bluuebunny/crossref_metadata_2025 huggingface dataset:
Wind tunnel experiments for investigating wake effects in atmospheric boundary layers using a simplified miniature model wind turbine
Authors: Zúñiga Inestroza M A , Mattuella J M , Wittwer A R , Loredo-Souza A M
Published: June 2022

The clustering of wind turbines within a prospective site is prone to significant wake interactions, which have a negative impact in terms of loads and plant performance. Further improvements in the design and control of wind farms can be achieved through a detailed assessment of wake effects under site-specific conditions. The aim of this paper was to investigate wake effects in atmospheric boundary layer (ABL) flows using a simplified model wind turbine. First, we performed experiments under smooth- and turbulent-uniform inflows to highlight the wake features of the model. Second, we evaluated the interactions with two different neutrally stratified ABL: i) ABL Type-I (α = 0.11, TI = 6 %), ii) ABL Type-II (α = 0.23, TI = 13 %), where α is the incident wind shear and TI is the turbulence intensity. For each case, hot-wire anemometry was used to obtain wake mean velocity, turbulence intensity and power spectrum profiles at four different downstream positions. We also included a comparison with three analytical wake models to get further indications on the experimental measurements. First results showed that the simplified model is well-suited for its intended purpose. The largest discrepancies between experiments and model predictions were in the near-wake region and in cases with lower background turbulence. We also corroborated a strong influence of the incoming wind profile on how fast the wake recovers to the undisturbed conditions. Furthermore, this study provided an initial framework to explore the ABL wind tunnel at Universidade Federal do Rio Grande do Sul (UFRGS) as a viable tool for research in wind energy.
Measurements and modeling of friction torque of wind turbine blade bearings
Authors: Menck O , Behnke K , Stammler M , Bartschat A , Schleich F , Graßmann M
Published: June 2022

The friction torque of rotor blade bearings is a required parameter for the design of pitch actuators and may provide information about continued degradation and impending failure of the bearing. The torque is heavily influenced by the operating conditions and external loads acting on the bearing. Test results for real-size bearings under realistic loads are rare. This paper presents test results of various double-row four-point bearings of three different diameters, ranging from 0.7m up to 5m. They are loaded with bending moments and axial loads, and their behavior at different speeds is compared. For the same bearing type, large differences are observed at zero load that decrease for higher loads. At lower contact pressures, the change from four-point into two-point contact is clearly visible and results in a temporary decrease of the torque. To monitor potential degradation of the bearing, an empirical model that can be fit to a particular bearing is proposed.
An aeroacoustics-based approach for wind turbine blade damage detection
Authors: Zhang Yanan , Avallone Francesco , Watson Simon
Published: June 2022

In this work, aimed at the development of an aeroacoustics-based wind turbine blade damage detection approach, the noise scattered from two airfoils with damage at the trailing edge or at the leading edge is investigated. Four trailing edge cracks (with width of 0.2, 0.5, 1.0 and 2.0 mm) and four leading edge erosion configurations (consisting of gouges and delamination) are investigated for a NACA 0018 and a DU96 W180 airfoil. Experiments are carried out under clean and turbulent inflow conditions. Acoustic measurements are performed in an anechoic wind tunnel with a microphone array. The trailing edge crack causes a tonal peak at trailing-edge-thickness-based Strouhal number approximately equal to Sth
∼ 0.1 under clean and low turbulence intensity inflow conditions (e.g. ∼4% in this study). For a higher turbulence intensity (e.g., ∼7%), the tonal peaks are not detectable. For the leading edge erosion case, under clean inflow conditions and minor damage levels, the amplitudes of the harmonics in the trailing edge noise spectra increase compared with the baseline. For moderate damage levels, the harmonics on the suction side shift to higher frequencies with lower amplitudes. For the highest damage levels, only broadband characteristics are present, where low-frequency contributions increase and high-frequency contributions decrease as the damage level increases. When introducing turbulent inflow, the leading edge impingement noise level decreases at medium-high frequency (above 1000 Hz) with increasing levels of erosion.
Wake rotation impacts on wake decay
Authors: Schutz W M , Naughton J W
Published: June 2022

This study considers the behavior of a well-conditioned swirling turbulent wake. A custom swirling wake generator was used to produce a swirling wake absent of a tower that complicates the flow regime. Four swirling wakes were studied with swirl numbers ranging from 0.19 to 0.37 and compared to a non-swirling counterpart. Two-component Laser-Doppler Anemometry was used to measure mean and turbulent quantities in the wake. Measured profiles were analyzed based on similarity theory applied to a swirling wake. The results show that wake behavior was impacted by the initial swirl strength. Generally, wake growth rate and axial deficit decay rate increased with higher levels of swirl, and tangential velocity decay rate increased with lower levels of swirl. However, the wake that diffused fastest was the case with a swirl number of 0.27.
Design and testing of a model-scale yaw mechanism for an experimental wind turbine model
Authors: Mühle Franz V. , Campagnolo Filippo , Llobell Buigues Juan , Bottasso Carlo L.
Published: June 2022

