bluuebunny/crossref_metadata_2025 huggingface dataset:
Multi-row extremum seeking for wind farm power maximization
Authors: Rotea Mario A , Kumar Devesh , Aju Emmanuvel J , Jin Yaqing
Published: June 2024

This paper presents results from wind tunnel experiments to evaluate power gains from wake steering via yaw control. An experimental scaled wind farm with 12 turbines in an aligned rectangular array is used. Wake steering is performed by yawing turbines using a closed-loop algorithm termed the Log-of-Power Proportional Integral Extremum Seeking Control (LP-PIESC). Two configurations are considered. In the first configuration, the turbines in the first two upstream rows are controlled. In the second case, yaw control is applied to the turbines in the first upstream row and the third row. For both cases, uncontrolled turbines have no yaw misalignment. The results show that by independent parallel maximization of the power sum of a reduced number of turbines, it is possible to obtain a close approximation of the true maximum power. The data shows that the LP-PIESC algorithm can converge relatively fast compared to traditional ESC algorithms.
Robustness of LiDAR-assisted controller towards measurement uncertainty
Authors: Svenstrup Mikael Aslak , Thomsen Jesper Sandberg
Published: June 2024

Many studies have concluded that LiDAR-assisted control can be used to increase annual energy production (AEP) as well as reduce extreme loads and fatigue damage on blades, tower, and main bearing components. However, most studies assume perfect LiDAR measurements and do not consider large measurement uncertainties on LiDAR measurements. In this study we evaluate the potential benefits in terms of fatigue load reduction from using a simple robust feedforward controller. In addition, we evaluate the impact of measurement uncertainties exemplified by a time shift between the expected time the wind hits the rotor and the actual time it hits the rotor as well as a mean wind speed measurement bias. Results shows that fatigue loads can be reduced up to 7% on the tower sections and up to 4% on other main components. However, these results only applies when the LiDAR is assumed to give perfect information about what is hitting the rotor plane. The load reduction degrades with the magnitude of a time shift of the arriving LiDAR wind measurements. A delay up to 2.5 seconds still gives reasonable load reductions. Furthermore, a positive wind speed bias does not affect the performance of the LiDAR-assisted controller, whereas a negative bias significantly deteriorates performance.
Static Testing Using OFDR-based Quasi Continuous Fiber Optic Strain Sensing on the WiValdi Wind Turbine Rotor Blade
Authors: Knebusch J , Gundlach J , Meier D , Meier T , Govers Y
Published: June 2024

The German research wind farm WiValdi was taken into service in 2023 and enables researchers and industry to conduct full-scale experiments. It currently consists of two highly instrumented wind turbines and multiple wind measurement masts. In one of the six wind turbine rotor blades, four Fiber Optic Strain Sensors (FOSS) are installed [1].

The strain which a wind turbine rotor blade exhibits under load is a key factor in the design, the certification and the assessment of its remaining service life as well as its further development and optimization. The aerodynamic optimization of wind turbine rotor blades leads to longer, and increasingly slender rotor blades and therefore increasingly relevant static and dynamic structural mechanical challenges.

Usually the strain of wind turbine rotor blades during static deflection tests is measured with electrical strain gauges (SGs) or Fiber-Bragg-Gratings (FBGs). In the present study, quasi-continuous Fiber-Optic-Strain-Sensors (FOSS) are used in parallel with FBGs. The underlying technology is called optical frequency domain reflectometry (OFDR) and since only glass-fibers are used, it is inherently lightning protected [2]. Besides that, it is commercially available and delivers strain results at a spatial resolution of 2.6 mm. For this study, there are four 50-meter-long glass fibers bonded to the wind turbine rotor blade on suction and pressure side of each main spar of a 57 m rotor blade. This contribution shows how the FOSS measurements are post-processed and validated. The validated data is then used to obtain the relation between strain and introduced force over the length of each FOSS. With the accurate prediction of root-bending-moments, one of many possible use-cases of the FOSS results, is presented.

Besides the computational model updating of the corresponding Finite-Element model in future investigations, the results will also be used to judge the current structural mechanical state of the wind turbine rotor blade at recurring service inspections. In that, results of the then to be acquired state at a given time and the shown base-line state will be compared. This will lead to high resolution insight into the deterioration of the structural integrity of the wind turbine rotor blade over time.

