Table of contents

The actuator line (AL) is a lifting line (LL) representation of aerodynamic surfaces in computational fluid dynamics (CFD) applications. The AL blade forces are computed from 2D airfoil polars and the CFD velocity vector extracted at the line position, as the self-induction at the very centre of the bound vortex should, following vortex theory, be nil. Yet, this is not the case in CFD, which leads to errors in the angle-of-attack computation. We derive an expression for the error in the lift force from vortex considerations and show it to be a function of chord, the smearing length scale used in distributing the AL forces over the numerical domain and the number of grid cells per smearing length scale. Thereby demonstrating that the required number of grid cells-contrary to current belief-needs to grow faster than the inverse of the smearing length scale refinement to maintain the error level. We additionally show that the error can be large for the commonly used ratio of 2 grid cells per length scale, especially if the latter is relatively small with respect to the rotor radius. Ultimately, the recommendation is to always run with the largest smearing length scale possible for the specific application in conjunction with a smearing correction, as this minimizes the error in the blade forces whilst reducing the computational resources required.

The failure of a wind turbine's main bearing leads to high maintenance costs which affects the economic competitiveness of wind energy as renewable energy source. Therefore, research is conducted to replace today's used roller bearings by plain bearings. These can be designed segmented so that in case of a failure the segments can easily be exchanged on tower. As part of a former research project a 1 MW plain bearing with conical sliding surfaces was developed and tested on a full-scale system test bench. This contribution presents a methodology how this concept can be developed further and optimized to design a bearing in 3-point suspension or a moment bearing for high power classes.

The issue of leading edge erosion (LEE) of wind turbine blades (WTBs) is a complex problem that reduces the aerodynamic efficiency of blades, and affects the overall cost of energy. Several research efforts are being made at the moment to counter erosion of WTBs such as-testing of advanced coating materials together with development of high-fidelity computational models. However, the majority of these studies assume the coated surfaces as flat, while the surface curvature and the shape of the aerofoil at the blade's leading-edge exposed to such rain fields is neglected. The present study questions the assumption of a flat surface, in the context of LEE of WTBs, and provides guidelines for erosion modelling. The critical parameters associated with rain droplet impingement kinematics on leading edge are compared for blade impact with (a) flat surface assumptions together with (b) the effects of the blade's surface curvature. A parametric study is performed which includes WTBs of varying sizes and power ratings ranging from 750 KW to 10 MW, different positions along the blade length, and different rain droplet radii ranging from 0.1 mm to 5 mm for a land based wind turbine operating at rated wind speed. It is found in the study that droplet impingement kinematics are influenced by the surface curvature at the leading edge, the effect of which is significant for representing erosion at the blade tip for smaller blades, and for exposure to rainfall intensity with larger rain droplet size. A master curve describing the threshold level along the blade length is established for various WTBs and rainfall conditions, where flat surface approximation of the surface yields noticeable error and violates the impingement process. The results of the study are expected to aid the modeller in developing advanced numerical models for LEE for WTBs.

Wind turbines (WT) are highly coupled and complex systems. The dynamic interaction between various components is especially pronounced for multi megawatt wind tur-bines. As a result, the design process is generally split into several phases. [3]

The first step consists of creating a global aero-elastic model that includes essential dynamics of structural components using the minimum possible number of degrees of freedom (DOFs), with the most important simplifications concerning the drivetrain and rotor nacelle assembly. This approach is not suitable to analyse the generator air gap of a direct drive wind turbine. In this paper a more detailed simulation model is depicted, which calculates realistic deformation of the wind turbine and in particular the displacement between the generator-rotor and generator-stator. For these analysis a detailed multibody simulation (MBS) model of a generic direct drive WT is set up and used to analyse the generator air gap [3].

Several concepts of a direct drive WT are available on the market. One of them includes a coupling between the hub and generator-rotor to decouple the input wind loads from the generator and thus to decrease the impact on the generator air gap deflection [5]. The aim of this paper is to compare such a concept with a conventional concept without coupling.

Wind turbines operate in all sorts of weather conditions around the globe, exploiting installation sites with high power production potential. These machines operate within the atmospheric boundary layer and are therefore subject to impacts with rain, hail dust particles and insects, often leading to rapid blade deterioration and performance drops. This wok aims to explore different ways of modelling wind turbine blade leading edge erosion with a medium-fidelity approach and to show how different models impact performance and loads. This provides a quick way of evaluating the effects of blade deterioration. A 2D airfoil erosion model is developed and lift and drag coefficients are calculated with Computational Fluid Dynamics (CFD). An aero-servo-elastic model of the DTU 10MW Reference Wind Turbine (RWT) is then used to evaluate the impacts of the models on performance. Differences in airfoil performance are discussed as well as differences in the turbine's power coefficient and fatigue loading levels.

Vortex methods like vortex-lattice or vortex-panel methods are particularly promising to enhance the industrial aerodynamic design process of modern wind turbines. However, despite their advantages over low order methods, like the blade-element-momentum theory, vortex methods share an essential disadvantage. Their computational cost rapidly increases, which is due to their n-body problem characteristics. To overcome this issue, a method that neither relies on multipole expansions nor on a multi-grid approach is presented. Based on the aerodynamic simulation of the MEXICO rotor, it is shown that the proposed vortex pseudo-particle method (VPPM) is able to reduce the computational cost related to the n-body problem of vortex methods to O(n × log(n)). However, its application is not only advantageous in terms of the reduction of the computational cost. Since vortex-and pseudo-particles share the same properties, the VPPM's implementation is quite simple compared to that of fast multipole methods. Furthermore, no method specific boundary-conditions need to be imposed as in the case of multi-grid methods. Therefore, the presented pseudo-particle approach is an attractive alternative to fast multipole or multi-grid methods.

During the development process of wind turbines, there is a rising demand for detailed flow and multi-physics analyses. Especially the system level computational fluid dynamics (CFD), focusing not on a single component or aspect, but on the turbine as a system of geometric and dynamic properties can be used to improve the turbine quality and time to market. In this paper the integration of the system level CFD into the development process of Enercon wind turbines is introduced. Further two example applications are detailed presenting the benefits of CFD based nacelle anemometer calibration and a risk assessment study based on aero-servo-elastic CFD simulations.

Gearbox testing is an important and complex task that will become even more challenging as the wind industry moves towards ever-growing turbines. The burden of this task can be decreased by using reduced-scale models with similar characteristics as its industrial-scale equivalent. This work presents a step-by-step procedure to down-scale a gearbox to different fractions of its rated power while preserving its core properties: structural safety and frequency distribution. The parameters to be scaled are sub-divided according to their relation to the system's integrity and dynamic behavior. After performing an overall scaling, it is possible to fine-tune the scaling factors, according to the user precision requirements. Simulations show that it is possible to down-scale a gearbox to 0.01 % of its rated power while having less than 10 % relative deviation on its pitting safety factor. These preliminary results show that wind turbine drivetrain testing can become more affordable by using down-scaled models in a structured manner.

Reducing the structural mass of low speed multi-MW electrical machines for renewable energy purposes have become an important object of study as with the drop in mass a substantial decrease in the machine capital cost can be achieved. Direct-drive wind turbine electrical generators need very robust and heavy supporting structures able to cope with the demanding requirements imposed by the environment and the forces and moments transmitted by the wind turbine rotor in order to maintain the air-gap clearance open and stable. It is estimated that at least 2/3 of the total machine mass corresponds to the supporting structure. The main aim of this investigation is to minimize the structural mass of a 3 MW direct-drive wind turbine permanent magnet electrical generator, which dimensions have been previously optimized, making use of topology optimization techniques. Easy to manufacture structures made of cast iron capable of complying with the requirements were generated for both rotor and stator.

In their lifetime of 20-25 years, wind turbine rotor blades are subjected to high-cycle fatigue. Hence, new rotor blade designs need to undergo cyclic fatigue tests to show that they fulfill the reliability requirements. For these tests, a blade prototype is loaded consecutively in its flapwise and lead-lag directions, which does not represent the actual loading in the field very well. This applies especially for the off-axis load directions of the blade, i.e., the load directions between flapwise and lead-lag directions. By combining the two consecutive tests into one biaxial test, these loads can be improved. Exciting both directions at the same time also generates loads in the off-axis directions.