In this study a new yaw mechanism for a small existing wind turbine model is designed and tested. The special requirement for this yaw device is its compact design due to the small length scaling factor of the model turbine. Such a dense design is crucial in order for the wake of the model turbine not to be influenced by the yaw mechanism. Furthermore, it has to have a fast system response and high accuracy to take into account the time scaling. Different concepts are investigated and a design located at the tower base is realized. To limit its drag, the structure is being concealed in an aerodynamically shaped cover. Wind tunnel measurements confirmed that the wake of the turbine is not influenced by the structure. A system analysis, using a Simulink model of the turbine helps finding the right motor and shows that the device has a fast system response with high accuracy. Consequently, the designed yaw mechanism is suited for the small wind turbine model, which can be used for experimental wake and wind farm control studies in the future.
Inflow and pressure measurements on a full scale turbine with a pressure belt and a five hole pitot tube
Authors: Madsen H. Aa. , Barlas T. , Fischer A. , Olsen A.S. , Gomez Gonzalez A.
Published: June 2022

We present an autonomous add-on measurement system for detailed aerodynamic measurements on full scale turbines. From the measured data we can derive the local aerodynamic coefficients for the blade section and e.g. compare with wind tunnel data for a similar section. This forms the basis for evaluating how well the airfoil performs on a rotor in the turbulent inflow and rotating environment. We describe the measurement system which comprises a pressure belt with 15 taps, a flyboard with the data acquisition system and a five hole pitot tube measuring the local inflow to the blade section. The system was used on a SWT-4.3-120 DD turbine in a short campaign in June 2021. A trailing edge flap system is installed on that turbine and a particular objective with the measurements was to evaluate the static and dynamic performance of the flap system. The aerodynamic measurements were correlated with the SCADA and flap actuation data. Overall the measurement system performed well and provided good data like smooth pressure distributions. The derived flap performance in the form of the delta lift coefficient was close to what has been measured in wind tunnel tests. Finally, the aerodynamic and aeroelastic dynamic time response of the flap actuation could be characterized with good precision.
Effect of turbulence on the performance of a pair of vertical-axis wind turbines
Authors: Talamalek Ayoub , Runacres Mark C. , De Troyer Tim
Published: June 2022

The focus of this study is to investigate under high turbulence intensity the performance and wake characteristics of a pair of closely spaced counter-rotating vertical axis wind turbines (VAWTs) in wind tunnel-controlled conditions. To achieve this, wind tunnel experiments for an isolated VAWT and dual VAWTs are conducted. A squared-mesh wooden grid is placed upstream of the wind turbines to increase the level of free stream turbulence. The wind tunnel tests confirm the increase in power coefficient of paired VAWTs in turbulent flows compared to smooth flows. Additionally, this study analyses the near-wake downstream of the paired turbines in both smooth and turbulent flows to better understand the flow physics.
Dynamic optical deformation measurements on wind turbines
Authors: Helming P , Poeck N , v Freyberg A , Sorg M , Fischer A
Published: June 2022

Because of an increase in the size of wind turbines in recent years, efficient structural health monitoring of the turbine’s structure has become critical. Wind turbine towers deform elastically as a result of the loads due to wind and inertial forces acting on the revolving rotor blades. To adequately assess these deformations, an earthbound device is desired that can measure the tower’s movement in two directions from a very large working distance of over 150 m and a single site. For this task, a terrestrial laser scanner in line-scanning mode with horizontal alignment is used to measure the tower cross-section and to determine its axial (in the line of sight) and lateral (transverse to the line of sight) position using a least-squares fit. The method is validated by comparing the data obtained by the laser scanner with continuously measured wind speed data. To test its application, the method is then applied to two separate wind turbines with hub heights of 61.4 m and 128 m and the experimental results are compared with a reference video measurement that records the nacelle movement from below and determines the nacelle movement using point-tracking software. The results are compared for various operating circumstances, such as different wind speeds. The laser-based and the reference measurement have a strong correlation. The measurement uncertainty is determined to be 9.6 mm in the lateral direction and 10.3 mm in the axial direction at a measurement distance of 160 m, respectively. As a result, the proposed approach is identified as an effective tool for the in-process structural health monitoring of wind turbine towers.
Field Study of Turbulence Intensity measurement by Nacelle Mounted Lidar (NML)
Authors: Liang Zhi , Cote Robin , Cariou Jean-Pierre
Published: June 2022

In this paper, Nacelle Mounted LiDAR (NML) was used to study Turbulence Intensity (TI) by dataset from three measurement campaigns, using two approaches: the white box test and the black box test. The white box test was to compare TI LOS using each Line Of Sight, while the black box test was to compare TI Hub using all LOS by relative reconstruction algorithm at wind turbine hub height. The result of white box test showed that the correlation coefficient of TI LOS varied from 0.923 to 0.955 and the slope varied from 0.978 to 1.030. The result of black box test showed that the correlation coefficient of TI Hub is 0.8337∼0.8924 and the slope is 0.904∼0.935. This study provided some promising results to wind energy community, which could be the evidence base for the further investigation for the temporal-spatial turbulence structure in low Atmospheric Boundary Layer.
Evaluation of a Hardware-in-the-loop Test Setup Using Mechanical Measurements with a DFIG Wind Turbine Nacelle
Authors: Neshati Mohsen , Zhang Hongkun , Thomas Philipp , Heller Marcel , Zuga Adam , Wenske Jan
Published: June 2022