Considering that standard telecommunication glass fibers can be used as sensors, the cost per sensor (i.e. fiber) is relatively low (circa 1500 € for 50 m). Despite the high cost of the required measurement system, the low prices of the fibers make FOSS measurements not only a scientifically interesting but also a potentially economically viable solution for measuring strain on wind turbine rotor blades.
Influence of solar radiation and outside air temperature on the thermographic flow visualization of wind turbines
Authors: Dorszewski Caroline , Dieckmann Jakob , Balaresque Nicholas , Freyberg Axel V. , Fischer Andreas
Published: June 2024

Infrared thermography (IRT) can be used to visualize the boundary layer flow on wind turbines – during operation and with no need of blade or turbine modification. So far, no other technique is capable of this. IRT uses the temperature difference between the rotor blade and the surrounding air to visualize the inherent difference in convective heat transfer, i. e. the resulting difference in blade surface temperature, which corresponds to the different boundary layer flow regimes. For onsite IRT measurements on wind turbines in operation, the heat convection is mostly caused by the heat of the absorbed solar radiation. Hence, the measurement quality is highly dependent on the solar radiation and presumably also on the air temperature. To understand the capability and availability of IRT-based flow visualizations over day and year, the paper aims to quantify the achievable measurement quality according to the solar radiation and air temperature. Both the contrast and the contrast-to-noise ratio for visualizing the laminar-turbulent transition are studied and quantified by means of theory and experiment. As a result, the maximal contrast over day and year for a wind turbine in the north of Germany is estimated to be in the range of a few Kelvin and a contrast-to-noise ratio in the order of 50 is feasible. These theoretical findings agree well with experimental results. Furthermore, the measurability of the flow transition over one winter and one summer day is assessed, which shows strong variations in the measurement quality and clarifies the IRT measurement capability.
Chemical factors for premature failures of roller bearings in wind turbines due to white etching cracks: A review from literature and industry experience
Authors: Harling Jörn Christian , Jacobs Georg , Lehmann Benjamin
Published: June 2024

This paper investigates the chemical influencing factors leading to premature bearing failures in wind turbine gearboxes. Chemical factors like the lubricant composition and the boundary layer of the bearing material play a crucial role in enabling premature failures caused by white etching cracks (WEC). They are then eventually triggered by energy input due to mechanical or electrical loads. Even though reproducing WEC on test rigs (e.g. FE8 test rig or three-disc-tribometer) is possible a reproduction or transfer of the knowledge to full-size test rigs or real systems remains challenging. This is due to the system-specific contact conditions which result from the respective loads, the bearing geometry and the chemical influencing factors. To collect the industry’s perspective on chemical influencing factors and their significance in the field, the state of the art was analysed and based on the results a survey was prepared. The survey interviews are conducted among leading manufacturers of lubricants, bearings, gearboxes and OEMs in the wind energy sector. From those results the influence of hydrogen, particles and contact temperature were identified as particularly important.
Method for accelerated testing of wind turbine gearbox bearings based on frictional energy in the rolling contact
Authors: Yazici E , Jacobs G , Röder J , Zweiffel M
Published: June 2024

This paper investigates a method for a novel accelerated test procedure for rolling bearings in wind turbine gearboxes. Established test procedures e. g. highly accelerated lifetime tests (HALT) mainly reflect fatigue damage and not slip-induced damage patterns (e.g. smearing). A commonly used criterion to rate the severity of an operating point regarding slip-induced damage is the frictional energy. Frictional energy is introduced into the rolling contact as soon as there is a significant combination of simultaneously occurring slip and pressure. Up to now only critical thresholds for this criterion that must not be exceeded have been identified on small-scale component test rigs. However, the permissible amount and duration of overshoots of the critical threshold that lead to damage in the actual application are not understood yet. Therefore, the aim is to conduct tests on full-size test rigs in which frictional energy is applied until slip-induced damage occurs. In order to perform these tests in a reasonable time, it is essential to accelerate the test procedures. Thus, this paper introduces a method for accelerated test procedures based on frictional energy in the rolling contact. The requirement is that the same cumulated frictional energy as in field operation is applied in a shorter time on the test rig. A further requirement is that the frictional power in the accelerated test procedure never exceeds the maximum frictional power occurring in the field. This paper shows the theoretical background regarding frictional energy and the transfer to the test procedure.
Inclusion of low-frequency cycles on tower fatigue lifetime assessment through relevant environmental and operational conditions
Authors: Faria Bruno Rodrigues , Sadeghi Negin , Dimitrov Nikolay , Kolios Athanasios , Abrahamsen Asger Bech
Published: June 2024

Structural Health Monitoring (SHM) campaigns aiming at accurate consumed fatigue damage estimations play a pivotal role in the decision-making of wind turbines approaching their design lifetime. This study focuses on developing a sensor-based fatigue damage counting approach for wind turbines to calculate the consumed lifetime of the tower. The reliability of strain gauges for long-term operation is studied for an onshore V52 and an offshore turbine Adwen AD 5-116. Interestingly, the offshore strain gauges showed more variability compared to the onshore case. Moreover, an investigation on the coupling of low-frequency fatigue damage (LFFD) with Environmental and Operations Conditions (EOCs) through damage matrices highlighted that not only wind speed variability but varying operational conditions of a turbine increase tower fatigue consumption and should be accounted for in the design phase and optimized operational strategies aiming at lifetime extension.
How accurate are ultrasonic anemometers, calibrated in a laminar wind tunnel, under turbulent conditions?
Authors: Glabeke G , Gigon A , De Mulder T , Van Beeck J
Published: June 2024