This work investigates the loading of a biaxial fatigue test using elliptical biaxial resonant excitation. By changing the phase angle of the elliptical excitation, the loading of the blade can be altered to better represent the operational conditions of the blade.

A blade shape distortion develops during the manufacturing process. The distortion is defined as the difference between the target blade shape as per the design and the shape under operational temperatures. This initial distortion varies under operational temperatures due to the different thermal coefficients of expansion of the various blade materials. The resulting manufacture-induced distortion and operational temperatures affect the twist angle, the cross-sectional shape, and the sweep of the blade. Young's modulus of the blade's raw materials, i.e., the matrix of the fiber-reinforced polymers and the adhesive material, changes during the manufacturing process, complicating the determination of the distortion. This work calculates the shape distortion for a reference temperature on the basis of a thermal stress analysis using a full 3D finite element blade model. It performs an aero-elastic load simulation for two models: one with the target blade shape and one with the distorted shape. The simulation reveals that the turbine with the distorted shape has an energy yield which is 0.5% lower and a lifetime extension of additional 4. 4 years.

Placing two counter-rotating rotors of a Vertical Axis Wind Turbine (VAWT) can lead to a significant power enhancement and a faster wake resorption. This global power output is directly related to the spacing between both rotors permitting a mutual confinement effect. In addition, the relative direction of angular velocity of both rotors can strongly impact the overall performances of the machine. A range of two-dimensional (2D) Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations has been managed in order to study the aerodynamic interactions occurring in a pair of VAWT. By comparing with a single-rotor of VAWT, it has been shown than the global power enhancement of a double-rotor VAWT is linked with an extension of the lift production range in one of the two first quartiles of the upwind path. Moreover, the region of the extra power generation seems to be dependant on the relative rotational directions of counter-rotating rotors. In all cases, the extent of lift generation can be associated with a suppression of the cross-stream velocity induced by the confinement of the neighbouring turbine. This local flow perturbation, closed to the inner region, leads to an augmentation of the incidence experienced by the blades in the upwind path, increasing the global lift and torque recovered by the turbine.

The second field test of a plasma-assisted 300-kW wind turbine was performed to investigate the flow-control authority of plasma-actuation technology at Reynolds numbers (Re) exceeding 10⁶. The turbine rotational speed was regulated via pitch activation in the power-saving operation mode to limit Re to approximately 10⁶. Additional generator-torque control was adopted to make the angles of attack exceed those allowable via original control. The leading-edge plasma electrode was activated at 10-min intervals and turbine characteristics with and without plasma actuation were compared using 1-min-averaged SCADA data, including wind conditions measured by a nacelle anemometer, obtained during a 6-day test. Results of this study reveal that the tip speed ratio (TSR) reduces under high-wind-speed conditions owing to additional torque control. Further, the power coefficient drops in the low TSR operating range, and improves upon plasma actuation. This has been statistically proven for at least one TSR bin. This result implies the existence of leading-edge separation under the additional torque control, and that plasma-actuation technology can effectively control the flow in the 1.6–1.8 × 10⁶Re range. Results of this study demonstrate the potential of and need for further investigation of the proposed technology to facilitate its application for multi-megawatt wind-turbine.

Wind turbines can exhibit flutter-like edgewise instabilities past a critical rotor speed. These edgewise instabilities are dominated by an edge-torsion coupling due to flapwise blade deflection. This paper experimentally validates the predicted stability limit of a 7 MW machine. State of the art simulation software is used to compare against recorded test data and characterize the observed flutter mechanism. The critical rotor speed at which the edgewise instabilities occur at in field measurements can be predicted by time domain simulations and stability analysis with very good agreement.

Wind turbine leading edge erosion is a complex installation site-dependent process that spoils the aerodynamic performance of wind turbine rotors. This gradual damage process often starts with the formation of pits and gouges leading ultimately to skin delamination. This study demonstrates the application of open source parametric CAD functionalities for the generation of blade geometries with leading edge erosion damage consisting of pits and gouges. This capability is key to the development of high-fidelity computational aerodynamics frameworks for both advancing knowledge on eroded blade aerodynamics, and quantifying energy losses due to erosion. The considered test case is an offshore 5 MW turbine featuring leading edge pit and gouge damage in the outboard part of its blades. The power and loads of the nominal and damaged turbines are determined by means of a blade element momentum theory code using airfoil force data obtained with 3D Navier-Stokes computational fluid dynamics. An annual energy loss between about 1 and 2.5 percent of the nominal annual energy yield is encountered for the considered leading edge damages. The benefits of adaptive power control strategies for mitigating erosion-induced energy losses are also highlighted.

Reliable predictions of the aero-and hydrodynamic loads acting on floating offshore wind turbines are paramount for assessing fatigue life, designing load and power control systems, and ensuring the overall system stability at all operating conditions. However, significant uncertainty affecting both predictions still exists. This study presents a cross-comparative analysis of the predictions of the aerodynamic loads and power of floating wind turbine rotors using a validated frequency-domain Navier-Stokes Computational Fluid Dynamics solver, and a state-of-the-art Blade Element Momentum theory code. The considered test case is the National Renewable Energy Laboratory 5 MW turbine, assumed to be mounted on a semi-submersible platform. The rotor load and power response at different pitching regimes is assessed and compared using both the high-and low-fidelity methods. The overall qualitative agreement of the two prediction sets is found to be excellent in all cases. At a quantitative level, the high-and low-fidelity predictions of both the mean rotor thrust and the blade out-of-plane bending moments differ by about 1 percent, whereas those of the mean rotor power differ by about 6 percent. Part of these differences at high pitching amplitude appear to depend on differences in dynamic stall predictions of the approaches.

Two-bladed turbines offer a promising opportunity for rotor cost savings, especially considering the ongoing growth trend in rotor size. An increased chord and airfoil thickness of a two-bladed turbine's blade results in potential structural improvements caused by a rapidly growing second moment of area. Compared to a three-bladed turbine's blade, the blade structure would theoretically require less material, while withstanding 50% higher flapwise loads. An analytical method of progressive structural scaling for three-dimensional rotor blade structures, based on equal material stresses, is introduced to calculate the modified structural thickness properties of the two-bladed turbine's blade. It simplifies the airfoil-shaped structure to a thin-walled rectangle, utilizes a fixed initial flapwise load factor, and scales the edgewise loads proportionally to the required blade mass. To evaluate the validity of this analytical approach, a progressively scaled and an iterated 20 MW two-bladed turbine's blade are examined with finite element analyses for static loads. The outcomes are then compared to corresponding analyses of a three-bladed turbine's reference blade. Overall, the static stress comparisons at different blade positions show good agreement with the analytical results. Nevertheless, the buckling analyses performed reveal stability issues, which subsequently will lead to a readjustment of the blade mass.

In aerodynamics, as in many engineering applications, a parametrised mathematical model is used for design and control. Often, such models are directly estimated from experimental data. However, in some cases, it is better to first identify a so-called nonparametric model, before moving to a parametric model. Especially when nonlinear effects are present, a lot of information can be gained from the nonparametric model and the resulting parametric model will be better. In this article, we estimate a nonparametric model of the lift force acting on a pitching wing, using experimental data. The experiments are done using the Active Aeroelastic Test Bench (AATB) setup, which is capable of imposing a wide variety of motions to a wing. The input is the angle of attack and the output is the lift force acting on the NACA 0018 wing. The model is estimated for two different types of input signal, swept sine and odd-random multisine signals. The experiments are done at two different pitch offset angles (5° and 20°) with a pitch amplitude of 6°, covering both the linear and nonlinear aerodynamic flow regime. In the case of odd-random multisines nonlinearity on the FRF is also estimated. We show that the level and characterisation of the nonlinearity in the output can be resolved through a nonparametric model, and that it serves as a necessary step in estimating parametric models.

