bluuebunny/crossref_metadata_2025 huggingface dataset:
Visual biospeckle analysis of just torn plant leaf using frequency filtering
Authors: Koley Chhanda , Nirala Anil Kumar
Published: June 2022

Biospeckle activity in leaves arise due to the contribution of several phenomena like water content, respiration, transpiration etc. Our present research work deals with the biospeckle activity of a plant leaf using the laser biospeckle technique and wavelet transform just after removal from the plant, to isolate physiological activities like respiration, transpiration etc over water activity for the first time to the best of our knowledge. Visual analyses of biospeckle activities of Mangifera indica and Tabernaemontana divaricata leaves have been done with the help of spectral activity maps obtained using fujii and parameterized geometrical mean of temporal difference algorithms. Mid-vein and other veins are mainly responsible for water transportation whereas different physiological activities occur in both veins and lamina parts of the leaf. As the biospeckle activity due to high water content suppresses the activity due to other physiological phenomena of the leaf, the spectral activity map without any frequency filtering shows the higher activity of the mid-vein than that of the lamina part of the leaf. It has already been established by researchers that the exclusion of lower frequency bands represents physiological activities of biological samples. Spectral activity maps considering the individual frequency bands starting from 13 and 12 onwards in the case of M. indica and T. divaricata leaves are able to represent the physiological phenomena of the leaves respectively. In comparison to the results obtained considering the individual frequency bands, the results obtained by removing the frequency band groups 1–12 (6.25–3.25 Hz frequency range) for M. indica leaf and 1–11 (6.25–3.50 Hz frequency range) for T. divaricata leaf clearly represent the physiological activities over the water activity of those leaves respectively. Also, biospeckle activities using the individual frequency bands along with the exclusion of band groups 13–25 (3.25–0.00 Hz frequency range) and 12–25 (3.50–0.00 Hz frequency range) represent the water activity of the above two leaves respectively.
Asymmetrical photonic spin Hall effect based on dielectric metasurfaces
Authors: Geng Guangzhou , Pan Ruhao , Zhu Wei , Li Junjie
Published: June 2022

The photonic spin Hall effect has attracted considerable research interest due to its potential applications in spin-controlled nanophotonic devices. However, realization of the asymmetrical photonic spin Hall effect with a single optical element is still a challenge due to the conjugation of the Pancharatnam–Berry phase, which reduces the flexibility in various applications. Here, we demonstrate an asymmetrical spin-dependent beam splitter based on a single-layer dielectric metasurface exhibiting strong and controllable optical response. The metasurface consists of an array of dielectric nanofins, where both varying rotation angles and feature sizes of the unit cells are utilized to create high-efficiency dielectric metasurfaces, which enables to break the conjugated characteristic of phase gradient. Thanks to the superiority of the phase modulation ability, when the fabricated metasurface is under normal incidence with a wavelength of 1550 nm, the left-handed circular polarization (LCP) light exhibits an anomalous refraction angle of 28.9°, while the right-handed circular polarization (RCP) light transmits directly. The method we proposed can be used for the flexible manipulation of spin photons and has potentials in high efficiency metasurfaces with versatile functionalities, especially with metasurfaces in a compact space.
Progress and challenges in magnetic skyrmionics
Authors: Du Haifeng , Wang Xiangrong
Published: June 2022

Magnetic skyrmions are two-dimensional localized topological spin-structures characterized by the skyrmion number that measures the number of times of spins wrapping the Bloch sphere. Skyrmions behave like particles under an external stimulus and are promising information carriers. Skyrmions can exist as an isolated object as well as skyrmion condensates in crystal structures, helical/conical states, mazes or irregular stripy states with emergent electromagnetic fields. Thus, skyrmions provide a nice platform for studying fundamental physics, other than its applications in spintronics. In this perspective, we briefly review some recent progress in the field and present an outlook of the fundamental challenges in device applications.
Simulation based on a modified social force model for sensitivity to emergency signs in subway station
Authors: Cai Zheng-Yu , Zhou Ru , Cui Yin-Kai , Wang Yan , Jiang Jun-Cheng
Published: June 2022

The subway is the primary travel tool for urban residents in China. Due to the complex structure of the subway and high personnel density in rush hours, subway evacuation capacity is critical. The subway evacuation model is explored in this work by combining the improved social force model with the view radius using the Vicsek model. The pedestrians are divided into two categories based on different force models. The first category is sensitive pedestrians who have normal responses to emergency signs. The second category is insensitive pedestrians. By simulating different proportions of the insensitive pedestrians, we find that the escape time is directly proportional to the number of insensitive pedestrians and inversely proportional to the view radius. However, when the view radius is large enough, the escape time does not change significantly, and the evacuation of people in a small view radius environment tends to be integrated. With the improvement of view radius conditions, the escape time changes more obviously with the proportion of insensitive pedestrians. A new emergency sign layout is proposed, and the simulations show that the proposed layout can effectively reduce the escape time in a small view radius environment. However, the evacuation effect of the new escape sign layout on the large view radius environment is not apparent. In this case, the exit setting emerges as an additional factor affecting the escape time.
Analysis of the effects of scanning trajectory parameters on minute-scale lidar forecasting
Authors: Ortensi M , Theuer F , Kühn M
Published: June 2022