Two ultrasonic anemometers, namely the Gill HS-100 and the Anemoment TriSonica Mini, were placed in the VKI L-1A wind tunnel to assess their accuracy and precision. A comparison was made between these instruments and a Prandtl probe, using a range of wind speeds up to 20 m/s, eighteen azimuth angles (wind directions) at 20° intervals, five tilt angles ranging from -10 to 10°, and two levels of turbulence. As the normal level of turbulence in the wind tunnel, determined by hot wire anemometry, is less than 1%, a grid was installed to allow a second level of turbulence of 6.1% to be evaluated. The analysis shows that the Gill HS-100 accurately measures wind speed in low turbulence conditions in accordance with the manufacturer’s specifications. However, in a turbulent atmosphere, the measured wind speeds are not only overestimated by 2-8% compared to the Prandtl probe but also show a systematic shift compared to the low turbulence case. As expected from the manufacturer’s specifications, the TriSonica Mini is less accurate and precise than the HS-100. In a low-turbulence environment, it fails to meet the manufacturer’s specifications. In a more turbulent atmosphere, however, it matches and even outperforms the HS-100, making it suitable for real-world applications. However, it did show reduced accuracy at non-horizontal wind angles, requiring careful consideration for drone-based measurements. More complex calibrations involving multiple turbulence levels could be considered for both anemometers. However, the feasibility of this approach remains uncertain in light of the results obtained.
Experimental Characterization of Common-Mode Voltages and Currents in Gearbox Bearings of a Doubly-Fed Induction Generator Wind Turbine on a Test Bench
Authors: Zweiffel Maximilian , Jacobs Georg , Bach Amandus , Röder Julian , Herzog Fabian , De Doncker Rik W.
Published: June 2024

Wind turbine (WT) drivetrain failures can lead to expensive repairs and downtime, significantly impacting the Levelized Costs of Electricity (LCOE). Gearbox failures are a major contributor to maintenance costs, with High-Speed-Shaft (HSS) bearings having the highest failure rate. Frequently occurring premature HSS bearing damage indicates design inadequacies. Currently, bearing design standards (DIN/ISO 281) are primarily based on fatigue. Other damage mechanisms like adhesion or damages due to electrical currents passing through the bearings are not considered. WT gearbox bearings are exposed to electrical currents, mainly caused by common-mode (CM) voltages. Bearing currents lead to surface damages and also may promote the formation of white etching areas (WEA) or cracks (WEC) and thus need to be investigated further. The CM voltage is an inherent feature of the power converter, which generates CM currents in the electrical machine. The resulting high voltages at the HSS bearings can lead to electrical discharge currents in the rolling contact, which damage the raceways of the bearing. This paper presents an experimental method for characterization of the currents and voltages that pass from the generator to the HSS of the gearbox in a multi-megawatt WT drivetrain on a test bench. CM currents and voltages measured at the converter are correlated with the shaft voltages at the HSS bearing. The results indicate, that the CM currents directly pass the HSS and its bearings into the gearbox housing. The CM currents show peaks indicating discharge events with over 100 A. Furthermore, at the discharge events, high-frequency oscillations with over 20 MHz are observed in CM current and bearing voltage.
Detecting abnormal small-scale events by nacelle lidar at the AD8-180 prototype turbine
Authors: Meyer Paul Julian , Adorno Ramiro , Wegner Anna , Gottschall Julia
Published: June 2024

Identifying anomalous wind events that lead to high aerodynamic loads is essential for mitigating extreme loading on wind turbines. Building upon a recently published method, we herein introduce an extended methodology for early small-scale abnormal wind event detection employing a pulsed nacelle-lidar, potentially triggering preemptive control actions within the wind turbine system. The extended method integrates the consideration of multiple measurement ranges within the induction zone and the evaluation of a horizontally asymmetric wind detection parametrized through a difference in lidar line-of-sight (LoS) measurements. A comparative assessment of various detection algorithms is conducted, leading to the identification of optimal parameter configurations and an evaluation of their reliability. With the proposed methodology, the false positive rate can be reduced with respect to the original methodology, although not all extreme loading events are detected.
Dynamic wake conditions tailored by an active grid in the wind tunnel
Authors: Onnen D , Neuhaus L , Petrović V , Ribnitzky D , Kühn M
Published: June 2024

Well characterized test environments are required for novel wind turbine and wind farm control concepts. Aeroelastic simulations are mostly used to model turbine aerodynamic and structural response. For wind farm control, also the wake behaviour needs to be represented, including the impacts of dynamic wind direction changes, wake meandering and the interaction of wakes with the atmospheric boundary layer. This paper shows how wake-like inflow conditions can be emulated with an active grid in a wind tunnel, exciting a broad band of turbulent scales. The artificial wake conditions can be used as inflow for an exposed model turbine. A focus is put on the meandering dynamics, which are driven by large transversal turbulence patterns in the atmospheric boundary layer. Following the conjecture of the Dynamic Wake Meandering (DWM) model, such turbulent scales must have the size of multiple rotor diameters, to impact the entire wake deficit like a passive tracer. In conventional wind tunnel experiments, such spatial scale ratios are hard to reach, since wind tunnel sizes are bounded while the model turbines must be sufficiently large to have appropriate aerodynamic scaling and instrumentation. In this work, quasi-stochastic meandering trajectories are created, using scaled Ornstein-Uhlenbeck processes. Thus, the intrinsically stochastic process of wake meandering is made repeatable. The paper focuses at a thorough characterization of the inflow conditions with both lidar and hot-wire measurements, considering wake shape and spectral features. The results show an approximately axi-symmetric Gaussian deficit, which meanders as a coherent structure while having spectral features similar to a turbine wake.
Multiscale-simulation method for the wear behaviour of planetary journal bearings in wind turbine gearboxes
Authors: Decker Thomas , Jacobs Georg , Graeske Carsten , Röder Julian , Lucassen Mattheüs , Lehmann Benjamin
Published: June 2024