The impact of system test benches on the development and validation of wind energy converters is significantly increasing, and that is due to the higher availability as well as the provided powerful actuator and emulation systems. However, the question remains to be answered about how consistent measurements in the laboratory are. Especially in the case where no field measurements are available for prototypes. The challenge in this regard is the unknown accuracy of simulations and the lack of an experimentally calibrated torque measurement in the hardware-in-the-loop (HiL) setup. The presented paper handles this topic and presents an evaluation method to elaborate measurement consistency independent from field tests. A transparent and comprehensive comparison is provided, taking advantage of the deterministic test environment enabled by the HiL system and the dedicated designed torque measurement adapter. Provided results demonstrate consistent measures obtained from the real-time simulation, HiL control system, and the electrical and mechanical measurements, where a low deviation of up to 2 % is partly observed.
Lifetime fatigue response due to wake steering on a pair of utility-scale wind turbines
Authors: Dana Scott , Ivanov Hristo , Doubrawa Paula
Published: June 2022

Quantifying the impacts on turbine loads during wind farm control is an important consideration in assessing power production benefits. Wake steering controls aim to improve total wind farm performance by coordinating the control actions of individual turbines, wherein an upstream turbine is intentionally yawed at an offset angle from the measured wind direction. Consequently, this redirects its wake for improved power production and potentially reduces fatigue loads of the downstream turbines. This paper studies the lifetime fatigue loads associated with wake steering by using utility-scale wind turbine experimental data to conduct an analysis on a pair of wind turbines. This study was part of a large field experiment in which a group of five GE 1.5-MW SLE CWE turbines were selected as targets for conducting wake steering research. A standard loads instrumentation package and data acquisition system were installed on two turbines within the cluster to measure turbine fundamental loads. The time-series databases were used to calculate loads statistics as well as short-term and lifetime damage equivalent loads. Fatigue calculations followed the guidance in Annex H of the International Electrotechnical Commission standard 61400-1, Edition 4. Lifetime fatigue calculation results are presented in this analysis; three methods of assessing lifetime fatigue were used to determine percent difference for blade root moments, main shaft moments, main shaft torque, and tower top torque. For all three fatigue treatments, some components’ lifetime fatigue increases for the controlled turbine; however, the downwind turbine experienced a reduction in lifetime fatigue and combined effect for the turbine pair results in a reduction of fatigue when wake steering controls are applied.
Comparison of dynamic stall on an airfoil undergoing sinusoidal and VAWT-shaped pitch motions
Authors: Brunner C E , Kiefer J , Hultmark M
Published: June 2022

The aerodynamics of vertical axis wind turbines (VAWTs) are inherently unsteady because the blades experience large angle of attack variations throughout a full turbine revolution. At low tip speed ratios, this can lead to a phenomenon known as dynamic stall. To better characterise the unsteady aerodynamics and represent them in models and simulations, data from studies of individual static or pitching airfoils are often applied to VAWT blades. However, these studies often involve sinusoidally pitching airfoils, whereas the pitching motions experienced by VAWTs are more complex. Here, the pressures and forces on an airfoil undergoing VAWT-shaped pitch motions corresponding to various tip speed ratios are compared to those of a sinusoidally pitching airfoil in order to assess whether a sinusoidal motion represents a reasonable approximation of the motions of a VAWT blade. While the lift development induced by the sinusoidal motion yields good agreement with that induced by the VAWT-shaped motion at the higher tip speed ratios, notable discrepancies exist at the lower tip speed ratios, where the VAWT motion itself deviates more from the sinusoid. Comparison with sinusoidal motions at reduced frequencies corresponding to the upstroke or downstroke of the VAWT-shaped motion yield better agreement in terms of the angle of stall onset.
A strongly coupled Eulerian Lagrangian method applied in unsteady 3D external flows around Wind Turbine rotors
Authors: Spyropoulos Nikos , Vlastos Dimitris , Papadakis George , Prospathopoulos John M. , Riziotis Vasilis A.
Published: June 2022