The definition of the lidar scan parameters for a measurement campaign can be complex and unsure due to the distinct effects that they cause on the lidar output. In order to study those effects on minute-scale lidar forecasting, we simulated several lidar trajectories in a large eddy simulation wind field. We analysed the effects of changes in range gate spacing, and angular and temporal resolution on the accuracy of wind field reconstruction and on the forecast skill of 5 minute ahead deterministic wind speed forecasts. We found that the range gate spacing has no influence on forecast skill. Furthermore, we discussed the forecasting improvement caused by the increase in angular resolution, and its unexpected worsening caused by the reduction in temporal resolution. We concluded that high values for range gate spacing do not reduce the forecast skill, and that the effects of changes in angular and temporal resolution on forecast accuracy can differ among the wind turbines.
Modeling extreme weather events for offshore wind in the North Sea: a sensitivity analysis to physics parameterizations in WRF
Authors: Vemuri Adithya , Munters Wim , Buckingham Sophia , Helsen Jan , Beeck Jeroen van
Published: June 2022

Extreme weather events such as storms, cold fronts, and mesoscale convective systems, are capable of producing extreme and sudden precipitation, strong wind gusts and fast changes in wind direction, which are potentially harmful for the operation, power production and maintenance of wind farms. This study aims to provide insight into the modelling of three such events for the offshore wind farms located in the Belgian North Sea. This work extends the ongoing work on generalizing WRF model sensitivities to multiple extreme weather events. This sensitivity analysis considers three cases consisting of Storm Ciara, a low-pressure system and a cold front. These events are modeled using five model setups comprising different combinations of WRF physics parameterizations. The simulated wind direction and speed are evaluated against 10-min averaged SCADA data. The results indicate no single best model setup to accurately simulate all events. The combination of physical parameterization is observed to play a major role in model sensitivities. The use of scale-aware physical parameterization show potential for better performance but is heavily influenced by the combination of model physics and the type of event. No conclusive set of inferences are obtained from a qualitative evaluation of precipitation against observed radar data.
A numerical investigation of a wind turbine wake in non-neutral atmospheric conditions
Authors: Baungaard M. , Abkar M. , van der Laan M.P. , Kelly M.
Published: June 2022

Wind turbine wakes cause energy losses and increased blade fatigue loads in wind farms. The magnitude of these effects depend strongly on the atmospheric conditions. In nonneutral atmospheric conditions, there is a turbulence kinetic energy (TKE) contribution from buoyancy, either positive (convective boundary layer, CBL) or negative (stable boundary layer, SBL). In this work, both conditions are analyzed with new large-eddy simulation (LES) data of a single wind turbine wake in flat, homogeneous terrain to quantify the effects of buoyancy. It is found that the buoyancy contribution is negligible compared to the shear production in the wake region and the role of buoyancy is therefore mainly to alter the inflow profiles. This fact is used in a simple Reynolds-averaged Navier-Stokes (RANS) turbulence model, which shows reasonable results for wake velocity deficit compared to LES data.
Influence of nacelle-lidar scanning patterns on inflow turbulence characterization
Authors: Fu Wei , Sebastiani Alessandro , Peña Alfredo , Mann Jakob
Published: June 2022

Nacelle lidars with different number of beams, scanning configurations and focus distances are simulated for characterizing the inflow turbulence. Lidar measurements are simulated within 100 turbulence wind fields described by the Mann model. The reference wind turbine has a rotor diameter of 52 m. We assume homogeneous frozen turbulence over the lidar scanning area. The lidar-derived Reynolds stresses are computed from a least-square procedure that uses radial velocity variances of each of the beams and compared with those from a simulated sonic anemometer at turbine hub height. Results show that at least six beams, including one beam with a different opening angle, are needed to estimate all Reynolds stresses. Enlarging the beam opening angle improves the accuracy and uncertainty in turbulence estimation more than increasing the number of beams. All simulated lidars can estimate the along-wind variance accurately. This work provides guidance on designing and utilizing nacelle lidars for inflow turbulence characterization.
Impact of Canonical Perturbations in the Inflow on Wind Turbine Loads
Authors: McGhee Jaylon , Wagh Divya , Farnsworth John , Vijayakumar Ganesh
Published: June 2022

The turbulent velocity field in the atmospheric boundary layer creates inflow fluctuations at a wide range of spatial and temporal scales that must be considered for resilient turbine design. This study is focused on separately characterizing the wind turbine’s response to canonical spatial or temporal oscillations in the inflow with only one wavelength or frequency. Temporal fluctuations in the inflow produce a response that is up to an order of magnitude greater than that of purely spatial fluctuations. The response to spatial oscillations in the inflow is also highly dependent on the alignment of the spatial waveform in the inflow to the rotor hub (i.e. phase angle). Different phase angles produced up to a factor of 30 difference in load variations. The integrated wind turbine loads show the largest response to spatial oscillations at wavelengths around 100-140m which is approximately four times the response at the chord scales. Structural deformations largely dampen the response to spatial oscillations in the inflow while increasing the response to temporal fluctuations near the blade rotation frequency. The wind turbine control system also amplifies the load response to temporal oscillations near the blade rotation frequency.
A fast, heuristic method for generating offshore wind farm turbine layouts
Authors: Bech K. H.
Published: June 2022