Wind energy is one of the most important renewable energy sources. To maintain competitiveness wind energy needs to be cost effective. Increasing the performance of wind turbines can help to reduce the total cost of wind energy. The power output can be increased by an increase in turbine rotor size since the power increases disproportionately with the rotor diameter. The increase in rotor size inherently comes with an increase in overall wind turbine size to cope with the increasing loads. Since the nacelle weight cannot be increased without limitation, e.g. due to the effect on the tower and costs, an increase in power density is necessary. The usage of journal bearings instead of rolling bearings for the application in planetary gearboxes in wind turbines is one way to increase the power density and reliability of the drivetrain. When designed and operated correctly, journal bearings have a longer service lifetime than rolling bearings. Wear simulations aid in the design of journal bearings. This work presents a simulation method for the wear prediction of planetary journal bearings in wind turbine gearboxes. The method is validated with measurements on a component test rig. The transfer to the wind turbine gearbox is shown by means of simulation.
Towards vortex-based wind turbine design using GPUs and wake accommodation
Authors: Blondel Frédéric , Joulin Pierre-Antoine , Guern Caroline Le
Published: June 2024

Vortex methods are often cited as a promising alternative to the classical Blade Element Momentum (BEM) theory for wind turbine design, overcoming some of its limitations such as imposing steady-state, uniform, and aligned inflows. BEM assumptions are being questioned, especially for large rotors with very flexible blades or even floating wind turbines. However, vortex methods are not yet widespread due to their computational costs. The following work demonstrates how free vortex wake methods using filament discretizations can become an affordable and viable alternative to BEM. This is achieved by combining filament number reduction as well as graphical processing units (GPUs) computations. The efficiency and accuracy of the proposed approaches are demonstrated against simple test cases, including an elliptical wing and a rigid horizontal axis wind turbine (HAWT). Finally, the approaches are applied to the aero-hydro-servo-elastic simulation of floating horizontal axis wind turbines.
Synthesis of realistic non-homogeneous non-Gaussian turbulent wind fields
Authors: Gallego-Castillo C , Elagamy M , Cuerva-Tejero A , Lopez-Garcia O , Avila S
Published: June 2024

This paper presents a sequential method for generating synthetic non-homogeneous non-Gaussian turbulent wind fields with a prescribed time-space covariance structure. The proposed methodology is based on the optimisation of restricted multivariate autoregressive (VAR) models, and the quantile-to-quantile transform between statistical distributions. The considered case study is a non-homogeneous non-Gaussian turbulent wind field over the roof of a high rise building simulated with LES. Results show a notably good matching in terms of the reproduced wind statistical distributions, Covariance Matrix Function (CMF) and Cross Power Spectral Density Matrix (CPSDM). In addition, the synthetic wind field reproduced accurately the recirculation bubble close to the roof. The main advantages of the proposed method are that, once the VAR model is computed, the synthesis of several realisations is computationally very cheap, which is useful for performing several aeroelastic simulations of the same analysis case, as suggested by the standards. The critical point is that, to characterise the statistical features for a specific case study (such as wind turbine wakes or turbulence due to obstacles), an LES simulation of the wind field is required as input. The software employed in this work is open source and it is available on GitHub.
Stabilization of SIMPLE-like RANS solvers for computing accurate gradients using the complex-step derivative method
Authors: Dicholkar A , Lønbæk K , Zahle F , Sørensen N N
Published: June 2024

The Reynolds-averaged Navier-Stokes (RANS) approach in Computational Fluid Dynamics (CFD) is increasingly vital for aerodynamic design across wind turbines, gas turbines, aircraft, and rotorcraft. Enhancing the process through CFD-based design optimization requires numerous design variables, favoring gradient-based methods for better scalability than gradient-free methods. This research uses numerical differentiation techniques, particularly the complex-step derivative method, to compute gradients. Challenges arise during aerodynamic shape optimization when unconventional shapes disrupt RANS solver assumptions, causing convergence failures. These failures undermine optimization, prompting the need to enhance solver convergence for robust optimization. The modified-Boostconv method is a residual recombination method bolstering unstable eigenvalues to stabilize convergence of iterative solvers for nonlinear systems of equations. This study extends the modified-Boostconv method to combine it with the complex-step derivative technique, creating robust optimizations via RANS-CFD with accurate gradients, even for these numerically unstable cases. The main issue encountered during the complexification of the modified-Boostconv method is how to correctly ensure that the model reduction leading to the least-squares problem satisfies the complex-step derivative method. We test whether the dot product operation involved in the model reduction should be a Hermitian or non-Hermitian inner product. The problem is first tested for a simple analytical case using the logistic equation in combination with the modified-Boostconv method. It shows that the non-Hermitian inner product should be used. This is also confirmed with a similar study using the RANS solver.
Investigation of Nonlinear Effects in the Tower Structure of Wind Turbines and Their Influence on System Behavior
Authors: Witter S , Schelenz R , Jacobs G
Published: June 2024