High-fidelity two-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) simulations are employed to investigate the influence of boundary layer suction through a slot located near the leading edge of a vertical axis wind turbine operating in dynamic stall. The analysis includes both steady and unsteady suction with different frequencies. The results shows that: (i) when the suction slot is located within the chordwise extent of the laminar separation bubble, dynamic stall can be avoided with minimal suction amplitude; (ii) the most promising suction location is the most upstream suction location studied, at 8.5%c where c is the blade chord length; (iii) the suction only needs to be applied during the azimuthal angles when dynamic stall occurs; (iv) the oscillation frequency of the suction velocity has insignificant influence on the obtained turbine power gain; (v) applying unsteady suction is interesting as it reduces the energy consumption of the suction system, thus the net power gain.

Dynamic stall modeling remains one of the most difficult topic in wind turbine aerodynamics despite its long research effort in the past. In the present studies, comprehensive investigations involving CFD, engineering models and experimental data were performed within the joint collaborative work DSWind. Both static and dynamic characteristics of a pitching wind turbine airfoil under a turbulent boundary layer flow were considered. The results indicate that optimized time-accurate CFD calculations can serve as reference data for verifying the engineering models, which can be potentially used for tuning the models under different air flow conditions.

In this paper, the icing effect on wind turbine is investigated. The sectional aerodynamic performance of iced blade was measured in wind tunnel with icing airfoil models. The analysis conducted by using FAST using the measured sectional performance was conducted to investigate the degradation in wind turbine performance and added load due to icing. Frequency analysis was also conducted with the time series data calculated by using FAST. As the results, the power output at the rated wind speed for the icing case is smaller than that for the clean case and the maximum rotor thrust for the icing case is smaller than the clean case. The wind speed at which the turbine's rated power is reached for the icing case was higher than that for the clean case.

In this study, the clarification of the effect of plasma flow control on the rotor performance was carried out as a wind turbine control technology. The flow fields around rotor and the loads acting on wind turbine were measured by wind tunnel test using a test wind turbine with plasma actuator. As a result, in the low tip speed ratio, the power coefficient of Plasma On is larger than that of Plasma Off and the thrust coefficient of Plasma On slightly increased. In particular, the remarkable increase in the power coefficient was 24% at the most effective tip speed ratio. The wind turbine thrust increased slightly in the low tip speed ratio regime, but was less affected by the plasma flow control. In addition, when the power coefficient and thrust coefficient were calculated using the velocity components of the flow field around the wind turbine, the same tendency as in the wind turbine performance test was shown.

In this paper, four noise propagation models including the parabolic-equation based WindSTAR model, ray-tracing based Nord2000 model, Danish regulation BEK 135 model and ISO 9613-2 standard model are validated against flow and acoustic measurements of a sound source created from a speaker located at a turbine hub of 109 m height. The flow was measured with a fully instrumented met-mast at 350 m and 218 degrees from the turbine tower base. The sound was measured with 11 microphones: 8 were along a line of 45 degrees and a distance up to 1200 m away from the sound source, 3 were located at IEC positions, and 1 microphone close to the speaker, which was used to measure the source strength. White noise and 1/1 band-limited white noise sound at 2 different wind shears with exponents of 0.12 and 0.23 are used for validation. Results show that an overall agreement between experiment and computation is reached for all the numerical models. Among the 4 numerical models, Nord2000 gives the best prediction for the nearfield microphones of mic 4-mic 6 and WindSTAR gives the best prediction for the far-field microphones of mic 7 and mic 8.

This article presents the consolidated results of a comprehensive validation campaign of a pneumatic active flap system (AFS) developed within the scope of the Induflap2 project [1] in a collaboration effort between Siemens Gamesa Renewable Energy A/S (SGRE), the Technical University of Denmark (DTU), and Rehau GmbH. The validation results presented herein include wind tunnel measurements, measurements in a rotating test rig under free atmospheric conditions, as well as the validation at full scale on a 4MW wind turbine with 130m rotor diameter (SWT-4.0-130). This article is of a summarizing character and the reader is referred to further literature where applicable. During the course of the project, several revisions of the AFS were developed and tested at different levels of fidelity. Along with the different variations of the AFS, the impact of variations of the geometric and material related design parameters were studied. Two of the latest revisions of the AFS were tested at full scale on a wind turbine. The measurements presented herein focus solely on the latest revision of the AFS (internally referred to as the FT008rev10 concept). The different tests and measurements were performed in the period from 2016 to 2019. During the full scale measurements, a wind speed dependent load impact between 5% and 10% was measured for the blade root flapwise bending moments, demonstrating thus a large potential for turbine loading adjustments to perform active load control on modern wind turbines.

Due to a small cross-sectional dimension compared to their diameter, pitch bearings have a comparably low stiffness. Therefore, surrounding structures strongly influence the internal load distribution of the bearing. By the use of stiffening plates, the load situation of pitch bearings can be positively affected. With a gradient-based optimization procedure regarding contact force and contact angle distribution, an optimized plate shape is determined. This optimized shape provides at the same time a more equalized load distribution, while minimizing the effect of contact ellipse truncation.

The analysis of wind turbines relies on aero-hydro-servo-elastic simulation tools. OC3, 4, 5, 6 projects showed that much effort is required to obtain consistency in numerical models set up and the agreement in the simulation results. To mitigate the uncertainty in the model setup and to reduce the time spent on its verification, a robust verification procedure is necessary. The presented verification procedure provides a structured and efficient way of checking and comparing aeroelastic wind turbine models. In case a discrepancy between the two models or the model and documentation is found, a procedure for finding the source of this discrepancy is suggested. During several successful applications of the presented verification procedure, it proved to reduce the effort, the number of iteration loops and thus the consumed time that is required to achieve a verified model.

The development of floating offshore wind turbines opens the way for various new design types, and the platform, tower and turbine can benefit from its floating foundation. Self-aligning platforms, where the entire structure follows the wind direction are a promising concept. A single point mooring with turret system allows for free rotation around the vertical axis. Aerodynamic forces of rotor and tower induce the self-aligning moment. In the present study, the operating principle of a passive platform design with airfoil-shaped tower and downwind rotor is analyzed under steady conditions using a boundary element method (BEM). Rotor cone angle and the tower dimensions have a major influence on the yawing moment. They must be large enough to dominate the hydrodynamic forces induced by seaway and current. The passive self-aligning capability is shown in an integrated simulation for various current velocities and wind-current offset angles.

Unsteady airfoil experiments were conducted in a high-pressure wind tunnel at chord Reynolds numbers of Re_c = 3.0 × 10⁶. A moderately thick NACA0021 airfoil was pitched from rest beyond the static stall angle in six individual ramp tests with increasing and decreasing angles of attack. The variant types of motion of the pitching maneuvers were characterized by constant angular velocity, angular acceleration and angular jerk, respectively. The ramp-up experiments revealed a substantial and time-dependent excess of the aerodynamic forces from static values in all three test cases and exhibited a distinct time delay as a consequence of the variant motion types. Similarly, the ramp-down experiments were largely impacted by the progression of the pitching motion, resulting in pronounced differences in the temporal development of lift and drag. Results are shown as time series of integrated forces and surface pressure distributions.

Since the introduction of the 3rd edition of the IEC Standard 61400-1, designers of wind turbines are required to apply statistical extrapolation techniques, to estimate the extreme (ultimate) load values corresponding to fifty-year return period. In the present paper, the certification procedure is assessed under the uncertainty of the material properties using simulated load time series of the NREL 5MW Reference Wind Turbine. The uncertainty of the material properties is introduced in the elastic properties of the composite blades by using input data from the OptiDAT composite material database. Comparison of the estimated extreme loads and deflections of the blades as well as maximum stresses, also in connection to the Tsai Wu failure criterion, is performed for different material sets. It is found that the variability of the material properties does not affect the estimated ultimate flapwise moments (difference <1.5%) but affects the maximum flapwise deflection (differences ∼8%). It is concluded that for the levels of variation considered in the composite material properties, the coefficient of variation of the extreme stresses and the Tsai Wu failure criterion are in the order of 2% and 8% respectively.