This paper describes a heuristic method for generating turbine layouts for offshore wind farms, applying a physical analogy between particle motion in a potential field and turbine placement in a wind kinetic energy field. The method is fast, and relies on industry-standard wake models to account for internal and external wake. The method is heuristic in several senses; it is based on physical analogy, it does not solve the wake model explicitly during the iterative process, and it does not necessarily provide an optimal solution at the end of iteration. The current method does not aim to outperform a full optimization with an objective function, explicit wake model solution and an heuristic optimizer algorithm, but the computational speed and simplicity makes it well suited for screening purposes and as initial solutions for more time-consuming methods.
Surrogate modelling of wind fields from point-wise atmospheric turbulence measurements
Authors: Friedrich Jan , Moreno Daniela , Sinhuber Michael , Wächter Matthias , Peinke Joachim
Published: June 2022

We present an advanced model for the generation of synthetic wind fields that can be understood as an extension of the well-known Mann model. In contrast to such Gaussian random field models which control second-order statistics (i.e., velocity correlation tensors or spectra), we demonstrate that our extended model incorporates the effects of higherorder statistics as well. In particular, the empirically observed phenomenon of small-scale intermittency, a key feature of atmospheric turbulent flows, can be reproduced with high accuracy and at considerably low computational cost. Our method is based on a recently developed multipoint statistical description of a turbulent velocity field [J. Friedrich et al., J. Phys. Complex. 2 045006 (2021)] and consists of a superposition of multivariate Gaussian statistics with fluctuating covariances. Furthermore, we explicitly show how such superstatistical Mann fields can be constraint on a certain number of point-wise measurement data. We give an outlook on the relevance of such surrogate wind fields in the context of fatigue loads on wind turbines.
Enhanced resource assessment and atmospheric monitoring of the research wind farm WiValdi
Authors: Wildmann Norman , Hagen Martin , Gerz Thomas
Published: June 2022

Prior to the installation of wind turbines at the Krummendeich research wind farm (referred to as WiValdi) which is developed by the German Aerospace Center (DLR), we conducted measurements with a Doppler wind lidar (DWL) and a microwave radiometer (MWR) for 16 months, starting in November 2020. The remote sensing data was validated against radiosonde measurements in a short-term campaign in September 2021. We present a statistical analysis of wind speed, direction, shear and veer as derived from the DWL as well as static stability from temperature retrievals of the MWR in comparison to model results of the New European Wind Atlas (NEWA). The observational data from 2021 shows a higher occurence rate of winds from northwest in comparison to the longer-term statistics of NEWA (2011-2018). From the overall statistics, cases with the occurence of low-level jets (LLJ) are detected and separated. The LLJs appear in 49% of the observed nights and predominantly in conditions with seaward wind direction. The LLJ increases shear and veer in the rotor swept area and thus the potential loads significantly. Future research at WiValdi will allow to analyse the effects of such load cases on the wind turbines in detail.
Evaluation of two mesoscale wind farm parametrisations with offshore tall masts
Authors: Garcia-Santiago Oscar M. , Badger Jake , Hahmann Andrea N. , Lucia da Costa Graziela
Published: June 2022

We use offshore tall mast measurements in the North and Baltic Sea to evaluate wind farm parametrisations in the Weather Research and Forecasting (WRF) model. We identify periods before and after the construction of neighbouring wind farms around the tall mast for evaluation purposes. We test multiple WRF model set-ups for the pre-construction period to obtain the best configuration representing the region’s wind climate. Our best set-up is then used with the wind farm parametrisations to investigate wind farm wake effects in the post-construction phase. We use two wind farm parametrisation schemes implemented in the WRF model: the Fitch scheme and the Explicit Wake Parametrisation. We select and prioritise cases of low variability in wind speed and direction to isolate the wake effects. Our results indicate an improvement in the wind speed, as a function of height, using both schemes, with the Fitch scheme creating deeper wakes than EWP. Both parametrisations perform well in cases when the mast is aligned far-downwind of the turbines, with mean wind speed differences of less than 0.2 ms−1. In near-wind cases, the performance of the scheme varies with the site. Finally, our research provides a possible baseline for testing the improvements in the wind farms parametrisations schemes in these offshore regions.
Comparison of time averaged wake flow fields between LiDAR measurements and DWM
Authors: Da Costa Louis , Bernard Valentin , Zamanbin Arezoo
Published: June 2022