In multibody simulation (MBS) software, large structural components such as the tower are often modeled as flexible bodies using the Craig-Bampton method. While being very computationally efficient, nonlinear effects cannot be modelled in this way. The flange connections between the segments of tubular steel towers are one such source of nonlinearity. The magnitude of this nonlinearity increases with damages at the flanges as flange gaping becomes more common. The nonlinearities in the flanges lead to a changing stiffness and hence changing eigenfrequencies of the structure as a function of its deformation. In this paper, a nonlinear model of a tower with and without damages is being developed and validated based on an FEM model. The model is assembled from linear flexible bodies linked with nonlinear rotatory springs. The damages considered are broken bolts as well as axially symmetric angular flange gaps. It is shown that the MBS model closely emulates the FEM model including all nonlinear effects. The nonlinear tower model is incorporated into an existing MBS model of a commercial wind turbine in the 4 MW range. The eigenfrequencies of the full turbine model are being calculated using the build-in eigenvalue solver in Simpack. The results show a small but measurable decrease in eigenfrequencies compared to the linearized model, which scales with the extend of the damage.
Superelement-based acceleration of finite-element simulations of wind turbine rotors
Authors: Schleich F , Chen Z , Graßmann M , Balzani C , Stammler M
Published: June 2024

Large numbers of wind turbine rotor finite-element simulations are required for blade bearing raceway and ring fatigue calculations. Finite-element rotor models come along with a complex nonlinear model behaviour and a high number of degrees of freedom due to the necessity of considering the blade bearing’s surrounding structures. For that reason, accelerating such simulations is of particular interest for the iterative design process. This study focuses on different superelement configurations for the rotor model of the IWT 7.5-164 reference turbine. The blade bearing’s resulting contact forces and contact angles are analysed for 18 load steps throughout an exemplary rotor rotation and the respective model results are compared to each other. The results show that implementing superelements in the rotor model significantly increases the computational efficiency with an acceptable loss of accuracy in terms of the blade bearing’s internal loads. Furthermore, it is shown that such models outperform the acceleration and especially accuracy achieved by the usage of a one-third rotor model.
Integrating COAWST and OpenFAST for wind turbine loading
Authors: Vemuri Adithya , Porchetta Sara , Munters Wim , Gebel Jakob , Nejad Amir , Helsen Jan , Van Beeck Jeroen
Published: June 2024

Modern research endeavors in wind energy have been increasingly focused on achieving accurate representations of wind turbine loading across diverse atmospheric conditions. Recent advancements in numerical weather prediction techniques make it possible to downscale weather conditions for operational use, underscoring the importance of including air-sea interactions using models such as the Coupled-Ocean-Atmosphere-Wave-Sediment Transport model to improve wind energy predictions. Nevertheless, challenges of high computational costs, the elusive ”gray zone” in simulations, and creating accurate wind turbine digital twins for predictive modeling remain.

The current research addresses this challenge by proposing a novel modeling framework integrating the COAWST model with the OpenFAST aeroelastic solver. The current research strives to bridge the gap between the different dynamic regimes involved under diverse atmospheric conditions to achieve real-world representative wind turbine loading. Therefore, a scaled wind turbine model is implemented in the OpenFAST aeroelastic solver. Further, this research tests and discusses the proposed model framework’s capabilities and limitations under extreme weather phenomena.
A multi-fidelity approach for wind farm simulations and comparison with field data
Authors: Yu W. , Euzenat F. , Maheshwari P. , Baiza Martinez L. A. , Eneau C. , Pallud M.
Published: June 2024