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.

Surface degradation of the wind turbine blades lead to a reduction while on the other hand blade add-ons like, vortex generators, lead to an increase in the aerodynamic performance. Within this study, both the reduction due to leading edge roughness and the increase due to vortex generators in the aerodynamic performance are quantified individually first, and then it is investigated if the vortex generators would compensate for the losses due to roughness. Roughness models for the Spalart-Allamaras (SA) and k — ω SST turbulence models are implemented in the open source CFD suite SU2 and validated against theoretical predictions and experimental data. The roughness model is then applied to a commonly used airfoil section, DU97-W-300 and steady RANS simulations are carried out with SU2. Four different conditions are considered-no erosion (clean), eroded (rough), clean blade section with VGs and rough blade section with VGs. Numerical simulations are validated with experimental data for the clean airfoil section and airfoil section equipped with vortex generators. Finally, a preliminary analysis is presented for each of the cases considered on the effect of power production.

This paper exposes a method of modelling and analysis to help verifying design iterations of non-cylindrical shaped floating wind technologies. In order to do so, the Morison based quadratic hydrodynamic drag forcing contribution within the HAWC2-WAMIT coupled model, is modified to account for a panel-based input geometry. The implementation is applied to the P80 platform, and a verification of the hydrodynamic responses is performed. A reduced load case analysis is carried out, in which floating wind technologies can be assessed efficiently during early stage design iterations. The final platform assessment includes the wind and wave coupled effect using three load cases from IEC 61400-3 standards. The results of the simulations show that the chosen cases provide significant information relating to the platform motions and the tower top acceleration, in order to provide feedback for later design iterations of the platform.

Vertical axis wind turbines have several attractive features in the context of energy production in urban areas, but the inherent aerodynamic complexity of the flow around them has challenged their development on a larger scale. They generally operate at low tip speed ratios, where dynamic stall occurs on the blades. The vortex shedding associated with dynamic stall causes highly transient and heavy stress cycles that reduce the aerodynamic performance and increase the risk of fatigue and failure. The flow around an airfoil undergoing VAWT blade angle of attack variations was investigated using particle image velocimetry and force measurements. The formation of vortices and the lift force were studied for different tip speed ratios. A special focus was put on the effect of the asymmetry of the motion. The role of dynamic stall vortices on aerodynamic coeffients was evidenced by comparing experimental data to analytical predictions obtained from Theodorsen's model. For the lowest equivalent tip speed ratio clockwise and counter clockwise rotating dynamic stall vortices formed on the airfoil with increasing and decreasing angle of attack. The asymmetry in motion lead to an asymmetry in size of the clockwise and counter-clockwise vortices. As the asymmetry in motion has a strong effect on the flow behaviour, the local pitch rate was proposed as a governing parameter. The increase of extrema with increasing pitch rate varies for increasing and decreasing angle of attack, indicating an additional influence of the history of the flow development.

The work describes modelling of erosion of the IEA 15 MW Reference Wind Turbine. Five-year worth of historical time series including wind speed, rain intensity and price of energy was used in the modelling. The work included erosion-safe operation of the turbine where the tip speed was limited during rain events to prevent erosion. The tip-speed limit was defined as a function of rain intensity and price of energy, the distribution of which was subject to an optimization. The objective for the optimization was to maximize the profit. The factors taken into account in the modelling and optimization were: the loss in power curve due to erosion, and the cost of repair and associated downtime. The results indicated that for this particular turbine it would be sufficient to define the tip-speed limit as a function of rain intensity alone. The model showed that 88% of the overall profit loss due to erosion could be saved by running the turbine in the erosion-safe operation. Although some inaccuracy may be associated with the modelling, the results strongly indicate that a significant amount of money could be saved by utilizing the erosion-safe operation on offshore turbines.

As the offshore wind industry moves into deeper waters, the number of different floating wind platform concepts continues to expand. Accurately predicting the motion characteristics of these novel platforms is crucial to the safe and economic conversion of wind energy into electricity. A challenge in the currently used numerical simulation methods involves successfully predicting the slow drift motions and damping, especially in complex realistic environmental conditions which include multi-directional spectral waves, current and wind loading. Of most uncertainty to these predictions is determining the low frequency viscous damping which dominates the resonant surge and sway motions. In this paper, full-scale motion observations of the Fukushima FORWARD's floating wind substation are used to determine the platforms viscous drag forces. These drag coefficients are used in a combined potential flow and Morison's equation model to capture the observed behaviour. This validated numerical model is then used to describe the required fidelity of mooring line models, in order to aid platform developers in the early stages of design.

Aeroelasticity is one of the biggest challenges in wind turbine rotor design, as the length of rotor blades increases which comes along with a slenderer design. The knowledge of the aeroelastic turbine behavior is of great importance. A comparison to field measurements is of huge importance when validating aeroelastic tools. However, the measurement of deformation and torsion in the field is not trivial and the conduction of realistic post-test simulations is a challenge. One crucial factor for these simulations is the wind field, which needs to be captured in a high spatial and temporal resolution. In this paper, the results of deformation measurements conducted in the field with an optical measurement method called Digital Image Correlation (DIC) on one rotor blade will be shown and compared to aeroelastic post-test simulations using highly resolved wind fields measured with a SpinnerLidar.

Bearing failures in wind turbine gearboxes still occur which results in downtimes of wind turbines and hence increase the levelized costs of energy. One of these affected bearings is the generator sided planet carrier bearing as it is still subject to unexpected fatigue related failure modes. To reduce the occurrence of these failures a methodology is presented to determine the most influencing wind loads on this planet carrier bearing using the finite element method. Therefore a 1 MW wind turbine drive train model is used and the influence of several parameters influencing the load distribution of the bearing are investigated. These parameters are the surrounding stiffness of the bearing seat, the influence of the individual components of the wind loads and the circumferential position of the planet carrier. The results show that a realistic surrounding of the bearing seat leads to an unequal circumferential load distribution while the bearing itself is less loaded than using the generic surrounding of the bearing seat. The different components of the wind loads do not have much impact on the load distribution while the force acting on the rollers change. The different positions of the planet carrier also do not affect the circumferential load distribution.

Passive load alleviation on wind turbine blades can be achieved through geometric bend-twist coupling, for example by sweeping the blade backwards. In order to obtain the correct load distribution of a curved blade with in-plane sweep and/or out-of-plane dihedral, the influence of the blade shape on the aerodynamics must be modelled correctly. This includes the influence of the curved bound vortex, and it is especially important when designing a wind turbine blade with aeroelastic tailoring. In this paper, the background for modelling the curved bound vortex influence will be described in detail and a modified method is proposed. The proposed method of bound vorticity modelling is compared for curved and straight translating wings as well as wind turbine blades with results from a panel code and a Navier-Stokes solver. From this comparison, the advantages of the current modification with respect to the other lifting-line implementations are shown. The method proposed in the present work is general and applicable to any lifting-line like model.

The power output of a wind turbine depends on both wind speed and turbine control. There is a fluctuation in the natural wind speed, which changes the turbine power output. In this paper blade pitch control based on detailed monitoring of the inflow wind, including vertical velocity distributions, has been experimentally evaluated in a field test wind turbine. The inflow wind speeds were observed by 11 sets of ultrasonic anemometers. In this first test, the blade pitch is changed to keep optimum angle of attack based on the local inflow wind speed in the rotor plane. The blade pitch is independently controlled as a reference radial position of 80% of rotor radius. As a result of the pitch controls, the wind turbine power output was increased and the power output fluctuation was decreased for the cyclic pitch control based on the local wind speed. With the cyclic pitch control, the fluctuation in rotor thrust and average rotor thrust were also reduced compere to collective pitch control.

This paper deals with the 3D effects of a vertical-axis wind turbine caused by the tip vortices. In this study, the VAWT rotor is simplified by the infinitely bladed actuator cylinder concept. The loads are prescribed to be uniform and normal to the surface and are distributed between the upwind and downwind half. Depending on the load configuration, the tip vortices are shed at different locations. This causes the wake induction field and the induction at the rotor to be significantly different. The 3D effects cause a power loss that may go up to 15% depending on the load configuration and aspect ratio. Starting from an aspect ratio of 5, the rotor induction and power is approaching 2D results.