This paper presents a hybrid CFD solver that couples a standard Eulerian approach that solves the compressible Navier-Stokes equations under a cell-centered finite volume discretization, with a Lagrangian one, based on particles representation which carry mass, pressure, dilatation and vorticity. The velocity field is calculated using the Helmholtz’s decomposition theorem. Computational performance is enhanced by employing the Particle Mesh (PM) technique in order to solve the Poisson equations for the scalar potential ϕ and the stream function ψ. Even though validated for 2D flows over airfoils, this specific solver is used for the first time in order reproduce the flow around a wind turbine rotor. The validation simulations concern axial flow over the wind turbine model rotor used in the New MEXICO experimental campaign. Results of the hybrid solver, presented as blade pressure distribution and axial flow velocities are compared against the ones produced by its pure Eulerian counterpart and experimental measurements. PM grids of up to 5 points per chord of the blade section at 75% radius have been used. Comparison with the standard Eulerian solver suggest that the produced blade loads are over-predicted by approximately 7% near the tip and 14% near the root. However, the calculated velocity field is much closer to the experimental measurements as compared to the one produced by the Eulerian approach, which is attributed to the reduced numerical diffusion of the Lagrangian-vorticity formulation.
Sensitivity analysis of geometrical design parameters on the performance of conical plain bearings for use as main bearings in wind turbines
Authors: Rolink Amadeus , Jacobs Georg , Pérez Alex , Bosse Dennis , Jakobs Timm
Published: June 2022

The failure of a wind turbine’s main bearing causes very high repair costs, as the entire drivetrain has to be disassembled with the help of an external crane. Just the costs for a crane for an offshore turbine can sum up to 200.000€ per day [1]. In contrast, segmented hydrodynamic plain bearings offer the advantage that individual bearing segments can be replaced on the tower without the use of an external crane, which offers great potential for savings in repair costs. However, this application requires new, innovative plain bearing concepts for which the current design guidelines are not applicable. This paper presents a simulation-based design approach for a plain bearing concept with conical sliding surfaces. Such a simulation-based approach with a multi-objective optimization is necessary, because due to the geometric complexity the manual, iterative design process is not efficient. In this paper, a bearing geometry optimized in terms of load distribution and friction losses was developed by applying this approach.
Impact of rotor size on aeroelastic uncertainty with lidar-constrained turbulence
Authors: Rinker Jennifer M.
Published: June 2022

Nacelle-mounted lidar measurements offer the opportunity to tailor turbulence to specific conditions, reducing aeroelastic uncertainty in one-to-one wind turbine validation and opening the door for novel lidar-based control methodologies. Despite this, the use of lidar to generate constrained turbulence is not commonplace, partially due to a lack of readily available tools. Kaimal-based constrained turbulence methods—such as the one implemented in the open-source constrained-turbulence generator PyConTurb—can be used to easily generate turbulence constrained to measurements in a matter of minutes. Unfortunately, the limitations of the Kaimal-based methods prevent the direct use of the lidar data as constraints. This paper therefore presents a preprocessing methodology to convert lidar data to PyConTurb-ready constraints. The method is demonstrated by quantifying the aeroelastic uncertainty for the DTU 10 MW and the NREL 5 MW without constraints and comparing it to the corresponding value with lidar constraints. The results show an excellent reduction in the one-to-one aeroelastic uncertainty and an adequate reduction on fatigue loads when lidar-constrained turbulence is used as inflow. The NREL 5 MW is found to have better reduction in uncertainty due to a rotor size that is better suited to the lidar geometry.
Characterization of a Fast Response Probe for Inflow Turbulence Measurements on Wind Turbine Blades
Authors: Sinem Özçakmak Özge , Fischer Andreas , Madsen Helge Aagaard
Published: June 2022

The performance of a wind turbine, its aerodynamic and acoustic predictions, and expected power output are affected by atmospheric turbulence. Previous studies showed that the installation of high-frequency measurement devices on blades is needed in order to analyze the effect of the inflow turbulence on boundary layer transition and on noise emissions. Therefore, the accuracy of a novel 5-hole high-frequency pitot tube designed to measure high-frequency velocity fluctuations is assessed by testing its capability to determine flow angles and velocity components in a well-defined wind tunnel setup. Moreover, the vortex shedding frequency and magnitude for the flow behind a cylinder and wire are analyzed to evaluate its performance for various turbulent flow scenarios. It is seen that the flow angles and the velocity components can accurately be calculated. The peaks observed in the spectra sampled at high frequencies that are relevant for boundary layer analysis demonstrate the characteristics of the flow with high turbulence intensity values. As the probe is validated for inflow turbulence measurements, it will be used on a wind turbine blade to identify the relevant frequencies that are induced by the atmospheric turbulence into the boundary layer and to improve wind turbine noise emission models.
Test of an active flap system on a 4.3 MW wind turbine
Authors: Gomez Gonzalez A. , Enevoldsen P. , Barlas T.K. , Madsen H. Aa.
Published: June 2022

This article describes a field test performed on a 4.3 MW wind turbine with one blade equipped with an active flap system. The results presented focus primarily on the steady state and periodic characterization of the active flap system in different deflection states. The tests were performed between 2020 and 2021 on a test turbine in northern Denmark in close-to-shore conditions. The active flap system is primarily characterized based on the load response of the flapwise bending moment of the blades. Flap actuations are performed both as open-loop on-off swaps (shifting between two different flap positions at fixed time intervals) and as closed-loop 1P activations (cyclic activation of the flap both in phase and counterphase with the blade azimuthal position). For the steady state swap tests, the impact on flapwise bending moment ranges between 3% and 20% depending on the flap actuation level and the wind speed. For the cyclic flap tests, the load impact is based on blade-to-blade fatigue load comparison, showing a potential load reduction of approximately 13%.
Innovative aerodynamic rotor concept for demand-oriented power feed-in of offshore wind turbines
Authors: Ribnitzky Daniel , Berger Frederik , Kühn Martin
Published: June 2022