The prediction of energy production and structural loads within wind farms is of high interest for wind industries to optimize the wind farm design and control under a variety of atmospheric conditions. However, it is still a key challenge to predict them accurately and efficiently due to the complex interactions between wind turbines and turbulent flows. Nowadays, high-fidelity simulations using Reynolds-Averaged Navier-Stokes (RANS) or Large-Eddy Simulation (LES) coupled with actuator disk or actuator line method are widely developed and used in wind farm analysis, but it still needs a huge computational resource, especially in the application of a commercial wind farm with large number of turbines. In this context, a mid-fidelity simulation tool based on the Dynamics Wake Meandering (DWM) model called FAST.Farm has been developed by National Renewable Energy Laboratory (NREL) in order to tackle this challenge. It allows to capture essential physics at the turbine scale as well as at the farm scale in a computational efficient manner. This study focuses on the calibration of DWM model which is the key to minimize inaccuracies between mid-fidelity and high-fidelity simulations regarding key performance indicators, e.g. thrust and power production. The calibration is made for DTU10MW wind turbine and shows a good improvement in accuracy compared to default parameters. The calibrated DWM model is finally employed to forecast the power generation for two turbines within a reference wind farm. A good improvement on the prediction is also obtained thanks to the calibrated parameters.
Multi-fidelity actuator-line modelling of FOWT wakes
Authors: Firpo Agnese , Sanvito Andrea G , Persico Giacomo , Dossena Vincenzo , Schito Paolo , Zasso Alberto
Published: June 2024

Large Eddy Simulations (LES) are considered the proper tool for predicting the physics of a wind turbine wake, thereby establishing a solid foundation for investigating the interaction among floating turbines within wind farms. In this work the Actuator Line Model, implemented in the OpenFoam environment, is combined with both U-RANS and LES simulations to underline the differences in accuracy when reproducing the near and far wake of a single turbine. Both a fixed-bottom and a surge motion case are tested to highlight the wake phenomena strictly generated by the platform motion. The use of LES simulation becomes fundamental by virtue of its ability to accurately simulate turbulence and mixing with free-stream flow, hence, this research aims at advancing the knowledge of wake dynamics from multiple perspectives while ensuring reliability thanks to the use of the experimental wake data from the UNAFLOW test campaign on the scaled laboratory model of the DTU 10 MW. In the near wake, limited flow unsteadiness and similar mean velocity are predicted by the two models. In the far wake, instead, the LES approach estimates a strong rise of flow unsteadiness which, in case of surge motion, affects the mean velocity value making evident the difference between LES and RANS estimation.
A generalised implementation of eN
transition method for airfoils and wind turbine blades
Authors: Kassem Hassan , Theron Johannes Nicolaas , Höning Leo
Published: June 2024

Prediction of the boundary layer transition from laminar to turbulent is essential for accurately calculating the aerodynamic forces on airfoils. Yet, it is still a challenging task for most RANS turbulence models (Reynolds-averaged Navier–Stokes) which are build on a fundamental assumption that the flow is fully turbulent. Although there are some derivatives of RANS models that promise to predict transition, the classical eN
method still holds its place as strong alternative, especially at high Reynolds numbers. The eN
method is usually applied in conjunction with boundary-layer codes for flow over 2D airfoils. In wind energy applications of flow over thick airfoils, the high fidelity CFD models play a central role in the analysis and design workflow. This work is an attempt to couple eN
method with an unstructured CFD code, namely OpenFOAM. Most of the previous attempts to achieve this coupling was limited to structured CFD codes and often with boundary-layer codes as an intermediate layer in-between. The newly implemented framework is applied on 2D airfoil cases with Reynolds numbers between 3 and 12 million. The method is extended to also allow for simulations using the Multiple Reference Frame (MRF) approach; this enables the user to calculate laminar-turbulent transition for wind turbine blades in axial flow. The 2D results showed very good agreement with XFOIL and wind tunnel data for forces and transition line prediction. The transition prediction of a 3D single blade of the IEA15MW turbine showed a good agreement with sectionwise transition values. The implemented concept proves the abilities of unstructured CFD based models coupled with the eN
method to predict transition accurately without compromising the robustness of the numerical setup.
Validation of new and existing methods for time-domain simulations of turbulence and loads
Authors: Doubrawa P , Rybchuk A , Friedrich J , Zalkind D , Bortolotti P , Letizia S , Thedin R
Published: June 2024

We seek to obtain a second-by-second match between the simulated and measured structural loads of a utility-scale wind turbine. To obtain the one-to-one load simulations, we start with the furthest upstream component of the modeling chain: the turbulent inflow. We consider new and existing methods to generate constrained-turbulence flow fields. The new method is based on large-eddy simulations (LES) and machine learning (ML). The existing methods include Kaimal-based TurbSim and the superstatistical wind field model. The inflow measurements used to constrain these simulations are obtained with a nacelle-mounted scanning lidar. We compare the flow fields for the different inflow simulation approaches and validate their associated load predictions against measurements collected in the Rotor Aero-dynamics, Aeroelastics, and Wake (RAAW) field campaign. We find that the rotor-position control developed for this study is key in enabling the time match between measurements and simulations. When this control approach is used, the load simulation performance tracks with the inflow simulation fidelity, with LES+ML yielding errors ≤ 4% for the damage-equivalent loads of flapwise bending moment, and tower fore-aft bending moments.
Data-driven time series forecasting of offshore wind turbine loads
Authors: Muhammad Amri Hafiz Ghazali Bin , Marramiero Daniela , Singh Deepali , Van Wingerden Jan-Willem , Viré Axelle
Published: June 2024