Since the first commercial projects, the development of vertical-axis wind turbines (VAWTs) has been impeded by the limited understanding and inability to accurately model VAWTs. This paper investigates and compares different aerodynamic modelling techniques for VAWTs in 3D. All considered models are using the same blade-element characteristics but use different descriptions to determine the induced velocity field. The H-and Φ-rotor are studied with various aspect ratios and rotor loadings. Both instantaneous azimuthal parameters as well as integral parameters, such as the thrust and power are investigated. The paper concludes that capturing the 3D effects of VAWTs is challenging and the trends to be expected are not straightforward due to the complex vortex system created by VAWTs. All model assumptions affect the results both at the mid-plane of the rotor as well as at the blade tips.

In boundary layer flow around rotating machines, a radial (or cross-flow) velocity exists due to Coriolis and centrifugal forces. This velocity component can be of great importance for laminar-turbulent transition. A series of direct numerical simulations (DNS) are performed to study the boundary layer flow transition on a rotating Horizontal Axis Wind Turbine blade. To quantify the effect of blade rotation, results are compared with that from airfoil DNS, where the section is taken from 3D blades and does not rotate. It is shown that the rotation gives rise to a small radial velocity and slightly modifies the shape of unstable waves. However, the transition location and mechanism of 3D blade boundary layer flow resemble 2D flow for the investigated case.

This paper compares the dynamic response of an OWT structure where the below ground pile-soil behaviour is modelled using (i) the conventional API 'p-y' approach and (ii) the 'PISA' approach. A nonlinear aero-elastic code is used to model the structural dynamics of the OWT and coupled to the geotechnical model. The dynamic behaviour (natural frequencies) and fatigue loads of the turbine tower and monopile are estimated and compared using both the API and PISA approaches. A limited number of load cases were considered in the dynamic analysis with varied met-ocean conditions. It was found that the stiffer springs estimated by the PISA approach reduce the mean displacement of the tower in both the fore-aft and side-to-side directions. However, the increased monopile stiffness leads to a slightly increased amplitude of oscillations, particularly in the lightly damped side-to-side direction.

The local aerodynamic loading on floating offshore wind turbines (FOWTs) is more complex than on bottom-fixed wind turbines due to the platform motions. In particular, the FOWT rotor may start to interact with its own wake and enter a so-called vortex ring state (VRS). However, it is still unclear when, and to what extent, the VRS may happen to floating offshore wind turbines. In this paper, we quantitatively predict the VRS using Wolkovitch's criterion during the operating conditions of different FOWTs simulated by FAST. The results show that the type of floating foundation has a significant influence on the aerodynamic performance of the rotor. Also, the probability of occurrence of VRS is bigger for the floating platforms that are more sensitive to wave excitations.

To enable the development of floating offshore wind farms, it is important to have a clear understanding of the aerodyamic forces applied on a floating offshore wind turbine. The paper presents comparisons between a lifting line free vortex wake method and an actuator line method in the case of a wind turbine in surge with blade-resolved CFD data as a reference. Each model is compared to a quasi-steady estimation of the loads to understand the variations due to the surge movement. The near-wake flow field is investigated in order to give an insight into the flow features leading to the observed behavior. Both methods predict a higher axial velocity at the position of the rotor and one radius downstream of the rotor in surge conditions compared to the fixed case.

Non-intrusive nonlinear reduced order modeling (ROM) techniques have been applied by researchers to obtain computationally cheap and yet accurate structural responses of aircraft panels. However, its application to wind turbine blades is new and challenging due to much larger deflections of wind turbine blades. This study improves a non-intrusive nonlinear ROM method for wind turbine blades going through large deflections. In the nonlinear ROM, the nonlinear stiffness is described by the quadratic and cubic functions, and the secondary motions induced by the primary large deflections are described by the modal derivative vectors in the reduction basis. The non-intrusive nature of the method requires a geometrically nonlinear solver, and HAWC2 is chosen in this study for the computation of nonlinear stiffness terms. Two examples, including a cantilever beam example and the NREL 5MW wind turbine blade model, are used to evaluate the accuracy and computational effectiveness of the nonlinear ROMs. The cantilever beam example shows that the nonlinear ROM can accurately capture the axial displacements due to large deflections reaching 20 % of span length as well as the torsion coupled with flapwise and edgewise motions. The NREL blade example shows that the nonlinear ROM is accurate for the tip displacements more than 5.9 m. Because the size of the nonlinear ROM is much smaller than that of HAWC2 model, a speedup factor of 8.5 for computational time is observed for the NREL blade example.

We analyze the new Danish Poul la Cour Tunnel (PLCT) facility using 2D and 3D Computation Fluid Dynamics simulations, with the aim to address the need for wind tunnel corrections. The assessment of tunnel corrections is based on 2D free simulations representing undisturbed airfoil flow, 2D tunnel configurations and a full 3D tunnel setup using laminar/turbulent Improved Delayed Dettached Eddy Simulations. By comparing lift, drag and pressure distributions and detailled flow patterns, the influence of the 2D and 3D tunnel is evaluated. In the present study a NACA 633-418 airfoil is investigated in the tunnel.

As rotor diameters and blade flexibility are increasing, current and future generation wind turbines are more susceptible to aeroelastic instabilities. It is thus important to know the prediction capabilities of state-of-the-art simulation tools in regards of the onset of aeroelastic instability. This article presents results of a code-to-code comparison of five different simulation codes using a representative wind turbine model. It is shown that the models are in good agreement in terms of isolated structural dynamics and steady state aeroelastics. The more complex the test cases become, the more significant are the differences in the results. In the final step of comparison, the aeroelastic stability limit is determined through a run-away analysis. The instability onset is predicted at different wind speeds and the underlying mechanisms differ between the tools. A Campbell diagram is used to correlate the findings of time domain simulation tools with those of a linear analysis in the frequency domain.

In this study, simulation results of two different computational fluid dynamics codes, Nalu-Wind and EllipSys3D, are presented for a wind turbine rotor in complex yawed and sheared inflow. The results are compared to measurements from the DanAero experiments, to validate computed pressures and azimuthal trends. Despite different code methodologies and grid setups, the codes agree well in computed pressures and integrated forces along the blade for all blade azimuthal positions, however with some discrepancy in the very yawed case. Additionally, both codes capture well the azimuthal trends and force levels seen in measurements. Investigation into discrepancies shows that expanding grids before the rotor, lead to smearing of the wind profiles, which is likely the main cause of the differences in the results between the codes. Additionally, omission of the ground constraint cause discrepancies in relative velocity seen by the passing blade, due to an over speeding beneath the rotor.

This study investigates how losses in energy production from wind turbines are influenced by leading edge roughness, rotor control and wind climates using computations. The performance of a NACA633-418 airfoil with five different damage types was predicted using Computational Fluid Dynamics (CFD) with bumps at the leading edge and grooves into the leading edge. Also, one type corresponding to a repair of a leading edge with an "overbite" was investigated. These airfoil characteristics were used in rotor computations where three different wind climates and five different maximum tip speeds were investigated. The rotor computations reflected that the Annual Energy Production (AEP) increased significantly with the average wind speed on the site. They also reflected that the relative AEP losses due to leading edge damages reduced with increasing average wind speed on sites. For the low wind speed site the losses were between 1% and 4% depending on the extent and the type of the blade damage. For the high wind speed site the losses were between 0.5% and 3%. Furthermore, the bigger the extent of the damages were the bigger the losses were. Finally, it was shown that increasing the maximum tip speed increased AEP and decreased the losses.