We introduce an aerodynamic rotor concept for a 15 MW offshore wind turbine which is tailored for an increased power feed-in at low wind speeds. The main objective of the conceptual design is to limit the stationary loads (blade flapwise root bending moment (RBM) and thrust) to the maximum value of the IEA 15 MW offshore reference turbine, while greatly increasing the swept rotor aera. The outer part of the blade (e.g. outer 30% of the rotor) is designed for a higher design tip speed ratio (TSR) and a lower axial induction than the inner part. By operating at the high TSR in light winds, the slender outer part fully contributes to the increased power capture. In stronger winds the TSR is reduced and the torque generation is shifted to the inner section of the rotor. Moreover, the blade is designed in a way that makes the limitation of the flapwise RBM through peak shaving aerodynamically more efficient. With static blade element momentum simulations the characteristics of the rotor are investigated and the economic revenue of the turbine is estimated, considering a wind speed dependent feed-in price. Our results show that the revenue can be increased by 30% compared to the reference turbine with an increase of rotor diameter by 36%.
Influence of soil properties on the shift in natural frequencies of a monopile-supported 5MW offshore wind turbine under scour
Authors: Jawalageri Satish , Jalilvand Soroosh , Malekjafarian Abdollah
Published: June 2022

This paper studies the change in the natural frequencies of the NREL 5MW monopile-supported offshore wind turbine (OWT) due to scour. The natural frequencies are derived from accelerations arising under wind and wave loads, for a range of different soil properties. A model of the wind turbine is developed using OpenFAST and SESAM software which considers aerodynamic and hydrodynamic loading with soil-structure interaction. The superstructure behaviour is modelled using OpenFAST and the foundation behaviour is modelled with SESAM, where soil-structure interaction is incorporated using p-y springs. The influence of soil density and scour hole depth on the obtained natural frequencies are studied. Accelerations are extracted at nine locations along the tower resulting from applied wind and wave loads. The first two system natural frequencies are estimated using frequency domain decomposition (FDD) applied to the acceleration time-histories for each scour depth and soil type. It is shown that the increase in scour depth around the foundation of the OWT leads to a decrease in natural frequencies when compared to the case where there is no scour.
Leading Edge Microspoilers for Load Dumping in Wind Turbine Rotors
Authors: Mertz B , Bortolotti P , Abbas N J , Johnson N , Paquette J , Jordan W
Published: June 2022

As rotor designs have grown in size and flexibility, new design challenges have arisen that could benefit from novel control strategies. Leading edge microspoilers (LEMS) have been shown to effectively reduce forces on airfoils by inducing stall at lower angles of attack. This paper investigates how LEMS can be used to reduce blade loading on large, flexible, downwind rotors during design-limiting design load cases. During shutdown in turbulent flows near cutout speed, LEMS could reduce the tip deflection spikes toward the tower by over 50% and keep flapwise root bending moments below normal operating conditions, thus eliminating these as design-driving load cases. In power producing load cases, deploying LEMS for a short period of time during a load spike can be effective to reduce toward-tower tip deflections without significantly impacting power produced, assuming these load spikes can be identified by a controller. Leveraging this technology could allow for lower specific power machines to be designed.
Repair of Wind Turbine Blades: Costs and Quality
Authors: Mishnaevsky Leon , Bendixen Brian , Mahajan Puneet , Fæster Søren , Johansen Nicolai Frost-Jensen , Paul Daniel , Fraisse Anthony
Published: June 2022

Repair and maintenance operations of wind turbines constitute a significant part of costs of wind energy. In this paper, technologies of structural repair of damaged wind turbine blades are reviewed. Costs of repair, and technological contribution to the costs are discussed. Technologies of repair are compared, including hand layup lamination, vacuum repair with hand layup and infusion, ultraviolet curing and high temperature thermal curing systems. Computational models of repaired blades, and curing as kinetic process are presented. Void formation during repair and curing, and the void influence on the post-repair reliability of blades is discussed.
Analytical determination of the influence of geometric and material design parameters on the stress and strain fields in non-prismatic components of wind turbines
Authors: Migliaccio G
Published: June 2022

Non-prismatic beamlike elements have long been used in engineering applications to optimize the performance of structures such as wind turbines, aircraft, and civil bridges, just to mention some examples. Unfortunately, engineering methods and formulas commonly used to analytically evaluate stresses and strains in prismatic beams do not hold and provide incorrect results for non-prismatic geometries. Large displacements and non-uniform material properties further complicate the analytical prediction. In order to determine the state of stress and strain in non-prismatic beamlike elements and its dependence on important design factors (such as taper parameters), a suitable mapping of the shape of such elements and a variational approach are used. The resulting field equations are exploited to derive application-oriented stress-strain formulas. Examples and comparisons with results of non-linear 3D-FEM analyses confirm the effectiveness of the modelling approach and of the new formulas proposed in this work.
Reduction of gearbox loads of a DFIG wind turbine during grid faults with optimized converter configurations
Authors: Röder J , Jacobs G , Bosse D , Izal J L R
Published: June 2022

