bluuebunny/crossref_metadata_2025_split huggingface dataset:
Wind turbine LIDAR-assisted control: Power improvement, wind coherence and loads reduction
Authors: Meng Fanzhong , Hou Lio Wai , Larsen Gunner Chr. 
Published: June 2022

Abstract
This paper aims to firstly review various LIDAR-assisted wind turbine control methods proposed in the past ten years for improving power production in the below rated wind speed region in order to clarify their performance and quantify the potential benefits and drawbacks of LIDAR-assisted control. Secondly, a new LIDAR-assisted control algorithm based on Extremum Seeking Control (ESC) using LIDAR preview information for tracking the optimal power coefficient value is presented and compared to the traditional control strategy. The results show that the newly proposed LIDAR-assisted control strategy is beneficial in the below rated wind speed region when the aerodynamic properties of blades are changed largely compared to the original design. Lastly, the effects of the wind field coherence between the LIDAR measured points and the rotor plane on the LIDAR-assisted control for loads reduction in above rated region is discussed. An important conclusion is that the major benefit of LIDAR-assisted wind turbine control is to improve collective pitch control for reducing fatigue loads in above rated region, when the coherence bandwidth between the LIDAR measured wind speed and the rotor effective wind speed is high.
Do ambient shear and thermal stratification impact wind turbine tip-vortex breakdown?
Authors: Hodgkin Amy , Laizet Sylvain , Deskos Georgios 
Published: June 2022

Abstract
Modern wind turbines experience uneven inflow conditions across the rotor, due to the ambient flow’s shear and thermal stratification. Such conditions alter the shape and length of turbine wakes and thus impact the loads and power generation of downstream turbines. To this end, understanding the spatial evolution of the individual wakes under different atmospheric conditions is key to controlling and optimising turbine arrays. With this numerical study we aim to obtain a better understanding of the fundamental physics governing the near-wake dynamics of wind turbines under shear and thermal stability, by examining their tip-vortex breakup mechanisms. Our approach considers scale-resolving simulations of a single turbine wake under a linear shear profile as well as the application of harmonic tip perturbations to trigger flow instabilities. For the subsequent analysis we use the proper orthogonal decomposition (POD) method to extract coherent structures from the flow, and we also calculate mean kinetic energy fluxes to quantify each coherent structure’s contribution to wake recovery. The wake’s helical spiral is found to hinder wake recovery for all studied ambient flow conditions, whereas the mutual inductance instability has positive MKE flux leading to an enhanced wake recovery. Finally, the ambient shear has the largest impact on the local MKE flux with respect to downstream location by changing the shape of the curve and location of extrema, whereas thermal stratification has only a minimal impact on the magnitude of the near-wake local MKE flux distribution.
Sensitivity analysis and Bayesian calibration of a dynamic wind farm control model: FLORIDyn
Authors: Dighe Vinit V. , Becker Marcus , Göçmen Tuhfe , Sanderse Benjamin , Wingerden Jan-Willem van 
Published: June 2022

Abstract
FLORIDyn is a parametric control-oriented dynamic model suitable to predict the dynamic wake interactions between wind turbines in a wind farm. In order to improve the accuracy of FLORIDyn, this study proposes to calibrate the tuning parameters present in the model by employing a probabilistic setting using the UQ4WIND framework. The strategy relies on constructing a surrogate model (based on polynomial chaos expansion), which is then used to perform both global sensitivity analysis and Bayesian calibration. For our analysis, a nine wind turbine configuration in a yawed setting constitutes the test case. The results of sensitivity analysis offer valuable insight into the time-dependent influence of the model parameters onto the model output. The model parameter tied to the turbine efficiency appear to be the most sensitive parameter affecting the model output. The calibrated FLORIDyn model using the Bayesian approach yield predictions much closer to the measurement data, which is equipped with an uncertainty estimate.
Turbulence Optimized Park model with Gaussian wake profile
Authors: Pedersen J G , Svensson E , Poulsen L , Nygaard N G 
Published: June 2022