Long Short-Term Memory Recurrent Neural Networks (LSTM) are used to build surrogate models to forecast time-series blade loads for both fixed and floating offshore wind turbines. In this paper, we train surrogate models on datasets generated with OpenFAST on the IEA-15MW-RWT under a range of metocean conditions. The aim of the surrogate models is to generate load forecasts inexpensively and accurately such that they can be used in a model predictive controller. Two cases are investigated with different model inputs: one with only measurements available to typical PI controllers and another one with additional wave elevation and deflection measurements (alongside the endogenous variable). The model performances are evaluated and compared. It was found that for the fixed turbine, the models predicted all three blade loads to a high degree of accuracy. The floating turbine surrogate models performed relatively worse, but edgewise and pitching moments are still reasonably accurate. The surrogate model forecasts the flapwise moment to a satisfactory accuracy only in 58% out of 400 test cases. The addition of wave elevation and blade deflection features did not significantly improve the prediction performance of the surrogate, demonstrating that just the information used by current PI controllers may be sufficient for forecasting blade loads.
Insights into the dynamic induction in FOWT surge motion using an actuator-line model
Authors: Sanvito Andrea G , Firpo Agnese , Schito Paolo , Dossena Vincenzo , Zasso Alberto , Persico Giacomo
Published: June 2024

This research investigates the impact of surge motion on Floating Offshore Wind Turbines (FOWTs) through dynamic inflow analyses using an in-house Actuator Line Model (ALM). The study explores surge reduced frequencies in the range from 1 to 10 to analyse the returning wake phenomenon occurring at blade passage frequency. The findings highlight the dampening effect of the rotor thrust and torque by 25% due to induction field inertia. Noteworthy observations include the filter-like effect on the span-wise induced velocity trends: with the frequency increment, the induction field is ’synchronised’ with the apparent wind and forces. The results of the analyses evidence the suitability of the induction parsed in the absolute reference frame to capture the induction field features, amplitude and phase shift, at increasing surge frequencies.
Three-dimensional stochastic dynamical modeling for wind farm flow estimation
Authors: Lingad M V , Rodrigues M , Leonardi S , Zare A
Published: June 2024

Modifying turbine blade pitch, generator torque, and nacelle direction (yaw) are conventional approaches for enhancing energy output and alleviating structural loads. However, the efficacy of such methods is challenged by the lag in adjusting such settings after atmospheric variations are detected. Without reliable short-term wind forecasting tools, current practice, which mostly relies on data collected at or just behind turbines, can result in sub-optimal performance. Data-assimilation strategies can achieve real-time wind forecasting capabilities by correcting model-based predictions of the incoming wind using various field measurements. In this paper, we revisit the development of a class of prior models for real-time estimation via Kalman filtering algorithms that track atmospheric variations using ground-level pressure sensors. This class of models is given by the stochastically forced linearized Navier-Stokes equations around the three-dimensional waked velocity profile defined by a curled wake model. The stochastic input to these models is devised using convex optimization to achieve statistical consistency with high-fidelity large-eddy simulations. We demonstrate the ability of such models in reproducing the second-order statistical signatures of the turbulent velocity field. In support of assimilating ground-level pressure measurements with the predictions of said models, we also highlight the significance of the wall-normal dimension in enhancing two-point correlations of the pressure field between the ground and the computational domain.
Data-Driven Infrastructure Planning for Offshore Wind Farms
Authors: Saxena Isha , Kazemtabrizi Behzad , Troffaes Matthias C.M. , Crabtree Christopher J.
Published: June 2024

Offshore wind farms are one of the major renewable energy resources that can help the UK to reach its net zero target. Under the 10 point plan of the green revolution, the UK is set to quadruple its wind energy production by increasing its offshore wind capacity to 40GW by 2030 [1]. Research needs to be conducted to study the failure and repair processes of wind turbines under various conditions as the current models make a simplifying assumption that the failure/repair rate remains constant over time. This research aims to create a more accurate model using SCADA data. In this research, different mathematical models are fitted to the time to failure and time to repair data of wind turbine components using frequentist methods (such as Maximum Likelihood Estimation) and Bayesian methods. Further analysis will be conducted using complex system analysis considering the failures of each electrical and mechanical component of the wind turbine. The aim of this project is to perform a more accurate reliability analysis that can help to further drive down costs of wind energy by potentially reducing the downtimes of the wind turbines.
An open-source analysis workflow for geometrically imperfect bolted ring-flanges in wind turbine support structures
Authors: Jorgensen J , Hodkiewicz M , Cripps E , Hassan G M
Published: June 2024

