large AoA values, offering various perspectives on the effects of surface curvature, (Formula presented.), and flow conditions on key aerodynamic parameters.

This paper studies the dynamic stall characteristics of thick flatback and nonflatback wind turbine airfoils. Two airfoils with a maximum thickness of (Formula presented.) were studied, with trailing edge thicknesses of (Formula presented.) and (Formula presented.), respectively. The static and dynamic experimental measurements were performed in the wind tunnel using surface pressure measurements for clean and tripped airfoils at the Reynolds number of (Formula presented.) and dynamic reduced frequency ranging from (Formula presented.) to (Formula presented.). The effects of the trailing edge gap, roughness, mean angle of attack, and reduced frequency on the dynamic stall characteristics of the airfoils were investigated. The results show that increasing the trailing edge gap delays the onset of dynamic stall. However, the lift loss after the onset of dynamic stall is for the flatback airfoil higher than the sharp trailing edge airfoil. Moreover, the flatback airfoil show higher lift overshoot compared to the sharp trailing edge airfoil in the dynamic stall condition. Increasing the reduced frequency affects the dynamic behavior both airfoils differently.

This study presents the experimental validation of regenerative wind farms (RGWFs), a novel wind farm concept designed to enhance overall wind farm performance. RGWFs employ multi-rotor systems with lifting devices (MRSLs), an innovative wind energy harvester engineered to stimulate strong vertical energy entrainment, thereby accelerating wake recovery. In the experiments, MRSLs are scaled for wind tunnel testing, with their rotors modeled using porous disks and their lifting devices represented by wings. The tested RGWFs comprise up to 3 × 3 MRSLs. Flow quantities within RGWFs and aerodynamic loads on MRSLs are measured using volumetric particle tracking velocimetry and strain gauges. Compared to conventional wind farms, flow analysis indicates that vertical energy entrainment is significantly enhanced in RGWFs, as evidenced by a more than 200 % increase in thrust on the second-row MRSLs and so on. These experimental results, which are in line with the previous numerical predictions, highlight the promising potential of RGWFs.

This study investigates the impact of zigzag tape on the aerodynamic performance and wake characteristics of the Delft Vertical Axis Wind Turbine (VAWT). The primary aim is to understand how the zigzag tape affects blade loads and the resulting aerodynamic wake. A comprehensive analysis was conducted using the Actuator Line Model (ALM) with airfoil characteristics measured in the wind tunnel at the Technical University of Denmark (DTU). Additionally, a 2-D CFD analysis with k-ω SST and γ-Re_θ turbulence models were employed to evaluate the influence of laminar transition phenomena on rotor characteristics. Results indicate that while the zigzag tape linearizes the lift coefficient characteristic, it leads to a notable reduction in aerodynamic efficiency due to increased drag and decreased lift below the critical angle of attack. The simulations were performed at a tip-speed ratio (TSR) of 4.5 to avoid a dynamic stall, as this operating condition ensures that the rotor blades remain below the static stall threshold and large offshore VAWTs are designed to operate near their maximum aerodynamic efficiency (C_P) for the majority of their operational time. The aerodynamic wake behind the rotor also shows significant changes, with the zigzag tape promoting asymmetry and affecting the wake recovery distance. The study's findings highlight the importance of considering surface contamination effects, represented by zigzag tape, in evaluating VAWT performance and wake behavior, offering valuable insights for wind turbine design and optimization.

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.

Future energy systems need offshore wind. However, large-scale deployment faces aerodynamic limits that constrain efficiency, energy yield, and grid integration. We present a closed-form analytical model for the theoretical upper limit to offshore wind farm production, expressed through a dimensionless Wind Farm Wind Factor. The model and limit are validated against data from 72 offshore wind farms over 420 cumulative years, demonstrating strong agreement including operational losses. We benchmark national policy targets in Europe and the US, revealing large overestimations of energy production—by nearly 50% in one case—underestimating energy costs, power variability and integration costs, curtailment, and policy risks. The model clarifies the critical design trade-offs between turbine height, specific power, and wind farm density. Our model provides a rigorous yet simple framework, readily usable by engineers, planners, and policymakers to forecast wind farm performance, support system planning, and to set realistic targets consistent with aerodynamic limits.

This study examined the effect of vortex generators on the dynamic stall characteristics of thick wind turbine airfoils with a relative thickness of 35% and trailing edge thickness of 10% and 2%. The experiments were conducted in the TU Delft LTT wind tunnel at a Reynolds number of Re=1×10⁶ and dynamic reduced frequency ranging from 0.032 to 0.096. The study investigated the impact of various factors on the dynamic stall characteristics of the airfoils, including the vortex generator's chord position, trailing edge gap, roughness, mean angle of attack, and reduced frequency. The study found that vortex generators delay dynamic stall for thick airfoils by stabilizing the flow during the upstroke phase. However, this can increase the maximum lift overshoot, particularly with flatback airfoils, resulting in a higher drop in lift during dynamic stall. This can potentially increase the dynamic loads on a wind turbine blade due to stall-induced vibrations. The study noted a significant difference in dynamic stall behavior between flatback and non-flatback airfoils. Overall, this research provides valuable insights into the dynamic stall and flow physics characteristics of thick wind turbine airfoils using vortex generators, aiding in more accurate rotor blade design.

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.

The aerodynamics of the multi-rotor system with lifting-devices (MRSL), an innovative concept of wind energy harvesting machine, is preliminary investigated using Large Eddy Simulation (LES) with actuator techniques. In the current setup, turbulent inflow conditions are considered, but inflow wind shear is excluded. Consistent with previous studies, the results demonstrate faster wake recovery of the MRSL compared to its conventional counterpart, namely the wind turbine system without the lifting-devices. Additionally, a set of high-fidelity simulations further reveals that the enhanced wake recovery is robust under both laminar and turbulent inflow conditions, remaining largely unaffected by variations in the ambient turbulence level. The present work provides proof-of-concept evidence that the effectiveness of MRSLs is not significantly hindered by ambient turbulence, motivating future research to evaluate their performance within a realistic atmospheric boundary layer.