The Dynamic Wake Meandering model (DWM) promises an accurate way of assessing wake effect through time-resolved simulation of wake flow fields. This study aims to validate the DWM model by comparing 10-minute time averaged wake flow fields to measurements from a LiDAR campaign carried out on the Lillgrund offshore wind farm. An implementation of the DWM model following the parametrization proposed in the IEC61400 standard is used. Two superposition assumptions are tested. Results show a good visual match and relative convergence of a quantitative error metric for downstream distances of 3 to 3.5 turbine diameters. Results in the immediate near wake show a clear mismatch, as expected considering model assumptions. Linear superposition of all wakes performs best, provided that local conditions at the wake emitting turbines are estimated. The present study uses a limited dataset of only one week of LiDAR data, making the results susceptible to measurement uncertainties.
A data-informed analytic model for turbine power prediction with anisotropic local blockage effects
Authors: Juniper Marcus C R , Nishino Takafumi
Published: June 2022

This paper presents a new analytic approach for estimating the local power coefficient of a turbine experiencing anisotropic local blockage effects. Data-driven methods are employed first to approximately obtain a known analytic expression for isotropic local blockage effects, and then deployed to find candidate expressions for anisotropic local blockage effects. The dataset for the analysis of anisotropic local blockage is collected from 3D Reynolds-averaged Navier-Stokes (RANS) simulations of an infinitely wide array of actuator disks for nearly 2,000 different blockage configurations. This study builds upon previous work in array optimisation of both tidal and wind turbines, where the local power coefficient may increase substantially for optimal array configuration. A brief discussion on the relationship between the local blockage and wind farm blockage is also provided. Other theoretical approaches and possible refinements to the presented analytic model are also discussed.
Wake impact of constructing a new offshore wind farm zone on an existing downwind cluster: a case study of the Belgian Princess Elisabeth zone using FLORIS
Authors: Munters Wim , Adiloglu Baris , Buckingham Sophia , Beeck Jeroen van
Published: June 2022

A first assessment of the potential wake impact of the future Princess Elisabeth wind-farm cluster on the existing Belgian wind farms is performed. We consider 3 different wake models coupled to a blockage model implemented in FLORIS, and study 15 design scenarios for the future cluster. Simulations show that, although intra-cluster wake effects are qualitatively comparable, inter-cluster effects differ strongly among model setups, confirming results in recent literature. With increasing new-zone capacity, a trend of higher existing-zone AEP loss caused by the new zone is observed, as well as an incentive to use turbines with higher individual rating. Quantitatively, AEP loss due to inter-cluster wakes can reach up to 0.8% for the full existing zone as compared to a reference case without the Princess Elisabeth zone. Further, worst-case conditions with west-southwesterly winds show the new zone induces an inter-cluster power loss of 6% for the entire existing zone, with extremes up to 20% for specific turbines.
Physics informed DMD for periodic Dynamic Induction Control of Wind Farms
Authors: Muscari C , Schito P , Viré A , Zasso A , van der Hoek D , van Wingerden JW
Published: June 2022

Dynamic Induction Control (DIC) is a novel, exciting branch of Wind Farm Control. It makes use of time-varying control inputs to increase wake mixing, and consequently improve the velocity recovery rate of the flow and the power production of downstream turbines. The Pulse and the Helix are two promising DIC strategies that rely on sinusoidal excitations of the collective pitch and individual pitch of the blades, respectively. While their beneficial effects are evident in simulations and wind tunnel tests, we do not yet fully understand the physics behind them. We perform a systematic analysis of the dynamics of pulsed and helicoidal wakes by applying a data-driven approach to the analysis of data coming from Large Eddy Simulations (LES). Specifically, Dynamic Mode Decomposition (DMD) is used to extract coherent patterns from high-dimensional flow data. The periodicity of the excitation is exploited by adding a novel physics informed step to the algorithm. We then analyze the power spectral density of the resulting DMD modes as a function of the Strouhal number for different pitch excitation frequencies and amplitudes. Finally, we show the evolution in time and space of the dominant modes and comment on the recognizable patterns. By focusing on the modes that contribute the most to the flow dynamics, we gather insight on what causes the increased wake recovery rate in DIC techniques. This knowledge can then be used for the optimization of the signal parameters in complex layouts and conditions.
Design of the American Wake Experiment (AWAKEN) field campaign
Authors: Debnath Mithu , Scholbrock Andy K. , Zalkind Daniel , Moriarty Patrick , Simley Eric , Hamilton Nicholas , Ivanov Chris , Arthur Robert S. , Barthelmie Rebecca , Bodini Nicola , Brewer Alan , Goldberger Lexie , Herges Thomas , Hirth Brian , Valerio Iungo Giacomo , Jager Dave , Kaul Colleen , Klein Petra , Krishnamurthy Raghu , Letizia Stefano , Lundquist Julie K. , Maniaci David , Newsom Rob , Pekour Mikhail , Pryor Sara C , Ritsche Michael T. , Roadman Jason , Schroeder John , Shaw William J. , Van Dam Jeroen , Wharton Sonia
Published: June 2022

