Vortex generators (VGs) are known to delay separation and stall, allowing the design of airfoils with larger stall margins, particularly for thick airfoil sections in the inboard and midboard regions of modern slender wind turbine blades. Including VG effects in blade design studies requires accurate VG models for fast lower-order techniques, like integral boundary layer (IBL) methods. Previous VG models for IBL methods have used engineering approaches tuned on airfoil aerodynamic data. The accuracy of these models depends on the availability of wind tunnel aerodynamic polar datasets for tuning, which are limited and time-consuming to expand for the relevant wind conditions, airfoil sections, and VG configurations being used in continuously growing wind turbine blades. This work proposes a VG model using IBL equations derived from flat-plate boundary layers under the influence of VGs. The new VG model empirically models the shape factor of the boundary layer and the viscous dissipation coefficient in the IBL framework to account for the additional momentum and dissipation in the boundary layer mean flow due to VGs. The model is developed from a wide range of flat-plate boundary layers and VGs to account for variations in VG vane size and placement on the turbulent boundary layer development influencing the airfoil aerodynamic characteristics. The new VG model, called RFOILVogue, is implemented in an in-house code RFOIL, an improvement over XFOIL, and validated with computational fluid dynamics (CFD) data and wind tunnel measurements of flat plates and airfoil sections equipped with VGs. Since it is derived from vortex dynamics in turbulent boundary layers, RFOILVogue better predicts both airfoil performance characteristics, such as positive stall angle, maximum lift, and drag, and boundary layer flow parameters, such as the separation location, compared to the existing tuned VG models. The VG model still suffers from some inherent drawbacks of reduced-order models like RFOIL, and future research directions for thick airfoils are proposed to overcome these drawbacks in VG modelling.

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.

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.

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 experimentally investigates the dynamic stall of a pitching NACA 643418 airfoil under reverse flow conditions, whereby the geometric trailing edge is located upstream of the geometric leading edge. The investigation focuses on the correlation between surface pressure, aerodynamic forces, and the evolution of the dynamic stall vortex (DSV) and the aerodynamic trailing edge vortex (TEV). A range of mean angles of attack from 5° to 25°, pitching amplitudes from 5° to 15°, and reduced frequencies between 0.05 and 0.21 were tested on an airfoil using a combination of particle image velocimetry (PIV) and surface pressure measurements. The results reveal a distinct dynamic stall mechanism in reverse flow conditions: multiple flow separations occur during both the upstroke and downstroke periods, yet the DSV maintains partial attachment. While the outer layer of the DSV sheds into the flow, its inner core remains anchored and is subsequently reinforced by shear layer feeding. This sustained vorticity injection leads to periodic DSV re-growth, which manifests in the force hysteresis loop as multiple distinct peak regions. In addition, it is also found that the largest difference between conventional and reverse flow dynamic stall lies in the initial stage of the DSV development. For reverse flow dynamic stall cases, the initial flow separation near the leading edge causes a gradual decrease in the pressure coefficient. However, for conventional dynamic stall cases, the laminar separation bubbles that occur before the DSV cause the maximum suction on the airfoil surface. Furthermore, by comparing the dominant modes from Proper Orthogonal Decomposition (POD) analysis, it is found that the type of flow regime depends mainly on the mean angle of attack within the tested range of pitching amplitudes and frequencies. In particular, the dominant vortex determines the flow dynamics. At low mean angles of 5° and 10°, the most energetic flow features are associated with DSV dynamics. At a mean angle of 15°, both DSV and TEV dynamics contribute. At higher mean angles of 20° and 25°, the flow is dominated more by TEV dynamics. Despite these mean-angle-dependent variations in modal dominance, all dominant POD modes share a consistent physical interpretation where they capture the growth or decay of DSV and TEV structures.

Accurately determining experimental blade loading distributions is crucial for analyzing rotor performance but challenging due to the limitations of conventional measurement techniques. This paper presents a so-called wake-informed lifting line model that estimates blade loading distributions from phase-locked velocity measurements in the slipstream, eliminating the need for blade instrumentation. The model is evaluated against computational fluid dynamics (CFD) simulations under both attached and separated flow conditions. For the attached flow condition, the model achieves excellent agreement with CFD, with errors in the peak value of thrust distribution below 1%. In the separated flow condition, the model captures radial gradients and the shape of the thrust distribution but exhibits discrepancies in absolute values, with a 10% error in the peak value. These differences arise from the inherent limitations of the potential flow model, the increased significance of drag, and the heightened influence of the spinner’s presence in separated flows. Incorporating profile drag through external polar data improves the model prediction, reducing the error to 4%. The model cannot reliably predict power distributions without external polar data for both attached and separated flows due to the crucial role of drag in the torque direction. The application of the model to experimental flowfield data shows a performance similar to that of the validation case. Therefore, the wake-informed lifting line model offers a promising approach for obtaining experimental blade loading distributions, overcoming the limitations of traditional methods.

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.