Grid faults in wind turbines (WT) with doubly fed induction generator (DFIG) result in dynamic generator torque excitations, which can lead to dynamic load changes within the gearbox. Dynamic load changes in combination with changing rotational speeds can increase the risk of damage in the gearbox. WT gearbox damage occurs mainly on the high speed shaft (HSS) components. The torque excitations have the highest influence on the HSS since it is coupled to the generator. Therefore, an investigation of the correlation between grid faults and gearbox damage is necessary. The torque excitation in DFIG WTs due to grid faults is dependent on the converter and its fault ride through capabilities. The load analysis in this paper is done for a state of the art converter configuration and for one that is optimized in order to stabilize the performance during grid faults. It is shown via simulation with a WT drivetrain model that dynamic load changes of the HSS gear wheel are prevented for symmetrical grid faults with the optimized configuration. The analysis of a HSS bearing shows that the smearing damage risk can be significantly reduced (minus up to 56 percent) by using the optimized configuration. Therefore, the possibility to decrease the gearbox damage risk during grid faults via an optimization of the converter configuration is shown in this paper.
Dynamic inflow and unsteady aerodynamics models for modal and stability analyses in OpenFAST
Authors: Branlard E , Jonkman B , Pirrung G R , Dixon K , Jonkman J
Published: June 2022

This work presents two aerodynamic models that are used to perform modal and stability analyses of wind turbines. Including aerodynamic effects in stability analyses is important to capture aerodynamic inertia, stiffness, and damping. The two models consist of a dynamic inflow model based on Øye’s model and an unsteady aerodynamics model based on a simplified Beddoes-Leishman dynamic stall model. Both models are written in a continuous-time state-space form, which is the form needed to perform linearizations within OpenFAST. In this article, we present the new models, perform some simple verifications against HAWC2 results, and then perform stability analyses. We investigate the contribution of the aerodynamic effects on the turbine modes, particularly the flapwise modes, which are the most affected. Because of the development presented in this work, Campbell diagrams accounting for dynamic stall and dynamic wake effects were generated for the first time using OpenFAST.
Potential of Mini Gurney Flaps as a Retrofit to Mitigate the Performance Degradation of Wind Turbine Blades Induced by Erosion
Authors: Papi Francesco , Melani Pier Francesco , Alber Jörg , Balduzzi Francesco , Ferrara Giovanni , Nayeri Christian Navid , Bianchini Alessandro
Published: June 2022

Leading edge erosion of wind turbine blades is still an important challenge for wind energy professionals, both at research and industrial level. While the efficiency and durability of materials and coatings are improving rapidly, it is important to explore innovative solutions at the aerodynamic design level to mitigate the adverse effects of surface erosion. For that, a preliminary analysis on the use of Mini Gurney Flaps (MGFs) is presented in the study. High-fidelity Computational Fluid Dynamic (CFD) simulations are used to evaluate the impact of severe leading edge (LE) erosion on the performance of the FFAW3-241 airfoil on the rotor blade of the DTU 10-MW Reference Wind Turbine, which is used as test case. CFD lift and drag polars of the eroded airfoil show that MGFs are able to partially recover aerodynamic efficiency caused by erosion; this suggested evaluating their use as a retrofit solution for blades that already experienced leading edge erosion damage. When tested on the DTU 10MW RWT blade, results show how, if sized correctly, MGFs perform as predicted: the lift curve is shifted back to its design value and performance is improved with respect to the eroded blade. Moreover, as one would expect, higher than optimal MGFs resulted in excessive lift increases and thus decreased performance, even in the case of LE-erosion. Although these devices behave as intended however, based on the results of this paper, performance decreases are noted at high tip-speed-ratios (TSR), due to the blade operating in off-design conditions.
How does the quantity, resolution, and scaling of turbulence boxes affect aeroelastic simulation convergence?
Authors: Liew Jaime , Larsen Gunner Chr.
Published: June 2022

Aeroelastic wind turbine simulations are extensively used in the turbine design process to estimate structural fatigue loading. While it is assumed that more stochastic realisations (seeds) and higher resolution simulations will converge to the ‘true’ result faster, a detailed study has not been published to support how many seeds and what resolution is required to achieve an arbitrary level of convergence. In this study, we perform a comprehensive study on the convergence of turbine fatigue statistics as a function of turbulence grid resolution, number of seeds, and the type of turbulence scaling. The study is performed using the HAWC2 aeroelastic code in conjunction with Mann turbulence fields of varying resolution. Key findings are that over 21 seeds are required to ensure a standard error of the mean of less than 5% for fatigues statistics, and a turbulence resolution of at least 2048×64×64 (in the longitudinal, lateral and vertical, respectively) is required to ensure statistical convergence of a 10 minute simulation on one turbine. The presented study provides a valuable insight into the uncertainties involved in turbine fatigue load calculations, as well as recommendations on how to mitigate them.
Effect of erosion morphology on wind turbine production losses
Authors: Saenz E , Mendez B , Muñoz A
Published: June 2022