The American WAKE experimeNt (AWAKEN) is a multi-institutional collaborative field campaign, starting in March 2022, that will gather an unprecedented data set including both atmospheric observations and wind plant operational data. This comprehensive data set will be used to characterize the wind plant performance and turbine loading in different operational and atmospheric conditions and validate the use of different wind plant control strategies and simulation frameworks. An extensive field campaign like AWAKEN requires proper coordination and long-term planning to be successful. In this paper, we review the major activities planned during AWAKEN to provide information for current and future project partners. Specifically, we provide information about the project sites, their planned instruments, and how these will relate to the scientific objectives of the overall AWAKEN project.
LIDAR based multivariable H∞ feedforward control for load reduction in wind turbines
Authors: Miquelez-Madariaga I. , Schlipf D. , Elso J. , Guo F. , Díaz de Corcuera A.
Published: June 2022

The performance of LIDAR assisted control is highly affected by the quality of the wind measurement. Consequently, it seems convenient to include some kind of uncertainty model during the design of the LIDAR-based feedforward controllers. Robust control methodologies provide a great tool to deal with uncertain systems. In this paper, three H∞ feedforward controllers have been designed based on linear frequency-domain models of the system. Because the control problem is multivariable and multiobjective, the controllers show different combinations of control actions -pitch and generator torque- and control objectives -generator speed regulation and tower base load reduction-. The controllers have been tested in a nonlinear simulation using the aerolastic code OpenFAST, a turbulent wind field and the onshore NREL 5MW wind turbine model.
Aerosense: Long-Range Bluetooth Wireless Sensor Node for Aerodynamic Monitoring on Wind Turbine Blades
Authors: Polonelli Tommaso , Deparday Julien , Müller Hanna , Fischer Raphael , Benini Luca , Barber Sarah , Magno Michele
Published: June 2022

Predictive maintenance and structural health monitoring are challenging and promising research fields today. In particular, cost-effective and long-term monitoring of wind turbines has been proven to be one of the key elements to successfully increase their efficiency. Accurate numerical modeling and real-time control-in-the-loop play an increasingly prominent role in understanding and optimizing blade aerodynamic and acoustic performances. A non-intrusive and modular measurement system is a prerequisite for long-term measurement campaigns in existing and future wind turbines. Current methods of performing aerodynamic and acoustic field measurements are cumbersome and expensive, leading to a shortage of aerodynamic and acoustic datasets on operating wind turbines. This paper demonstrates the ability of the new Aerosense system to operate successfully in the field. Aerosense is a long-lasting battery-operated and flexible wireless sensor node that can directly measure aerodynamic and acoustic effects on wind turbine blades. It consists of an array of state-of-the-art Micro-Electro-Mechanical Systems (MEMS) sensors, including 40 barometers and 10 microphones, combined with an ultra low power system-on-chip with wireless transmission over Bluetooth 5.1. Experimental results demonstrate the possibility of continuously acquiring data for up to four months on a single lithium battery of 8.7 Ah, featuring an absolute accuracy of 10Pa and an audio bandwidth of 6kHz.
Spatially distributed and simultaneous wind measurements with a fleet of small quadrotor UAS
Authors: Wetz Tamino , Wildmann Norman
Published: June 2022

The understanding of micro-scale flow in the atmospheric boundary layer is one major challenge in wind energy research. Besides the broad possibilities of numerical simulations, experimental data are necessary for tests of the flow conditions within a wind farm under real conditions. In wind energy and atmospheric science, a variety of measurement devices exist for measuring the wind speed. We propose a measurement system that enables completely flexible simultaneous wind measurements using a fleet of multirotor unmanned aircraft systems (UAS). This approach is validated through a two-week measurement campaign at the boundary layer field site Falkenberg of the German National Meteorological Service (DWD). The wind speed is calculated from UAS motions in hover state without additional wind sensors. The measurements are calibrated and validated against sonic anemometers mounted at a 99 m mast. The capability of highly accurate spatial distributed wind measurement with an improved wind algorithm is proven by a root mean square error (RMSE) of 0.25 ms−1 for the horizontal wind speed and < 5° for the wind direction. Further, turbulence measurements are presented showing valid results up to a frequency of 2 Hz in high turbulence conditions. Additionally, spatially horizontal distributed measurements with multiple UAS are examined in a case study of a gust front event.
Evaluation of the “fan scan” based on three combined nacelle lidars for advanced wind field characterisation
Authors: Meyer P J , Gottschall J
Published: June 2022

Nacelle-mounted lidars are finding more and more applications in the wind industry. Their use cases range from power performance testing to lidar-assisted control. Depending on the application, different wind field parameters are reconstructed from the lidar’s line-of-sight wind measurements. Hereby, the respective scan geometry of the lidar determines the capabilities and limitations for the reconstruction of wind field parameters. In this study, we introduce the fan scan, a scanning pattern made of multiple nacelle-mounted lidars with the aim of an advanced wind field assessment to reconstruct more than three wind field parameters, which has been implemented at the Testfeld BHV site in Bremerhaven, Germany. Analytically derived uncertainties as well as calibration results for both, line-of-sight speeds and reconstructed wind field parameters, are presented indicating a good agreement with reference measurements. Additionally, a method for the reconstruction of the horizontal wind shear is introduced and applied to the fan scan data, highlighting the extended utilization possibilities of nacelle lidars.
Design of a gust generator and comparison of model wind turbine and porous disc wake flows in a transverse gust
Authors: Yigili I. , Andirin M. A. , Akpolat M. T. , Baskan O. , Percin M. , Uzol O.
Published: June 2022