Bolted ring-flanges connect structures via two circular flanges, secured with pre-loaded bolts. In offshore wind turbine support structures the flanges connect the monopile and transition piece, the transition piece and tower, and adjacent tower sections. Fatigue analysis of the bolted ring-flanges in offshore wind turbine support structures is a critical step during design. The consideration of geometric imperfections of the flanges can be an influential factor in assessed fatigue damage over some given Fatigue Limit State load case. Schemes to analyse the effect of geometric imperfections using Finite Element Method simulations are available in literature. These schemes rely on the use of proprietary tools for the analysis. In this paper we introduce an open-source workflow to assess load transfer through geometrically imperfect bolted ring-flanges. The workflow relies on four open-source toolsets: the FreeCAD 3D parametric modelling package, the Code Aster / Salome-Meca Finite Element Method simulation environment, the ParaView post-processing and visualisation package, and the Python programming language. We present a case study of load transfer assessment in a monopile to transition piece bolted ring-flange typical of modern offshore wind turbine support structures. We decompose the load transfer across the flange under a number of geometric imperfection scenarios to determine the expected stress ranges for fatigue analysis. The results are compared with analytical methods specified in engineering standards. The workflow is configurable as per user needs, and is maintained as a GitHub repository at https://github.com/jhjorg/OpenBoltRF.
Integrated floating wind farm layout design and mooring system optimization to increase annual energy production
Authors: Mahfouz Mohammad Youssef , Lozon Ericka , Hall Matthew , Cheng Po Wen
Published: June 2024

As we cluster wind turbines in wind farms to gain energy from sites with high wind speeds, wake losses occur within the wind farm. Wake loss is a term used to describe the lower energy production of a downwind turbine that is totally or partially in the wake of an upwind turbine. To decrease wake losses inside the wind farm, the wind farm’s layout is optimized. However, a variety of factors constrain the wind farm layout optimization, such as the size of the lease area relative to the number of turbines to be placed, or the shape of the lease area. Therefore, many wind farms end up with a regular grid layout, such as the Horns Rev 1 wind farm in the North Sea. The ability of a floating offshore wind turbine (FOWT) to change its position based on the wind direction and its mooring system design presents an opportunity to further decrease wake losses in floating wind farms. In this work, we integrate the design of the FOWT mooring systems with the floating wind farm layout design with the goal of increasing the farm’s annual energy production. We use the Horns Rev 1 wind farm as a case study to demonstrate our method. The results show that allowing the FOWT to relocate can decrease wake losses up to 18%. Moreover, the newly developed mooring systems are less stiff and therefore allow larger motion of the FOWT; hence, the material cost of the mooring system decreases by an estimated 17%.
Fatigue assessment of a new concept for a floating barge with pendulum for offshore wind turbines in MOST
Authors: Sirigu Massimo , Ghigo Alberto , Petracca Ermando , Giorgi Giuseppe , Bracco Giovanni
Published: June 2024

Among floating foundations for offshore wind turbines, the barge is considered one of the most cost-effective platforms, owing to its ease of manufacturing, transport, and installation. However, the barge platform is susceptible to wave-induced stress due to its extensive waterplane area, potentially leading to fatigue in critical structural components such as the platform, tower, nacelle components and moorings. This paper delves into the prospect of mitigating fatigue damage in the barge concept by introducing a pendulum ballast, which can be installed at the offshore site using polyester ropes. The primary focus of this research is the assessment of the ‘barge-pendulum’ concept, aimed at reducing load cycles that could lead to fatigue failure, with a specific emphasis on the tower base as a representative indicator of device damage. The analysis is conducted using MOST, a time-domain simulation code developed in Matlab-Simscape environment. The findings show that the barge-pendulum concept has a poorer performance in terms of fatigue life, due to the increased sensitivity to wave forces.
Influence of wind-wave characteristics on floating wind turbine loads: A sensitivity analysis across different floating concepts
Authors: Reddy Likhitha Ramesh , Karystinou Dimitra , Milano Daniel , Walker John , Viré Axelle
Published: June 2024

Floating offshore wind turbines experience different operating conditions, such as wind and wave inflow characteristics. Accurate prediction of the loads acting on the floating wind system is essential for the system design and optimisation. However, there are a lot of uncertainties with the modelling input variables for time domain simulation tools such as OpenFAST to represent various hydro-aerodynamic and structural properties. The primary objective of this work is to identify the critical input parameters for different damage-equivalent load outputs for two substructure types: OC3 Hywind Spar and OC4 DeepCwind semisubmersible. The same rotor-nacelle assembly and tower (the NREL 5MW reference turbine) are used in both case studies. A sensitivity analysis based on the damage equivalent loads of six output quantities was conducted with 8 or 10 input parameters (depending on the floater). The dependent parameters were conditionally parameterised based on the independent inputs, such as wind speed and wind-wave misalignment.

The outcomes of this work show that the floater type affects the sensitivity levels of wave characteristics and hydrodynamic drag coefficients with no significant influence on the turbulence intensity, as expected. Further, the drag coefficient for spar-buoy configuration significantly influences mooring line tension compared to the semisubmersible because of their drag-dominant slender structure. The current velocity is the most dominating parameter for the mooring loads, irrespective of the floater type. While wave characteristics also influenced some turbine loads, it was almost independent of the floater type. Furthermore, the choice of the hydrodynamic model does not affect the sensitivity level rankings. A convergence study on the number of starting points was conducted to ensure a global sensitivity approach. As seen in this study, the results are floating platform-specific. This study provides valuable insight into design-driving input parameters, characterising substructure-specific wind-wave influence.