The impact of wind turbine (WT) blade erosion is still a major issue today because of the associated high maintenance costs and the loss of power production. The erosion process is influenced by many physical phenomena that still need to be understood better. Especially WT manufactures and owners need to know the potential impact of different levels of blade erosion on annual energy production (AEP). The damage produced by erosion modifies the aerofoil shape and therefore its aerodynamic properties, resulting in an AEP drop. In this study, the effect of various morphologies of erosion on AEP was obtained. First, a 3D characterisation of eroded blades using a laser scanner was performed. The erosion damages were grouped into two typologies: Typology 1 covers pits and gouges, and Typology 2 covers extended loss of top coat. Based on this, 2D section geometries were defined to evaluate the effect of erosion on the aerofoil aerodynamic 2D properties, which was done using the CFD code WMB. The aerofoil used as a reference was DU96W180. It was determined that the erosion damage alters the aerodynamic behaviour of the aerofoil by reducing the Cl max and the slope of the Cl- α curve, which resulted in a drop of the aerodynamic efficiency from 40% to 72%. These detrimental effects increased depending on the damage location (i.e., pressure side and closeness to the leading edge), damage size and shape (i.e., sharp transitions from the eroded to non-eroded surfaces). To evaluate the erosion effect on the AEP, the NREL 5 MW WT was selected. All its components were kept except the last 30% outer section of the 61.5 m blade whose NACA64_A17 aerofoil was replaced by a DU96W180. The erosion damages were modelled by extending the 2D eroded geometry along this outer section. The AEP was calculated using BLADED 4.5. It was concluded that erosion resulted in a decrease of the AEP, which can be up to 1% and 6% for damages of Typology 1 and 2, respectively. This study will help to standardize and estimate the effect of the type of erosion on AEP losses for different shapes and erosion typologies.
Impact of turbulent inflow and orography on the low-frequency noise sources of a wind turbine
Authors: Wenz F. , Lutz T. , Krämer E.
Published: June 2022

A high-fidelity process chain is used to numerically investigate the low-frequency emissions from a generic wind turbine under turbulent inflow conditions. It consists of the Computational Fluid Dynamics (CFD) solver FLOWer, which is coupled to the multi-body simulation software SIMPACK to take unsteady aeroelastic effects into account. A realistic flow field for the complex terrain of Perdigão, Portugal is obtained by including the orography and vegetation in the simulation and using a precursor simulation with E-Wind to generate a site- and situation-specific inflow. The acoustic emissions are calculated with the Fflowcs-Williams-Hawkings (FW-H) acoustic solver ACCO. The simulations show that the tower emits noise caused by the blade-tower interaction (BTI) equally strong in all directions. To the sides of the turbine, this contribution is dominant, regardless of the turbulent inflow. The blades, on the other hand, emit significantly more noise under turbulent inflow, especially in streamwise direction, where they become the dominant source. The main noise mechanism here is the low-frequency part of the inflow turbulence (IT), followed by the BTI. The flow field in Perdigão causes additional large-scale variations in IT noise over time.
The detection of generator bearing failures on wind turbines using machine learning based anomaly detection
Authors: Chesterman X , Verstraeten T , Daems PJ , Perez Sanjines F , Nowé A , Helsen J
Published: June 2022

In this research an early warning methodological framework is developed that is able to detect premature failures due to excessive wear. The methodology follows the data-driven Normal Behavior Model (NBM) principle, in which one or more data-driven models are used to model the normal behavior of the wind turbine. Anomalous behaviour of the turbine is identified by analyzing the deviation between the observed and predicted normal behaviour. The framework consists of two pipelines, a statistics and machine learning based pipeline. The former is based on techniques like ARIMA, OLS and CUSUM. The latter makes use of techniques like Random Forest, Gradient Boosting, … Each pipeline has its strengths and weaknesses, but by combining them in an intelligent way, a more capable detector is developed. The methodology is validated on 10-minute SCADA data from a real operational wind farm. The validation case focuses on generator (front/rear) bearing failures. The goal is to predict these failures well in advance (ideally at least a month) using the developed framework, which should allow for timely adjustments to the maintenance plan. The results show that the methodology is able to accomplish this reliably.
Modelling of turbine power and local wind conditions in wind farm using an autoencoder neural network
Authors: Dou Suguang , Dimitrov Nikolay
Published: June 2022