This paper outlines the similarities and differences between the wake characteristics of a porous disc and model wind turbine under transverse gust loading which is generated via a two-vane wind tunnel gust generator. Phase-locked two-dimensional two-component (2D2C) particle image velocimetry (PIV) measurements show that the capability of a porous disc in representing a wind turbine increases under gust loading. Compared to the uniform inflow case, normalized streamwise velocity plots of the porous disc and model turbine are in a better agreement under gusty inflow condition particularly in the near wake region. Comparison of the wake centerlines reveals a similar wavy wake pattern with no significant phase lag suggesting that the gust dynamics is the dominant factor in the determination of the wake dynamics.
Numerical Assessment of a BAY-Type Model for different Vortex Generator Shapes Applied on a Wind Turbine Airfoil
Authors: Seel Ferdinand , Blazejewski Lando , Lutz Thorsten , Krämer Ewald
Published: June 2022

Modern wind turbines are increasingly equipped with vortex generators in the inner part of the rotor blade. On large blades it is usual to install hundreds of these small passive devices to delay flow separation. Therefore, affordable numerical methods to include vortex generators in the design process are required. One very promising method based on the addition of source terms in the flow equations is the BAY model. In this work a BAY-type model is presented and assessed for three different vortex generator shapes (rectangular, delta and cropped-delta) placed on a DU 97-W-300 airfoil and computed with steady-state Reynolds-averaged Navier-Stokes numerical methods. In order to quantify the circulation of the main streamwise vortex, a method to identify the vortex core based on the λ2-criterion is presented and shows good results. The computations reveal very good agreement with the experimental data for attached flow in terms of lift and pressure distribution while drag is inherently underestimated by the BAY model due to the inviscid modelling of the vortex generators. It is displayed that this underestimation is proportional to the vortex generator surface area. Concerning the different shapes, it is shown that the delta shape reveals larger deviations from the fully resolved case than the other geometries and therefore requires higher mesh resolution.
Experimental evaluation of the mesh load factor (Kγ) of a 6MW wind turbine gearbox
Authors: Santiago Unai Gutierrez , Sisón Alfredo Fernández , Polinder Henk , Wingerden Jan-Willem van
Published: June 2022

The significant increase in rotor diameters seen in modern wind turbines has pushed gearbox manufacturers to introduce technological innovations to increase the torque density of current designs. Driven by the need to lower the cost of energy from wind and size limitations imposed by logistic constraints in onshore wind, a trend has emerged to increase the number of planetary stages and the number of planet gears per stage. One of the main challenges of next-generation gearbox designs is sharing the load evenly between a high number of planets. This paper presents an experimental evaluation of the mesh load factor of a modern 6MW wind turbine gearbox with five planets in the first planetary stage. Results from the traditional method, based on tooth root strain gauges, and from strain measurements in the outer surface of the ring gear are described and assessed. Both experimental approaches have yielded lower mesh load factor values than the default values required in the standard “Design requirements for wind turbine gearboxes” IEC 61400-4. Since the mesh load factor is used for gear rating and sizing, a lower value allows a more optimized gearbox design, which leads to a significant improvement in torque density and cost.
Performance analysis of wind turbines with leading-edge erosion and erosion-safe mode operation
Authors: Barfknecht N , Kreuseler M , de Tavernier D , von Terzi D
Published: June 2022

For offshore wind turbines, Leading-Edge Erosion (LEE) due to rain is posing a serious risk to structural integrity and can lead to a performance loss of the order of a few percent of the Annual Energy Production (AEP). A proposed mitigation strategy is the so-called Erosion-Safe Mode (ESM). In this work, the AEP losses caused by LEE or by operating in the ESM are compared for two reference turbines, i.e. the IEA 15MW and the NREL 5MW turbines. For both turbines, the performance is evaluated in uniform and sheared inflow conditions. The effects of erosion are modeled by creating clean and rough airfoil polars in XFOIL. It is assumed that erosion occurs once a critical blade element section speed is exceeded. Power curves for LEE and ESM are calculated by using the free-wake vortex method CACTUS. Results show that LEE negatively affects the power production below rated capacity, while operating in ESM predominantly sheds performance at rated power of the turbine. This study, therefore, shows that a break-even point for the ESM exists. The AEP loss due to erosion can be successfully mitigated with the ESM at sites with low mean wind speed, however, at sites with higher mean wind speed, operation with erosion leads to a lower AEP loss. The break-even point shows little sensitivity to the blade design and to mean shear variations, but strongly depends on the frequency ESM needs to be applied. The latter is driven by the predicted amount of damaging rain events. In conclusion, erosion-optimal operation is strongly governed by the site characteristics and much less by turbine design, and the viability of an ESM strategy can be significantly expanded by a better understanding of blade damage mechanisms and improved forecasting of the related weather events.
Identification of torsional frequencies of a large rotor blade based on measurement and simulation data
Authors: Wegner Anna , Huhn Matthias L. , Mechler Sebastian , Thomas Philipp
Published: June 2022

