bluuebunny/crossref_metadata_2025_split huggingface dataset:
High-fidelity simulations of airfoil vortex-induced vibrations: from 2D to blade-like aspect ratios
Authors: Fernandez-Aldama Ricardo , Papadakis George , Lopez-Garcia Oscar , Avila-Sanchez Sergio , Riziotis Vasilis A 
Published: June 2024

Abstract
Large edgewise vibrations due to vortex shedding can be suffered by a wind turbine blade when the rotor is stopped or idling and there is massively separated flow. High-fidelity simulations of these vortex-induced vibrations (VIV) with a full-blade model are very computationally expensive, and so currently reported results of this type are limited to short time series or reduced parameter spaces, far from being sufficient to predict the lock-in range and other characteristics of a full-scale blade response to VIV under real conditions. This computational cost has led researchers to, alternatively, study VIV using simplified approaches, like simulations of 2D and short-span airfoil sections. To help bridge the gap between expensive full-blade and short-span airfoil section simulations, in this work the VIV response of an airfoil section is obtained for three different aspect ratios —from 2D to a blade-like aspect ratio—, using CFD simulations coupled with a one-degree-of-freedom structural oscillator model. The results obtained show that inside the lock-in range the airfoil’s initial VIV response is very similar for all aspect ratios studied, despite notable differences in the Strouhal number obtained. In contrast, outside lock-in the aerodynamic forces vary substantially from one case to another.
On the role of induced power in turbine wake scaling: bridging 100-m to 0.1-m rotors
Authors: Major Desirae , Schmitz Sven 
Published: June 2024

Abstract
A wake scaling methodology is proposed in which turbine power is separated into its induced and profile power components. It is shown that preserving induced power as a function of thrust coefficient in conjunction with the radial circulation distribution are sufficient conditions for wake scaling. This work demonstrates the primary role of induced power in scaling turbine wakes over three orders of magnitude from full-scale down to model-scale rotors. The extended theory is verified using blade-element momentum theory (BEMT) and large-eddy simulation (LES), and an example set of model-scale turbine wake experiments is proposed for known wind tunnel facilities around the world.
Efficient Prediction of Turbulent Inflow and Leading-Edge Interaction Noise Using a Vortex Particle Method with Look-up Table Approach
Authors: Sharma Sparsh , Herr Michaela 
Published: June 2024

Abstract
This paper presents a new, efficient approach for predicting turbulent inflow, also known as leading-edge interaction noise. The method combines a low-fidelity Vortex Particle Method (VPM) with a look-up table approach. Its goal is to reconstruct realistic inflow turbulence and airfoil contours using stochastic control variables within a limited vortex window. To model far-field sound emission, Curle’s equation and a vortex sound database are employed. To increase confidence in the methodology, an analysis of inflow turbulence parameters and source characteristics was performed using systematic Large Eddy Simulations (LES). A generic NACA 0012 airfoil test case with different inflow turbulence grids was used for direct comparisons with semi-theoretical and semi-empirical predictions from the literature. The comparison is restricted to Amiet/Gershfeld predictions as the current model is only capable of dealing with homogeneous and isotropic turbulence. However, their usefulness is limited to narrower parameter ranges when compared to the more generally applicable new method. A satisfactory agreement of the results demonstrates the versatility of the proposed method.
The Influence of Floating Turbine Dynamics on the Helix Wake Mixing Method
Authors: Van Den Berg Daniel , De Tavernier Delphine , Gutknecht Jonas , Viré Axelle , Van Wingerden Jan-Willem 
Published: June 2024

AbstractWake mixing techniques like the Helix have shown to be effective at reducing the wake interaction between turbines, which improves wind farm power production. When these techniques are applied to a floating turbine it will excite movement. The type and magnitude of movement are dependent on floater dynamics. This work investigates four different floating turbines. Of these four turbines, two are optimised variants of the TripleSpar and Softwind platforms with enhanced yaw motion. The other two are the unaltered versions of these platforms. When the Helix is applied to all four floating turbines, the increased yaw motion of the optimised TripleSpar results in a reduction in windspeed whereas the optimised Softwind sees an increase in windspeed with increased yaw motion. From simulations using prescribed yaw motion at different phase offsets between blade pitch and yaw motion, we can conclude that this is the driving factor for this difference.
Assessing risks for the use of drones for wind turbine inspections
Authors: Kolios Athanasios 
Published: June 2024

Abstract
This paper presents a comprehensive risk assessment framework for integrating drones into wind turbine inspections, addressing technical, operational, human factors, environmental, and regulatory risks. Utilizing a multidimensional approach based on Failure Modes Effects and Criticality Analysis (FMECA), the study evaluates potential risks in drone-assisted wind turbine inspections, considering various factors like communication reliability, sensor functionality, battery performance, and environmental impacts. The research emphasizes the importance of systematic risk identification, evaluation, and prioritization in enhancing the safety and reliability of drone operations in the renewable energy sector. The framework’s practical application is demonstrated through a case study involving real-world offshore wind farms, engaging industry experts and employing structured data collection methods. The study identifies, assesses, and offers mitigation strategies for critical risks, underlining the significant potential of drones in optimizing maintenance processes and advancing sustainable energy production. Future research directions include refining risk assessment methodologies, advancing drone technology, and fostering international collaboration for broader adoption of drone-based inspections in renewable energy.
About control of bi-wind turbine single-point-moored floating offshore wind turbines
Authors: Sandua-Fernández I , Martín-San-Román R , Eguinoa I , Cascales-Gómez A , Iribas-Latour M 
Published: June 2024

Abstract
In the last years, several novel floating offshore wind energy concepts have appeared in the market, whose goal is the reduction of floating offshore wind energy costs. Among them, bi-wind turbine floating offshore wind turbines, which use a single floating platform to place two wind turbines, are one of the most promising ones. However, they face some challenges in their full deployment. For example, depending on the wind direction, one of the rotors could be found downstream of the other one and hence be affected by its wake. To avoid this, the turbines of this type of systems do not use a conventional active yaw mechanism, but a single-point-mooring configuration, to allow the platform rotation around the vertical axis. This way, the alignment of the rotors with the wind can be achieved, avoiding wake issues. However, as stated in the literature, at least for single-rotor wind turbines, this alignment is not straightforward and some yaw drift could appear in the platform. Therefore, the main goal of this paper is to confirm that this platform yaw drift could also arise in bi-wind turbine single-point-moored floating wind energy systems, and analyse its impact in terms of performance, while shedding some light on the performance improvements when applying new advanced control strategies.
Multi-row extremum seeking for wind farm power maximization
Authors: Rotea Mario A , Kumar Devesh , Aju Emmanuvel J , Jin Yaqing 
Published: June 2024

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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.