Abstract
A new analytical wake model for wind farm design is presented. The new model, TurbOPark, is based on the Park model but features non-linear wake expansion and a Gaussian deficit profile. The modelled wake expansion depends on the intensity of the local turbulence, which is a combination of ambient atmospheric turbulence and wake-generated turbulence. Moving downstream from the rotor, the intensity of the wake-generated turbulence decreases, and the wake expansion slows down. This leads to more persistent wakes in wind farms and clusters of wind farms with large distances between turbines. In the present study, we focus on the London Array wind farm and show examples of how results from Park and TurbOPark compare to SCADA data. We look at the array efficiency of the wind farm, the production of individual turbines, and the impact of neighbouring wind farms. We find that predictions from TurbOPark generally are closer to observations than predictions from the Park model.
Turbulence above offshore wind farms measured by aircraft
Authors: Haseeb Syed Abdul , Mann Jakob , Platis Andreas , Bange Jens 
Published: June 2022

Abstract
The flow inside and around large offshore wind farms may range from smaller and shorter structures present due to the mechanical turbulence generated by wind turbines to larger, and longer structures indicative of the mesoscale flow. In this study, we explore the variation in turbulence structures above large offshore wind farms located in the North Sea using the data obtained from a research aircraft. The aircraft was flown upstream, downstream, and above the wind farm clusters. We evaluate the normalized autocorrelation of the wind velocity component in the mean wind direction for transverse separations and the rate of energy dissipation in the inertial subrange for neutral and stable stratification. During the neutral stratification, there is high ambient turbulence in the atmosphere and large values of the energy dissipation rate. The intensity of small-scale turbulence structures is increased above and downstream of the wind farm and it dominates the mesoscale flow structures. While in stable stratification, mesoscale flow structures are not only dominant upstream of the wind farm but also in the downstream flow. Apart from the atmospheric stratification, the wind farm layout also affects the turbulent structures, the rate of energy dissipation, and time for wake recovery.
WakeNet 0.1 - A Simple Three-dimensional Wake Model Based on Convolutional Neural Networks
Authors: Asmuth Henrik , Korb Henry 
Published: June 2022

Abstract
Deep convolutional neural networks are a promising machine learning approach for computationally efficient predictions of flow fields. In this work we present a simple modelling framework for the prediction of the time-averaged three-dimensional flow field of wind turbine wakes. The proposed model requires the mean inflow upstream of the turbine, aerodynamic data of the turbine and the tip-speed ratio as input data. The output comprises all three mean velocity components as well as the turbulence intensity. The model is trained with the flow statistics of 900 actuator line large-eddy simulations of a single turbine in various inflow and operating conditions. The model is found to accurately predict the characteristic features of the wake flow. The overall accuracy and efficiency of the model render it as a promising approach for future wind turbine wake predictions.
LES verification of HAWC2Farm aeroelastic wind farm simulations with wake steering and load analysis
Authors: Liew Jaime , Juhl Andersen Søren , Troldborg Niels , Göçmen Tuhfe 
Published: June 2022

Abstract
Wind farm simulation tools are used for a multitude of purposes, including energy yield calculations, wind farm control optimization, layout optimization, structural load analysis, and many more. However, the vast majority of farm software either fails to capture the dynamic nature of both the flow and the turbine structural response or demands a high computational cost such as large-eddy simulations (LES). In this study, we present a new mid-fidelity aeroelastic wind farm simulation software, HAWC2Farm, which can perform dynamic wind farm computations at the same temporal and spatial resolution as LES using a fraction of the computational resources. Wind farm simulations are performed with wake steering, and statistics are verified with results from LES performed using Ellipsys3D.
Reduction and analysis of rotor blade misalignments on a model wind turbine
Authors: Jüchter J , Peinke J , Lukassen L J , Hölling M 
Published: June 2022

