bluuebunny/crossref_metadata_2025 huggingface dataset:
Single layer aligned semiconducting single-walled carbon nanotube array with high linear density
Authors: Liu Hao , Liu Fengjing , Sun Zhaolou , Cai Xiaoyong , Sun Huijuan , Kai Yuan , Chen Li , Jiang Chao
Published: June 2022

Highly ordered semiconducting single-walled carbon nanotubes(sc-SWCNTs) array with high purity, high linear density and controllable manner is strongly desired for carbon-based integrated circuits, yet it remains a big challenge. Herein, close-packed single layered and controllably aligned sc-SWCNTs arrays were obtained through dielectrophoresis using a high purity sc-SWCNT dispersion. Under optimized condition of length and average number of interconnecting junctions across the channel full of aligned sc-SWCNTs, field effect transistors (FETs) with high performance were achieved with both a high on/off current ratio and large carrier mobility. Based on the optimized channel length, by systematically optimizing the dielectrophoresis parameters of the frequency and duration of applied AC voltage (V
pp), the highly ordered sc-SWCNTs arrays with an ultra-high linear density of 54 ± 2 tubes μm−1 showed relatively high device performance of FET. The fabrication process optimized in this report can be further extended and applied in large-area, low-cost carbon-based integrated circuits.
Intentionally created localized bridges for electron transport through graphene monolayer between two metals
Authors: Daugalas T , Bukauskas V , Lukša A , Nargelienė V , Šetkus A
Published: June 2022

Monolayer graphene (1LG) is frequently unpredictably modified by supporting material so that it limits development of devices. Van der Waals interaction is dominant in the models describing the in-plane processes, including the electrical charge transport. However, the current flow perpendicular to the plane of the graphene is still less understood. This report analysed specific aspect of the perpendicular current and disclosed an original way to create transport bridges perpendicular to the plane across the 1LG. The most extraordinary finding is that the electron transport between two parallel metal surfaces can be shut down and opened if the metals are separated by the 1LG. The electron transmission can be intentionally varied in this metal–1LG–metal (M–G–M) system by pressure. In the experimental study the AFM force curve and tunnelling current measurements were combined when the external load force (0–1200 nN) and electrical potential (−1.5 V to +1.5 V) were used. It is proved that for low voltages (<±9 mV) a bridge is opened perpendicular to the graphene across the M–G–M systems by the external force, if the compression dependent Fermi level crosses electronic states in the interfaces and graphene. The localised bridges with diameter about 10–40 nm can be opened and kept continuously by the stabilised force in separated points of the system. However, the predictable changes can be produced in the system if the voltage and the force exceeded critical magnitudes. A combined model was proposed acceptable to explain the bridging and predictably modify the characteristics.
Superconductivity and weak anti-localization in nodal-line semimetal SnTaS2
Authors: Singh M , Saha P , Nagpal V , Patnaik S
Published: June 2022

Topological semimetals with superconducting properties provide an emergent platform to explore the properties of topological superconductors. We report magnetization, and magneto-transport measurements on high quality single crystals of transition metal dichalcogenide SnTaS2. It is a nodal line semimetal with superconducting transition below T
c = 2.9 K. Moderate anisotropy (γ = 3.1) is observed in upper critical fields along H//c and H//ab plane. In the normal state we observe large magneto-resistance and weak anti-localization effect that provide unambiguous confirmation of topological features in SnTaS2. Therefore, genuine topological characteristics can be studied in this material, particularly with regard to microscopic origin of order parameter symmetry.
A new MgB2 bulk ring fabrication technique for use in magnetic shielding or bench-top NMR systems
Authors: Moseley D A , Wilkinson D P , Mousavi T , Dennis A R , Speller S , Durrell J H
Published: June 2022

We report a new methodology in bulk MgB2 ring production for use in small-scale magnetic shielding or bench-top nuclear magnetic resonance systems. This process is a modified field-assisted sintering technique (mFAST) which enables direct formation of the rings without the need for machining or additives into the precursor powder. The shielding and trapped field capabilities of three mFAST MgB2 rings were determined using zero-field- and field-cooled magnetic experiments. Individual bulks trap magnetic fields up to 1.24 T at 20 K comparable to the highest published data for a ring sample. It is anticipated that for many applications, multiple rings will be stacked to form the required experimental structure. We find, for the three ring stack, a trapped field of 2.04 T and a maximum shielded field of 1.74 T at 20 K. The major factor limiting performance at low temperatures are flux jumps which cause rapid loss of the trapped field or shielding capability. Preliminary studies of magnetic field ramp rate dependence on flux jumps were conducted illustrating that even at very slow ramp rates (0.007 T min−1) they remain a significant issue. Despite this concern, we conclude that mFAST represents an exciting new fabrication methodology for bulk MgB2 rings.
Hesitant fuzzy partitioned Maclaurin symmetric mean aggregation operators in multi-criteria decision-making
Authors: Ali Jawad
Published: June 2022