Neural network models have been widely applied in wind energy to enable data-driven modelling of turbine’s power, loads, damage, and other quantities in wind farm. They have shown advantages in the flexibility, robustness, and efficiency, as well as the ease of application. An important task in data-driven modelling of wind farm is the characterization of wake-induced effects within wind farm. Because of the large number of wind turbines in wind farm and the broad range of conditions required for power output assessment, it has become relevant to use computationally efficient surrogate models that are based on parameterizations of the wind farm layout. Previous studies on surrogate models of wake-induced effects have focused on parameterizations based on manually extracted features from the wind farm geometry. The present study applies an autoencoder neural network to automatically learn the features representing the wind farm layout. Specifically, two autoencoders are studied and compared. The autoencoder provides dimension reduction and feature learning that allow representing the entire wind farm layout in just a few latent variables, thus allowing efficient and accurate surrogate models of wake-induced effects. The outputs of the autoencoder, together with any ambient wind condition variables, serve as inputs to a standard feedforward neural network that predicts the wake-induced effects for any turbine in the wind farm. Our results show that the autoencoder provides an automatic and effective way to parameterize the wind farm geometry, and facilitates the building of efficient surrogate models that are capable of accurate modeling of wake-induced effects in a wind farm with arbitrary layout.
Autoencoder and Mahalanobis distance for novelty detection in structural health monitoring data of an offshore wind turbine.
Authors: Weil Maximillian , Weijtjens Wout , Devriendt Christof
Published: June 2022

Structural Health Monitoring (SHM) has seen an explosion in data gathering in the last few years. This is illustrated in the offshore wind industry through an increase in the amount of placed offshore wind turbines (OWT), a higher rate of SHM instrumented OWTs and an increase in the sampling rate. The growing data gathering has led to the interest of big data techniques in the SHM industry. This paper introduces a new more robust unsupervised novelty detection pipeline combining an autoencoder and the Mahalanobis distance and comparing this combination to both methods separately. This reduces the high dimensional input to a one dimensional novelty index for detecting anomalies in the OWT. Additionally the research considers the challenges that the downtime of non operationally essential sensors poses, a method is introduced to guarantee high model availability without losing the benefit of high fidelity.
Region-based convolutional neural network for wind turbine wake characterization from scanning lidars
Authors: Aird J A , Quon E W , Barthelmie R J , Pryor S C
Published: June 2022

A convolutional neural network is applied to lidar scan images from three experimental campaigns to identify and characterize wind turbine wakes. Initially developed as a proof-of-concept model and applied to a single data set in complex terrain, the model is now improved and generalized and applied to two other unique lidar data sets, one located near an escarpment and one located offshore. The model, initially developed using lidar scans collected in predominantly westerly flow, exhibits sensitivity to wind flow direction. The model is thus successfully generalized through implementing a standard rotation process to scan images before input into the convolutional neural network to ensure the flow is westerly. The sample size of lidar scans used to train the model is increased, and along with the generalization process, these changes to the model are shown to enhance accuracy and robustness when characterizing dissipating and asymmetric wakes. Applied to the offshore data set in which nearly 20 wind turbine wakes are included per scan, the improved model exhibits a 95% success rate in characterizing wakes and a 74% success rate in characterizing dissipating wake fragments. The improved model is shown to generalize well to the two new data sets, although an increase in wake characterization accuracy is offset by an increase in model sensitivity and false positive wake identifications.
Transfer Learning of Model-Based Neural Network for Transfer Function Inversion and Load Monitoring of Wind Turbines
Authors: Delbos F , Caleyron F. , Leroy J-M.
Published: June 2022

One main concern when using a generic model of a wind turbine is how can we apply it on real measurements for different turbines of the same type. This paper gives a first answer by proposing an inversion approach of a transfer function. Then that approach is tested on a real test case. Two transfer functions from two different turbines of the same farm are inverted on SCADA measurements with transfer learning of a model-base neural network. The inversion results show better predictions than ones with a transfer function classically trained with LIDAR data. An application to the prediction of damage equivalent load with a generic model is showed: the results of applying the inverted transfer function is compared to real DEL measurements and to the transfer function trained from LIDAR data.
On the ill-conditioning of the combined wind speed estimator and tip-speed ratio tracking control scheme
Authors: Brandetti L , Liu Y , Mulders SP , Ferreira C , Watson S , van Wingerden JW
Published: June 2022

In recent years, industrial controllers for modern wind turbines have been designed as a combined wind speed estimator and tip-speed ratio (WSE-TSR) tracking control scheme. In contrast to the conventional and widely used Kω
2 torque control strategy, the WSE-TSR scheme provides flexibility in terms of controller responsiveness and potentially improves power extraction performance. However, both control schemes heavily rely on prior information about the aerodynamic properties of the turbine rotor. Using a control-oriented linear analysis framework, this paper shows that the WSE-TSR scheme is inherently ill-conditioned. The ill-conditioning is defined as the inability of the scheme to uniquely determine the wind speed from the product with other model parameters in the power balance equation. Uncertainty of the power coefficient contribution in the latter mentioned product inevitably leads to a biased effective wind speed estimate. As a consequence, in the presence of uncertainty, the real-world wind turbine deviates from the intended optimal operating point, while the controller believes that the turbine operates at the desired set-point. Simulation results confirm that inaccurate model parameters lead to biased estimates of the actual turbine operating point, causing sub-optimal power extraction efficiency.