Wind turbine blades in standstill or parked conditions often experience large angles of attack (AoA), where vortex-induced vibrations (VIV) may occur that increase the risk of structural damage. To better understand the VIV of airfoils at high AoA from an aerodynamic perspective, we conducted experimental investigations into the vortex dynamics of a surging airfoil at a 90∘ incidence undergoing forced vibrations. Experiments were conducted at two reduced frequencies (k) to demonstrate the lock-in effect, where the vortex shedding frequency aligns with the motion frequency. Results indicate distinct vortex shedding behaviors: at higher k value of 0.38, downstream wake vortices form when the airfoil is moving upwind, while upstream vortices emerge during the downwind motion, interacting with the downstream vortices and leading to an outward flow. At lower k value of 0.19, the wake remains directed to the downwind side, regardless of the airfoil’s motion direction. Lock-in is evident in both cases, with one vortex pair shed per cycle at lower k and two pairs at higher k. Furthermore, the study examines the influence of vortex dynamics on unsteady aerodynamic loads. The results show that drag peaks when the airfoil moves upwind near the center position of its trajectory; at higher k, negative drag occurs as the airfoil moves downwind near the center, driven by the interactions among convection, turbulent momentum, pressure, and viscous forces. A reduced-order load estimation model for a flat plate is applied to the experimental data, showing good agreement during the upwind motion of the airfoil, which is the design condition for the original flat plate model. However, during the downwind motion, as the flow condition does not match the original flat plate design condition, the circulatory part of the model is modified to account for the presence of two pairs of vortices in the flow field, yielding improved agreement with the drag values determined from the measured flow field. The findings highlight distinct flow patterns and vortex interactions for the two motion cases, offering insights into their impact on aerodynamic loads.

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 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.

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.

Conventional propellers operating at negative thrust conditions, even at 0 deg angle of attack, are characterized by flow separation and significantly different noise emissions than at positive thrust conditions. Operating the propeller at nonzero angles of attack at negative thrust conditions can further impact aerodynamic performance and far-field noise emission. This paper studies these effects using lattice-Boltzmann very large eddy simulations coupled with the Ffowcs Williams and Hawkings analogy. At positive thrust, operation at 10 deg angle of attack increases thrust along the freestream direction by approximately 3% compared to operation at 0 deg angle of attack, while efficiency remains constant. Conversely, the negative thrust condition shows approximately a 7% decrease in thrust magnitude and a 10% reduction in regenerated power. In this condition, the positively cambered blade sections exhibit dynamic stall, resulting in broadband fluctuations of up to 10% of the mean loading near the blade tip. The nonzero angle of attack induces opposite variations in absolute blade loading between positive and negative thrust conditions, resulting in opposite changes in the noise directivity. At positive thrust, noise increases in the region from which the propeller is tilted away (i.e., below the propeller at a positive angle of attack), while the opposite occurs at negative thrust. The varying blade loading over the azimuth results in destructive interference between loading and thickness noise for the negative thrust case at the 10 deg angle of attack. These findings highlight the crucial role of considering nonzero angles of attack in propeller design and optimization analyses.

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.

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.

Wake losses are a significant source of inefficiency in wind farm arrays, hindering the development of high-energy-density wind farms offshore. Studies have demonstrated the potential of vertical-axis wind turbines (VAWTs) to achieve high-energy-density configurations, due to their increased rate of wake recovery compared with their horizontal-axis counterparts. Recent works have demonstrated a wake control technique for VAWTs that utilizes blade pitch to accelerate the wake recovery, hereinafter referred to as the “vortex-generator” method. The present work is an experimental investigation of the wake topology using this control technique for the novel X-Rotor VAWT. The time-averaged wake topology of the X-Rotor has been measured by stereoscopic particle-image velocimetry at three fixed-pitch conditions of the top blades, namely pitch-in, pitch-out, and a baseline case with no pitch applied. The results demonstrate the wake recovery mechanism linked to the streamwise vorticity system of the rotor and the mechanisms that lead to a streamwise momentum recovery, where the pitched-in case injects high-momentum flow from above the rotor while ejecting the wake from the sides. In contrast, the pitched-out case operates in a mirrored fashion, with high-momentum flow injected into the wake from the sides while low-momentum flow is ejected out axially above the rotor. These modes of operation demonstrate a significant increase in the available power for hypothetical downstream turbines, reaching as high as a factor of 2.2 two rotor diameters downstream compared with the baseline case. The pitched-in case exhibits a higher rate of momentum recovery in the wake, compared with the pitch-out configuration.

The present study extends the idea of the vertical-axis wind turbine (VAWT) “vortex generator mode” for wake recovery on a wind farm scale, working towards the concept of “regenerative wind farming”, where upstream turbines entrain vertical momentum for those downstream. An experimental wind tunnel demonstration of the regenerative wind farming concept for an array of nine H-type VAWTs arranged in a 3×3 grid layout is performed. Volumetric particle tracking velocimetry measures the wake within the simulated wind farm while using two vortex generator modes achieved through a fixed blade pitch. The results demonstrate the strong dependence of the wake topology of a VAWT on the streamwise vorticity system, which can be effectively modified by pitching the blades and subsequently changing the load distribution of the different quadrants of a VAWT. An increase in momentum entrainment in the wake is observed for both vortex generator modes of operation, highlighting the potential of achieving regenerative wind farming. The derived available power within the farm increases by factors of 6.4 and 2.1 for the pitch-in and pitch-out cases compared to the baseline case, respectively, considering potential rotors directly in line with those upwind.

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.

Modern slender wind turbine blades use thick inboard airfoils and thicker trailing edges prone to flow separation. The increasing size of these flexible blades amplifies the importance of considering unsteady aerodynamics during the design phase. Environmental conditions result in Leading Edge Erosion (LER), further complicating the sectional unsteady aerodynamic behaviour. Although vortex generators are a well-studied method for passive separation control under steady conditions, their influence on unsteady aerodynamics for clean and rough blade sections is an area that requires further exploration. While some numerical studies exist, reliable experimental data is lacking in the literature. This work presents experimental results on the dynamic stall behaviour of a DU97W300 airfoil, a typical thick root section. The investigation covers both clean and rough conditions, both with and without VGs, to create an understanding of how VGs impact dynamic stall. Moreover, various VG array configurations are used to study the parametric dependence of dynamic stall phenomena on the VG array parameters.

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.