A new analytical model for representing body forces in numerical actuator disc models of wind turbines is compared to results from a BEM model. The model assumes the rotor disc being subject to a constant circulation, modified for tip and root effects. The advantage of the model is that it does not depend on any detailed knowledge concerning the actual wind turbine being analysed, but only requires information of the thrust coefficient and tip speed ratio. The model is validated for different turbines operating under a wide range of operating conditions. The comparisons show generally an excellent agreement with the BEM model, with, however, some deviations of the tangential force in the root area and at small values of the thrust coefficient.

Reference wind turbines (RWTs) that reflect the state-of-the-art of current wind energy technology are necessary in order to properly evaluate innovative methods in wind turbine design and evaluation. The International Energy Agency (IEA) Wind Technology Collaboration Platform (TCP) Task 37 has recently developed a new RWT geared towards offshore floating-foundation applications: the IEA Wind 15 MW. The model has been implemented in two aeroelastic codes, OpenFAST and HAWC2, based on an underlying common ontology. However, these toolchains result in slightly different structural parameters, and the two codes utilise different structural models. Thus, to increase the utility of the model, it is necessary to compare the aeroelastic responses. This paper compares aeroelastic loads calculated using different fidelities of the blade model in OpenFAST (ElastoDyn and BeamDyn) and HAWC2 (prismatic Timoshenko without torsion and Timoshenko with fully populated stiffness matrix), where both codes use the DTU Basic controller and the same turbulence boxes to reduce discrepancies. The aeroelastic responses to steady wind, step wind and turbulent wind (per IEC 61400-1 wind class IB) are considered. The results indicate a generally good agreement between the loads dominated by aerodynamic thrust and force, especially for the no-torsion blade models. Discrepancies were observed in other load channels, partially due to differences in the asymmetric loading of the rotor and partially due to differing closed-loop dynamics, and they will be the subject of future investigations.

This paper investigates the effect that adding constraints to turbulence simulations has on the uncertainty of resulting aeroelastic loads. The constrained turbulence is generated using the open-source constrained turbulence generator PyConTurb ("Python Constrained Turbulence"). A selection of constraint patterns were used to mimic the design of a met mast layout; i.e., the number of sonic anemometers and their locations throughout the rotor. A case study is presented to demonstrate in detail the effects of adding constraints before a larger numerical experiment is presented. The results of the numerical experiment indicate that adding constraints is extremely beneficial in reducing the mean absolute error of both operational parameters and loads. The reduction in mean absolute error ranged from 13% to 98%. The error in the extreme values and damage-equivalent loads were not impacted by the added constraints due to lack of gusts in the original signals and the similarity of the power spectra of the constrained and non-constrained signals, respectively.

The development of a new beam reference line to improve bend-twist response modelling is presented. The beam reference line is derived as the locus of the cross-section elastic energy centres under pure bending moment. The performance of the new reference line is assessed via experimental measurements performed on a Siemens Gamesa wind turbine blade. Bend-twist modelling capability is assessed by comparing the location of the zero-twist rotation line and zero-twist-rate lines.

In this paper a detailed comparison of the experimental and numerical results of a scaled wind turbine model in a wind tunnel subjected to fast pitching steps leading to the so-called dynamic inflow effect is presented. We compare results of an Actuator Line LES tool, a vortex code and four engineering models, to the experiment. We perform one and two time constant model analysis of axial wake induction and investigate the overshooting of integral loads. Our results show, that the effect is captured better by the two time constant models than by the one time constant models. Also the experiment and mid-fidelity simulations are best described by a two time constant fit. We identify the best dynamic inflow model to be the 0ye model. Different possibilities for the improvement of dynamic inflow models are discussed.

To carry out fatigue tests on the blade bearing test rig used in this work, knowledge of the maximum contact pressure and position of the contact ellipse in each rolling contact is necessary. Since these cannot be measured directly, a detailed FE-model of the blade bearings and the test rig components is set up. Previous studies use non-linear spring to simplify the raceway ball interaction. In this paper each rolling contact is modelled with a surface to surface interaction. This allows the evaluation of the maximum contact pressure and the distance between the contact area and the edge of the raceway. Furthermore, measurements of the relative displacement between the outer and inner ring for different load steps were carried out and show a good correlation with the simulation results.

To assess of the lifetime extension (LTE) of wind turbines, the remaining fatigue budget of different turbine components is predicted. The state-of-the-art approach to predicting the LTE uses a comparison of damage-equivalent loads (DEL) under site conditions and design conditions. The DEL-based approach entails several simplifications, i.e., neglecting the load direction and the mean load. An analysis based on stresses can mitigate the above simplifications. In this work, the LTE of a 1.5 MW turbine is assessed on the basis of the DEL-based approach and compared to that of the stress-based approach. The assessment focuses on components which are critical for the lifetime, i.e., the blade bolts, the blade root laminate, and the main shaft. Generic models of these components are implemented to calculate stress histories for the stress-based assessment. In addition, the main assumptions that may have to be based on estimates are outlined. The analysis shows that, depending on the component, the results of the stress-based approach may differ notably from those of the DEL-based approach. The stress-based approach can be used to improve model fidelity in LTE assessments.

In this study, the aerodynamic performance of a thick airfoil is analysed, after installing leading-edge roughness to emulate a severe state on the airfoil surface. The impact on aerodynamic coefficients has been quantified using two roughness methods: zig-zag tape and sandpaper. Wind tunnel tests are carried out at a Reynolds number of 3•10⁶. At low angles of attack, zig-zag tape and sandpaper provide comparable lift and drag coefficients but significant variations of these coefficients are obtained for high angles of attack. Stalled flow is the cause of the most significant variation on the airfoil performance between smooth and rough surface states. Vortex generators are adapted to recover the lift coefficient value previously given by the airfoil under smooth conditions. As a result, vortex generators are able to reduce the loss of lift and the sensitivity of the airfoil to the rough state.

Solvers based on Boundary Element Method (BEM) are fast and they have proven to provide accurate results in a wide range of applications. On the other hand, computational fluid dynamics (CFD) solvers are high-fidelity tools able to account for viscous effects. However, they are computationally demanding. In the present work, the limitations of BEM solvers are exploited considering the case study of a moonpool type floater in which the viscous effects near the sharp edges of the body (vortex shedding) are not negligible. The BEM results are compared with the results from an unsteady Reynolds averaged Navier-Stokes (URANS) CFD solver and experimental data, while viscous corrections of the BEM method are assessed.

A recurrent deep-neural network (DNN) surrogate model capable of modeling the unsteady aerodynamic response and dynamic stall behavior of wind turbine blades has been developed and validated for use in engineering design codes. The model is trained using a subset of the oscillating airfoil experiments conducted at the Ohio State University wind tunnel. The predictions from our DNN model show excellent agreement with the measured data and, in all cases, a marked improvement over the state-of-the-art unsteady aerodynamic models. The DNN-based unsteady aerodynamics model was integrated with OpenFAST to perform full-turbine load computations for the NREL-5MW rotor. The largest differences are observed for the inboard stations, particularly in the pitching moment response, when using the new surrogate model compared to the other models available in OpenFAST.

Slewing bearings of wind turbine blades pitch system allow the required oscillation while transferring complex dynamical loads from the rotor blades towards the hub. Most common for this application are double rowed four-point contact ball bearings consisting of two rings and two rolling element sets either equipped with a ring cage or with spacers. By equally distributing the rolling elements along the circumference, the bearing cage balances the loads along the raceways and thereby actively prevents damage mechanisms. However, cage stress analyses imply further optimization potentials leading to higher load capacities and the prediction of cage damaging mechanisms via dynamic simulations. This contribution presents a simulation procedure that calculates rolling element dynamics at the presence of a ring cage by taking the perpendicular movement of the contact ellipses due to the elastic deformations of the bearing rings into account. The procedure is carried out exemplary by the load spectra of a 3 MW reference wind turbine and the actual geometry of a 2.4m diameter bearing.