With increasing size of wind turbines, the aerodynamic and structural properties of the rotor blades become more and more important. However, studies investigating the torsional properties using measurement data and simulation results on full-size turbines are still scarce. In this study, torsional measurements are carried out on a large wind turbine with a rotor diameter of 180 m and compared with model results. In this study, torsional measurements on a large wind turbine with a rotor diameter of 180 m are conducted and compared with model results. Torsional parameters of a rotor blade are identified using two different measurement methods, the first based on strain gauges and the second based on accelerometers. Both methods show frequencies that can be associated with the natural frequencies of the two first torsional modes of the rotor blade. To extract the natural frequencies and to reduce external effects, a manual pitching procedure is evaluated. Here, the first two natural frequencies associated with torsional modes can be identified. In a second step, the behaviour of the blade during power production is investigated and compared with the simulation results.
Active rotor coning for a 25 MW downwind offshore wind turbine
Authors: Qin Chao (Chris) , Loth Eric , Zalkind Daniel S. , Pao Lucy Y. , Yao Shulong , Todd Griffith D. , Selig Michael S. , Damiani Rick
Published: June 2022

A two-bladed downwind turbine system was upscaled from 13.2 MW to 25 MW by redesigning aerodynamics, structures, and controls. In particular, three 25-MW rotors were developed, and the final version is a fully redesigned model of the original rotor. Despite their radically large sizes, it was found that these 25-MW turbine rotors satisfy this limited set of structural design drivers at the rated condition and that larger blade lengths are possible with conewise load-alignment. In addition, flapwise morphing (varying the cone angle with a wind-speed schedule) was investigated to minimize mean and fluctuating blade root bending loads using steady inflow proxies for the maximum and lifetime damage equivalent load moments. Compared to the fixed coned rotor case, morphing can provide an Annual Energy Production (AEP) increase of 6%, and the maximum blade root flapwise bending moment increases 21% (still under the constraint, i.e., 10% of the ultimate moments) as a trade-off. The resulting series of 25-MW rotors can be a valuable baseline for further development and assessment of ultra-large-scale wind turbines.
Simulating tip effects in vertical-axis wind turbines with the actuator line method
Authors: Melani PF , Balduzzi F , Bianchini A
Published: June 2022

Simulation of the complex, unsteady aerodynamics characterizing Darrieus rotors requires computational tools with a fidelity higher than the ubiquitous Blade Element Momentum (BEM) theory. Among them, the Actuator Line Method (ALM) stands out in terms of accuracy and computational cost. This approach, however, still fails to resolve the vortex-like structures shed at the blade ends, overestimating turbine performance at the higher rotational speeds. Moving from this background, in this study a comprehensive investigation on the ALM’s capability to simulate tip effects and their impact on rotor performance is carried out. To this end, the ALM tool developed by the authors in the ANSYS®FLUENT®environment (v. 20.2) and specifically tailored to the simulation of vertical-axis machines was employed. Both a steady finite wing and a fictitious one-blade Darrieus H-rotor, for which high-fidelity blade-resolved CFD data are available as benchmark, were considered as test cases. ALM simulations were first performed without any correction for different cell sizes and force projection radii, so that the limits of the original approach could be assessed. Then, two different sub-models were applied: the classical semi-empirical Glauert correction and a new methodology based on the Lifting Line Theory (LLT), which was recently proposed by Dağ and Sørensen (DS). The latter was here adapted to vertical-axis machines. Eventually, the blade spanwise load profiles coming from the three approaches were assessed and compared, proving the superior performance of the DS model.
Rapid approach for structural design of the tower and monopile for a series of 25 MW offshore turbines
Authors: Escalera Mendoza Alejandra S. , Todd Griffith D. , Qin Chris , Loth Eric , Johnson Nick
Published: June 2022

The goal of further reducing the Levelized Cost of Energy (LCOE) has driven the investigation of large-scale wind turbines. This work presents a simple, rapid and detailed approach for the structural design of the tower and monopile without a controller, but with frequency and high fidelity structural verification. The approach uses an optimization to reduce the mass of the structures while meeting strength, buckling and geometric constraints by using analytical equations. A verification of frequency constraints is performed with BModes, and ANSYS Mechanical APDL is used for high fidelity verification of stress and buckling. The approach is applied to study the design space of three 25 MW offshore wind turbines with different rotor diameters and cone angles, and to evaluate the nacelle center of mass fore-aft location effect. Results obtained show that the tower and monopile are more susceptible to changes in the rotor thrust than the overturning moment even for designs with high pre-cone angle and large distance of the nacelle center of mass from the tower axis. But it is possible to obtain structurally feasible tower and monopile designs for the three 25 MW turbines studied while not exceeding diameter and wall thickness limits. However, mass penalties can be decreased by 0.8-14%, to further reduce the cost of energy, by increasing the diameter limit which may require manufacturing technology development. The approach applied and studies serve to understand the design space of the tower and monopile for a 25 MW turbine, and provide baseline designs that can be used in the development of a controller and evaluation of a full suite of design load cases.
Field tests of a highly flexible downwind ultralight rotor to mimic a 13-MW turbine rotor
Authors: Loth Eric , Ananda Gavin , Chetan Mayank , Damiani Rick , Todd Griffith D. , Johnson Kathryn , Kianbakht Sepideh , Kaminski Meghan , Pao Lucy , Phadnis Mandar , Qin Chao (Chris) , Scholbrock Andy , Selig Michael , Simpson Juliet , Yao Shulong
Published: June 2022