A hesitant fuzzy set, enabling the membership of an element to be a set of various possible values, is highly helpful in describing people’s uncertainty in everyday life. Hesitant fuzzy aggregation operators are the standard mathematical tools for combining many inputs according to predefined criteria into a single result. The classic hesitant fuzzy aggregation operator-based approaches have been criticized because of the ignorance of criteria classification. In this work, we develop the conception of the hesitant fuzzy partitioned Maclaurin symmetric mean and hesitant fuzzy weighted partitioned Maclaurin symmetric mean operators spurred by the partitioned Maclaurin symmetric mean. Afterward, we analyze several features and peculiar instances of the formulated operators. A novel multiple criteria decision-making (MCDM) technique is propounded on the documented hesitant fuzzy weighted partitioned Maclaurin symmetric mean operator; the MCDM method chooses the optimal alternative from several alternatives. A case study of the best location selection for hospital construction is addressed to showcase the practicability of the presented technique. Eventually, we illustrate the devised approach is more widespread and efficacious than prevailing approaches via comparative and sensitive analyses.
Hybrid structures of rogue waves and breathers for the coupled Hirota system with negative coherent coupling
Authors: Du Zhong , Guo Chuan-Mei , Guo Qian , Yuan Yu-Qiang
Published: June 2022

Hybrid waves play the significant parts in the nonlinear physical systems. In this work, we study the hybrid waves for the coupled Hirota system with negative coherent coupling. Firstly, based on the generalized Darboux transformation method, we derive the Nth-order hybrid wave solutions with two spectral parameters, which can describe the mth-order rogue waves and (N−M)th-order breathers. According to such solutions, we graphically illustrate the properties of the second-, third- and fourth-order hybrid waves. Moreover, we exhibit five kinds of hybrid structures consisting of the rogue waves and breathers: (i) consisting of the first-order rogue waves and first-order breathers; (ii) consisting of the second-order degenerate breathers; (iii) consisting of the first-order rogue waves and second-order degenerate breathers; (iv) consisting of the second-order rogue waves and first-order breathers; (v) consisting of the second-order rogue waves and second-order degenerate breathers. In addition, we show that the angle between the two-hump bright breathers and the negative direction of the

x

-axis increases with the value of the real parameter in the coupled Hirota system with negative coherent coupling.
Real-time methods of hydrogen sulfide detection
Authors: Kireev S V , Kondrashov A A , Rybakov M A , Shnyrev S L , Sultangulova A I
Published: June 2022

A comparative analytical review of existing methods and means of measuring hydrogen sulfide content in gasses was performed. The method of infrared laser absorption spectroscopy, which uses semiconductor diode lasers tunable in the mid-infrared range as emission sources, is proving to be one of the most promising methods for solving these problems.
Photon-interactions with perovskite oxides
Authors: Yao Hongbao , Guo Er-Jia , Ge Chen , Wang Can , Yang Guozhen , Jin Kuijuan
Published: June 2022

Photons with variable energy, high coherency, and switchable polarization provide an ideal tool-kits for exploring the cutting-edge scientific questions in the condensed matter physics and material sciences. Over decades, extensive researches in the sample fabrication and excitation have employed the photon as one of the important means to synthesize and explore the low-dimensional quantum materials. In this review, we firstly summarize the recent progresses of the state-of-the-art thin-film deposition methods using excimer pulsed laser, by which syntactic oxides with atomic-unit-cell-thick layers and extremely high crystalline quality can be programmatically fabricated. We demonstrate that the artificially engineered oxide quantum heterostructures exhibit the unexpected physical properties which are absent in their parent forms. Secondly, we highlight the recent work on probing the symmetry breaking at the surface/interface/interior and weak couplings among nanoscale ferroelectric domains using optical second harmonic generation. We clarify the current challenges in the in-situ characterizations under the external fields and large-scale imaging using optical second harmonic generation. The improvements in the sample quality and the non-contact detection technique further promote the understanding of the mechanism of the novel properties emerged at the interface and inspire the potential applications, such as the ferroelectric resistive memory and ultrahigh energy storage capacitors.
Fundamental study towards a better understanding of low pressure radio-frequency plasmas for industrial applications
Authors: Liu Yong-Xin , Zhang Quan-Zhi , Zhao Kai , Zhang Yu-Ru , Gao Fei , Song Yuan-Hong , Wang You-Nian
Published: June 2022