Using flatback airfoils at the root of wind turbine (WT) blades is becoming more popular as the WTs increase in size. The reason is that they provide significant aerodynamic, aeroelastic and structural benefits. However, due to the blunt trailing edge (TE), the wake of such airfoils is highly unsteady and rich in three-dimensional vortical structures. This poses significant challenges on the numerical simulation of the flow around them, given the highly unsteady, three-dimensional turbulent character of their wake. In this work, computational predictions for a flatback airfoil employing both RANS and DES approaches on three successively refined grids up to 25 million cells are compared with available experimental data. Results suggest that even though URANS and DDES are in good agreement in terms of lift and drag, RANS simulations fail to accurately capture the turbulent wake unsteady characteristics.

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.

Simulation methods ensuring a level of fidelity higher than that of the ubiquitous Blade Element Momentum theory are increasingly applied to VAWTs, ranging from Lifting-Line methods, to Actuator Line or Computational Fluid Dynamics (CFD). The inherent complexity of these machines, characterised by a continuous variation of the angle of attack during the cycloidal motion of the airfoils and the onset of many related unsteady phenomena, makes nonetheless a correct estimation of the actual aerodynamics extremely difficult. In particular, a better understanding of the actual angle of attack during the motion of a VAWT is pivotal to select the correct airfoil and functioning design conditions. Moving from this background, a high-fidelity unsteady CFD model of a 2-blade H-Darrieus rotor was developed and validated against unique experimental data collected using Particle Image Velocimetry (PIV). In order to reconstruct the AoA variation during one rotor revolution, three different methods-detailed in the study-were then applied to the computed CFD flow fields. The resulting AoA trends were combined with available blade forces data to assess the corresponding lift and drag coefficients over one rotor revolution and correlate them with the most evident flow macro-structures and with the onset of dynamic stall.

Interest in offshore vertical axis wind turbines (VAWTs) has created a need to study VAWTs at much higher Reynolds numbers than they have previously been studied at. VAWTs are characterised by unsteady aerodynamics, and high Reynolds numbers have the potential to alter the blade aerodynamics significantly. Here, results are reported on an airfoil that is pitched sinusoidally around zero angle of attack at Re_c = 1 × 10⁶, at a reduced frequency k = 0.15 and amplitudes of 5° < α < 20°. Since the static stall angle is not exceeded, no stall effects occur. Nevertheless, lift, drag and moment coefficients show noticeable hysteresis. Despite the high amplitudes of oscillation, lift and moment coefficients show reasonable agreement with unsteady aerodynamic theories by Theodorsen and Motta et al. with regard to the width of the hysteresis loops, but have noticeably steeper slopes.

High-fidelity unsteady Reynolds-averaged Navier-Stokes (URANS) CFD simulation is employed to investigate the variations in the power performance of two tandem in-line floating offshore horizontal axis wind turbines for the scenario in which the upstream rotor is oscillating in surge motion and the downstream rotor is positioned in a distance of 3D (D: turbine diameter) and is stationary. The rotors are the NREL-5MW reference turbine. The platform surge period and wave amplitude are 9 s and 1.02 m, respectively. The results are presented for 100 full surge periods. It is found that the surge motion of the upstream rotor results in: (i) sinusoidal fluctuations in the power and thrust coefficients (C_P and C_T) of the upstream rotor with a standard deviation (std) of 9.7% and 5.5%, respectively; (ii) such fluctuations in C_P and C_T are less regular with a std of 4.2% and 2.8% for the downstream rotor, respectively. A low-frequency oscillating mode with a period nearly 10 times the surge period is also observed for the downstream rotor. The mean Cp and Ct of the downstream rotor are 28.9% and 38.5% of the upstream one.

Wind tunnel measurements are performed to investigate the potential for mitigation of aerodynamic load fluctuations on airfoils, as the main source of fatigue for wind turbines, using plasma actuators. The experiment consists of aerodynamic force measurements using six strain gauges and 2-component velocity measurements using particle image velocimetry (PIV). The analysis is focused on the aerodynamic loads, the mean flow, the turbulent kinetic energy and the proper orthogonal decomposition (POD) modes and their energy budget. The main findings are: (i) the actuation increased the lift coefficient for all the range of a, with an average and maximum increment of 0.05 and 0.1, per unit span of the actuator; (ii) the actuation deflects the airfoil near wake downward, resulting in the so-called virtual cambering effect; (iii) the actuation increases the TKE near the trailing edge, which could increase the airfoil trailing-edge noise; (iv) the POD analysis reveals that the actuation increases the size of the vortical structures in the near wake and the energy budget of the first POD modes, esp. at high angles of attack prior to stall.

A relatively simple method is presented to predict the maximum two-dimensional drag coefficient of an airfoil only using its shape. The method is based on a contribution related to the leading edge thickness in terms of the y/c coordinate at x/c=0.0125 and a contribution related to the trailing edge flow angle which appears also to be sensitive to the leading edge thickness. The relations were deduced from measurements in the Delft low-turbulence wind tunnel. The first contribution was established using 3 airfoil models with systematically varying leading edge y/c coordinates and a zero trailing edge angle. The second followed from measurements of one of these airfoils equipped with sheet metal flaps of various flap deflections. Compared to measurements found in the public domain differences are found up to ± 2.3% with an average of about -0.2%

For wind turbine rotor blades, the use of strain sensors is preferred over acceleration sensors for the purpose of permanent monitoring. Experimental modal analysis during operation is thus constrained to strain information, yielding strain modal data including strain mode shapes. For follow-up investigations such as aerodynamic load assessment or flutter monitoring it is however advantageous to have this information as displacement mode shapes or as displacements of the blade contour over time. This research applies a generic approach that converts strain mode shapes to displacement mode shapes utilizing an FE shell model as a basis for approximation. The accuracy of the approach is assessed by comparison with experimentally identified high-resolution displacement mode shapes which are acquired with accelerometers and serve as a reference. In the process the conversion procedure is illustrated with the help of strain data that has been obtained using a sensor instrumentation installed for certification testing of the blade. The requirements for successful usage of the employed conversion scheme and its suitability for rotor blade data are discussed.

A method is presented which aims to bridge the gap between overly simplified momentum-based wake models and overly demanding finite volume models of wind turbine wake evolution. The method has been developed to allow an essentially user-defined resolution of the wake. Beyond this, all dominant field quantities are automatically resolved by the solver including convection velocity, shear stress and turbulence intensity. Two distinct methods of solution are presented which both have strengths and weaknesses, the choice of which model being fidelity and application dependent. Both methods make use of multilevel spatial integration to allow greatly improved computational efficiency. The method is here presented for 2D flow in the symmetry plane of a vertical axis wind turbine as an initial demonstration of the potential of the method.

Low-fidelity predictions for vertical-axis wind turbines are affected by uncertainty due to the complexity of the rotor aerodynamics. In particular, in the most common operating conditions the blades undergo periodic excursions beyond the static stall limit, activating dynamic-stall effects. In this study we show how advanced dynamic-stall models, implemented in the frame of the Blade-Element-Momentum theory, are able to upgrade significantly the prediction of low-fidelity tools, both in deterministic and probabilistic terms. In particular, an uncertainty quantification is performed to investigate the epistemic uncertainty of the Strickland dynamic-stall model, introducing a large variability on the empirical parameters appearing in the formulation. The resulting variability in the power coefficient and torque exchange, compared to corresponding wind-tunnel and high-fidelity CFD values, remains relatively limited and, in the conditions around peak efficiency, it is comparable with the measurement uncertainty of the experiment. As a further relevant conclusion, the model uncertainty does not alter the general outcome of the deterministic model, thus demonstrating the robustness of the DMST predictions obtained in the present study.

In the present paper the methodology and the procedures for the implicit coupling of an Actuator Line (AL) aerodynamic code with a beam like structural code for the analysis of wind turbine rotors are detailed. Results from benchmark aeroelastic simulations of canonical inflow conditions, comparing the newly developed AL model against a standard Blade Element Momentum (BEM) model are presented in the paper. The two models provide very similar results in simple, uniform inflow, axisymmetric flow cases. The advantages of this newly developed tool emerge when more complex inflow conditions are addressed. In the present paper, besides axial flow conditions, operation under high yaw misalignment is also considered. BEM model accounts for the effect of the wake skewness through the application of an a posteriori engineering correction. Therefore, in this particular non symmetric flow case, deviations between AL and BEM are expected to be higher, especially as yaw misalignment angles increase. In the paper the above differences are assessed and interpreted.