Abstract
Model wind turbines with rotor diameters below 1 m often make use of a collective pitch control instead of an individual pitch control. As a result it is more difficult to achieve a high precision in the rotor blade pitch angle, especially when it comes to achieving the same pitch angle on each rotor blade. For the Model Wind Turbine Oldenburg 0.6 (MoWiTO 0.6) a rotor blade misalignment between the individual blades of up to 2.5 degrees was found. Due to the design, similar blade misalignments could also occur at other model wind turbines with a collective pitch mechanism. Here, it is shown that even small rotor blade misalignments influence the experimental results of small model wind turbines and should be avoided. In addition, a new mounting procedure is presented that serves to minimize blade misalignments when assembling the individual rotor blades in the manufacturing process. This procedure makes use of 3D printed parts that enclose the rotor blade during the mounting process and guarantee a precise pitch angle. The presented procedure is easily applicable to other model wind turbines as well. The subsequent experimental investigations of blade misalignments in the range of ±2.5 degrees show a significant influence on the turbine performance and thrust. A blade misalignment of +2.4 degrees for only one blade already decreases the mean power output of the turbine by up to 9%. Additionally, the mean thrust measurements show a clear influence of the blade misalignment (up to 17% difference) in comparison to the optimal pitch reference case. Furthermore the 1P (one-per-revolution) peaks of the thrust spectrum are significantly increased with present blade misalignments which suggests cyclic loads. These results underline the relevance of a precise rotor blade attachment for model wind turbines used in wind tunnel experiments.
Influences on the LVRT behavior of DFIG wind turbine systems
Authors: Herzog F , Röder J , Frehn A , Ruhe N , De Doncker R W 
Published: June 2022

Abstract
Grid faults introduce highly dynamic electrical and mechanical loads to a wind turbine (WT). Especially WTs with a direct grid connection like the doubly-fed induction generator (DFIG) are strongly affected. The behavior of a WT during grid faults can be tested in low voltage ride through tests (LVRT). But there are numerous influencing factors on the behavior of the DFIG during a LVRT which have not yet been fully investigated. The pre-fault operating point of the DFIG, the grid inductance, the pre-fault phase angle of the grid voltage, the fall time of the voltage as well as the start and end values of the voltage drop affect the electromagnetic torque and the short circuit current of the generator. Therefore, many LVRT test results for DFIGs are neither comparable nor representative. In this paper it is shown that the peaks of electromagnetic torque and currents during LVRTs can be reduced. A low pre-fault torque and rotational speed, a high grid inductance and a slow voltage drop can minimize the impact of a grid fault. The rotational speed is especially critical because it influences the slip of the DFIG and, thus, has an influence on the dynamics of the fault.
Effects of Lightning on Pultruded Carbon Fiber Wind Blades
Authors: Williams M , Tilles J N , Paquette J , Samborsky D , Clem P 
Published: June 2022

Abstract
Modern wind turbine blades incur occasional damage from lightning strikes. In non-conductive glass laminates, lightning damage causes severe charring, and is often obvious from visual inspection. In conductive carbon laminates, lightning strikes do not always result in visible damage, and the impact on strength and fatigue resistance is uncertain. Thus, it is difficult to evaluate a carbon fiber wind blade suspected of receiving a lightning strike. This experimental study investigates the effect of electrical current running longitudinally through pultruded carbon fiber wind blade specimens. Testing simulated the worst 1% of lightning strike in terms of peak current, time to peak, and action integral, and the worst 50% of lightning strike in terms of charge transfer. Post-test, specimens were inspected using nondestructive inspection (NDI) methods and were then mechanically tested for ultimate tension and compression, and tension-tension fatigue. Results show that carbon fiber pultrusions withstand severe lightning currents with no visible or NDI-indicated damage. Preliminary structural tests show a decrease in ultimate compressive strength at the most severe lightning test case.
Results from the FOCAL experiment campaign 1: turbine control co-design
Authors: Kimball R , Robertson A , Fowler M , Mendoza N , Wright A , Goupee A , Lenfest E , Parker A 
Published: June 2022

Abstract
This paper summarizes the scaling approach and results from the first experimental campaign of the Floating Offshore-wind and Controls Advanced Laboratory (FOCAL) Experimental Program, which is focused on developing public data sets to support the validation of numerical models for floating wind control co-design. A 1:70 Froude-scale performance-matched model of the IEA Wind 15-MW reference turbine was designed, built, and characterized on a fixed base. The National Renewable Energy Laboratory’s Reference Open-Source COntroller (ROSCO) was integrated into the scale testing to examine the impact advanced turbine and generator control strategies have on turbine performance and loads, enabling validation of control strategies for floating wind design optimization.
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

AbstractWe 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

Abstract
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.