Offshore extreme-scale turbines of 20–25 MW in size may offer reduced energy costs. The technical barriers at these extreme scales include escalating blade masses with increased flexibility as well as high gravity loads and tower-strike issues. These barriers may be addressed with a load-aligning downwind turbine. To investigate this type of design, a field test campaign was conducted with an aeroelastically scaled rotor, termed the Segmented Ultralight Morphing Rotor Demonstrator (SUMR-D). The tests were conducted on the Controls Advanced Research Turbine at the National Renewable Energy Laboratory. The paper gives an overviewof the experimental diagnostics, blade design, and results of the field campaign, as well as makes conclusions and recommendations regarding extreme-scale highly flexible downwind rotors.
Investigation of a delta wing Vortex Generator
Authors: Hansen M O L , Rogowski K
Published: June 2022

The flow and the effect on a DU08-W-210 airfoil using a conventional and a delta wing vortex generator (VG) has been investigated numerically using ANSYS Fluent and STAR CCM+ CFD solvers. The numerical results from the two CFD programs are very consistent. Both VG geometries delay boundary separation at the trailing edge of the airfoil and thus increase the lift and lift to drag ratio. The overall aerodynamic loads on the airfoil are quite similar for the two different VG geometries, but the created vortices give a slightly different distribution of the skin friction in the wake behind the VGs. The vortex created by the delta wing VG for the same conditions and size as the conventional triangular VG is slightly stronger and probably caused by the fact that the surface creating the vortices is placed higher up in the boundary layer and thus experience a higher inflow velocity. A method to measure the strength and core radius of the created vortices using a Lamb-Ossen vortex is described.
A simple model to predict the energy loss due to leading edge roughness
Authors: Bak Christian
Published: June 2022

In this paper a simple model to predict losses in annual energy production due to leading edge roughness or leading edge erosion from wind turbines is presented. Because detailed information about wind turbines not always are available, the input to this model requires only a few parameters: rated power, rotor radius, two Weibull parameters and categories of local damages. Therefore, some assumptions are made in the model. It was validated against a common implementation of a BEM code and showed good agreement in the loss of annual energy production.

Also, a categorisation methods was proposed. The categories ranged between a and e corresponding to an undamaged blade and a severely damaged blade, respectively. This categorisation was made locally on the blade. The blade is divided into annular elements and each annular element is graded with a category. By summing up the annular elements on the blade, each with their categories, the total loss in energy production was predicted.
Impact of wind turbine operation conditions on infrasonic and low frequency sound induced by on-shore wind turbines
Authors: Blumendeller Esther , Hofsäß Martin , Goerlitz Arne , Cheng Po Wen
Published: June 2022

In this paper, the influence of wind turbine operation conditions, like rotational speed, nacelle position and output power, on the low- and infrasonic sound emissions at the wind farm and sound immissions at residential buildings will be investigated. For this purpose, parallel measurements were carried out at a wind farm on the Swabian Alb in complex terrain and at four residential locations in the vicinity of the wind farm over a period of two months. Distinctive tones can be assigned to the blade passage at different rotational speeds. Furthermore, tones at 28.9 Hz (rated) and 18.3 Hz (below rated) with two higher harmonics can be attributed to the wind turbine generator. Wind farm infrasonic tones at the blade passing frequency were detected at the wind farm and residential buildings. At the residential buildings infrasonic tones were detected mainly for maximum rotational speed of the wind turbines and seem to be independent from wind direction.
Simulation based prediction of ring creep on a planetary bearing of a 1MW wind turbine gearbox
Authors: Gnauert Jonas , Krause Alexander , Jacobs Georg , Bosse Dennis , Zang Aaron
Published: June 2022

Wind turbines (WT) must be further optimized concerning availability and reliability. One of the major reasons of WT downtime is the failure of gearbox bearings. Some of these failures occur due to the ring creep. Ring creep is a failure mechanism, which is mostly detected in the planetary bearings. Ring creep describes a relative movement of the outer ring to the planetary gear. In order to improve the understanding of ring creep, the finite element method (FEM) is used to simulate ring creep in a full-scale planetary bearing (NJ2330) of a 1MW WT planetary gear. The simulation results are validated with experimental data, obtained on a nacelle test bench. The study shows that the simulative prediction of ring creep is of the correct order of magnitude, but no accurate prediction of ring creep can be made without accounting for manufacturing tolerance classes.