At DTU Wind Energy, a 12.6 m reference wind turbine rotor blade and the corresponding blade moulds were designed and manufactured. The entire blade design and the related test data will be published according to the FAIR principles. The blade moulds are intended to serve as a blade research and demonstration platform, allowing to realise theoretical concepts and testing these in full-scale on a wind turbine blade and thus enabling a comparison to numerical solutions. In this article, the preliminary blade design progress and process, initial load calculations and a 3D scan of the blade molds are presented.

The root area of a wind turbine rotor is usually constructed by very thick airfoils with a relative thickness of more than 35%. An airfoil at that thickness is characterized with strong flow separation. To solve this issue, vortex generators can be used as an active control in order to stabilize the airflow and improve the aerodynamic performance, consequently. Boundary layer control however, investigated employing numerical techniques, strongly depend on the employed method as well as the mesh especially due to a weak vortex conservation in the RANS model. Using the method of Delayed detached eddy simulation (DDES) the solution can benefit from effort-efficient boundary layer modeling as well as LES vortex resolving apart from the surface. The present studies aims to investigate suitable numerical approach mesh dependencies in comparison to experimental data, for application to thick airfoil in future studies.

This work aims to give an estimate of to heating power used in the anti-icing system of the NREL 5MW wind turbine rotor operating in a cold climate in the presence of suspended water particles in the air. In this work, the turbine blade was simulated using an in-house code implemented in OpenFOAM framework that uses compressible Lagrange-Euler approach to simulate water particles transport from the atmosphere to the blade surface. On the water film side, Shallow-Water Icing Model (SWIM) is used to simulate the icing and anti-icing process. The results show that at least 3% of the rated power of the rotor should be used just to keep the water film from freezing. It shows also that most of the heating power should be concentrated on the leading edge, lower surface of the blade, and the middle sections of the blade.

The offshore wind market is developing towards exploiting wind resources in deeper water sites. Inevitably, this fuels new research and feasibility studies on alternative solutions for the wind turbine support structures. Within this context, this work aims at comparing three different support structure design concepts for a 14 MW two-bladed downwind wind turbine: an XXL monopile, a hybrid jacket-tower and a lattice tower. To ensure a fair comparability of the three design concepts, a load capacity analysis is first performed to assess the yield strength of each concept and guarantee a similar material utilization. The comparative analysis is then carried out in terms of total mass, dynamic behaviour of the interaction between rotor and support structure, soil-structure interaction and resulting hydrodynamic forces. Based on the given design constraints, the preliminary results of this study favour a lattice tower solution. The study also highlights peculiar dynamic phenomena such as veering, mode hybridization and mode coalescence for the dynamic interaction between the lattice tower and the two-bladed rotor which need to be taken into account during the design phase.

Passive flow separation control with vortex generators (VG) is actively used over the wind turbine blade. In this paper, the effect of vortex generators is simulated on a full-scale 2-blade wind-turbine tested at the National Renewable Energy Laboratory. The simulation is performed using Very-Large-Eddy/Lattice-Boltzmann method (VLES/LBM). The analysis focuses on the effect of vortex generators on the aerodynamic performance and far-field noise. The simulation results without vortex generators are compared with the experimental results, reaching good agreement. The vortex generators produce counter-rotating vortices in the wake which effectively delay flow separation, leading to better aerodynamic performance. The acoustic analysis indicates that the dominant noise sources are the tonal noise produced by the flow separation and the turbulent-boundary-layer trailing-edge noise. Similar noise levels are obtained for the configurations with and without vortex generators.

Improvements to the Sandia blade aeroelastic stability tool have been implemented to predict flutter for large, highly flexible wind turbine blade designs. The aerodynamic lift and moment caused by harmonic edge-wise motion are now included, but did not change the flutter solution, even for highly flexible blades. Flutter analysis of future, large blade designs is presented based on scaling trends. The analysis shows that flutter speed decreases at a rate similar to maximum rotor speed for increasing blade sizes: ${\Omega }_{flutter}\,\infty \,{\Omega }_{rated}\,\infty \,\frac{1}{L}$. This indicates the flutter margin is not directly affected by blade length. Rather, it was innovative design technology choices that predicted flutter in previous studies. A 100 m blade, flexible enough to be rail transported, was analyzed and it exhibited soft flutter below rated rotor speed. This indicated that excessive fatigue damage may occur due to limit cycle oscillations for blades that incorporate highly flexible designs.

Historically, cost reduction in wind energy has been accomplished by increasing hub heights and rotor diameters to capture more energy per turbine. However, larger wind turbines cannot be expected to lead to lower LCOE without the addition of new technologies. Capital costs grow rapidly with rotor diameter, faster than the rated power, because as rotor diameter increases, the blades get heavier and more costly. The growth in blade mass with blade length is accelerated by the additional structure that must be added to withstand unsteady aerodynamic loads caused by turbulence, gusts, wind shear, misaligned yaw, upwind wakes, and the tower shadow. This paper presents a holistic design solution to integrate active load control via dielectric barrier discharge (DBD) plasma actuators into wind turbine rotors along with initial findings on load reduction, actuator development, and rotor mass reduction.

The U.S. offshore wind industry can expect higher costs due to the lack of domestic experience with offshore wind technology. A key factor of the capital expenditure related to offshore wind farms is the cost of the support structures of offshore wind turbines. Therefore, improvements to the reliability of support structures under ultimate and fatigue loading conditions will help reduce the levelized cost of energy of offshore wind. This study presents a framework that accounts for the wind directionality by assuming a distinct and independent wind speed distribution per each wind direction and investigates its effect on the fatigue life of offshore wind turbine support structures. A monopile support structure in a potential wind site close to a National Oceanic and Atmospheric Administration buoy in the north-eastern US waters is used in this study. Fatigue damage assessment is performed for the normal operational condition of wind turbine, and the results are presented considering both cathodic protection and free corrosion conditions at the mudline level of the monopile. The location and extent of the predicted fatigue damages are found to vary due to accounting for the wind directionality.

Floating offshore wind farms (FOWFs) are subject to wake effects similar to onshore and fixed-bottom offshore wind farms. Due to the smaller surface roughness, wake recovery is slower in offshore compared to onshore wind farms. Therefore, wake mitigation methods will become relevant for FOWFs as it is for onshore and fixed-bottom offshore wind farms. A common method is to apply yaw misalignment to steer the wake out of the rotor of the downwind turbine. However, it is necessary to investigate possible effects of yaw misalignment on a single turbine before farm-level investigations. This study focuses on the motion response of the platform and damage equivalent loads (DELs) of the mooring line tensions of the OO-Star Wind Floater Semi 10MW floating wind turbine. In order to satisfy environmental variability, the effect of yaw misalignment is investigated at different wind directions and wind/wave misalignments. DELs of three mooring line tensions at the fairleads were calculated using the rainflow algorithm and were reported as relative, normalized with respect to the aligned nacelle operation. Floater motions were analyzed to understand effects of misaligned operation. The results indicate that yawed inflow leads to a complex coupled response of floating offshore wind turbines (FOWTs). At slightly above rated wind speed, the yaw misalignment generally increases DELs due to the thrust increase. Conversely, yaw misalignment operation at below rated wind speeds leads to different responses on lines depending on environmental conditions. The steady state floater position has a large impact on DELs which may lead to a yet unknown response behavior of FOWFs. DEL trends in the presence of wind/wave misalignment are similar to those of aligned wind and wave.

Many factors that influence the effect of leading edge erosion on annual energy production are uncertain, such as the time to initiation, damage growth rate, the blade design, operational conditions, and atmospheric conditions. In this work, we explore how the uncertain parameters that drive leading edge erosion impact wind turbine power performance using a combination of uncertainty quantification and wind turbine modelling tools, at both low and medium fidelity. Results will include the predicted effect of erosion on several example wind plant sites for representative ranges of wind turbine designs, with a goal of helping wind plant operators better decide mitigation strategies.