Two classic radio-frequency (RF) plasmas, i.e., the capacitively and the inductively coupled plasmas (CCP and ICP), are widely employed in material processing, e.g., etching and thin film deposition, etc. Since RF plasmas are usually operated in particular circumstances, e.g., low pressures (mTorr–Torr), high-frequency electric field (13.56 MHz–200 MHz), reactive feedstock gases, diverse reactor configurations, etc., a variety of physical phenomena, e.g., electron resonance heating, discharge mode transitions, striated structures, standing wave effects, etc., arise. These physical effects could significantly influence plasma-based material processing. Therefore, understanding the fundamental processes of RF plasma is not only of fundamental interest, but also of practical significance for the improvement of the performance of the plasma sources. In this article, we review the major progresses that have been achieved in the fundamental study on the RF plasmas, and the topics include 1) electron heating mechanism, 2) plasma operation mode, 3) pulse modulated plasma, and 4) electromagnetic effects. These topics cover the typical issues in RF plasma field, ranging from fundamental to application.
The coupled deep neural networks for coupling of the Stokes and Darcy–Forchheimer problems
Authors: Yue Jing , Li Jian , Zhang Wen , Chen Zhangxin
Published: June 2022

We present an efficient deep learning method called coupled deep neural networks (CDNNs) for coupling of the Stokes and Darcy–Forchheimer problems. Our method compiles the interface conditions of the coupled problems into the networks properly and can be served as an efficient alternative to the complex coupled problems. To impose energy conservation constraints, the CDNNs utilize simple fully connected layers and a custom loss function to perform the model training process as well as the physical property of the exact solution. The approach can be beneficial for the following reasons: Firstly, we sample randomly and only input spatial coordinates without being restricted by the nature of samples. Secondly, our method is meshfree, which makes it more efficient than the traditional methods. Finally, the method is parallel and can solve multiple variables independently at the same time. We present the theoretical results to guarantee the convergence of the loss function and the convergence of the neural networks to the exact solution. Some numerical experiments are performed and discussed to demonstrate performance of the proposed method.
Exploring fermionic multiplet dark matter through precision measurements at the CEPC *
Authors: Gao Lin-Qing , Bi Xiao-Jun , Wang Jin-Wei , Xiang Qian-Fei , Yin Peng-Fei
Published: June 2022

New physics could be explored through loop effects using the precision measurements at the Circular Electron Positron Collider (CEPC) owing to its clean collision environment and high luminosity. In this paper, we focus on two dark matter models that involve additional electroweak fermionic multiplets. We calculate their one-loop corrections in five processes, i.e.,

, and

, and investigate the corresponding signatures at the CEPC with the projected sensitivity. We observe that the detectable parameter regions of these processes are complementary. The combined analysis shows that the mass of dark matter

in these two models can be probed up to

and

GeV, respectively, at a 95% confidence level.
The Science of Making Torque from Wind 2022 (TORQUE 2022)
Authors: Watson Simon J
Published: June 2022

Wind energy continues to make great strides in its contribution to the net CO2-zero targets of countries around the world. Both wind farms onshore and offshore are being built, but scientific challenges lie ahead if their generation is to be both reliable and economic. These challenges relate to better understanding the characteristics of the wind, how the wind inflow translates to loads and performance, and how best to build and operate the wind farms of the future so that their output can best be integrated into a 21st century energy system. Europe continues to be a hub for scientific research in wind energy and the European Academy of Wind Energy (EAWE) was created to bring together the top research institutes active in wind energy to cooperate, share knowledge and promote scientific excellence. As part of this remit, the conference, The Science of Making Torque from Wind (or TORQUE, for short) was inaugurated in 2004 in the beautiful city of Delft. The conference has gone from strength to strength and is probably the largest scientific conference devoted to wind energy in the world. History came full circle and the eighth edition, TORQUE 2020 was due to be held in Delft. Unfortunately, the global pandemic meant that this conference had to be held online, but the TU Delft Wind Energy Institute was given a further opportunity to host the ninth edition, TORQUE 2022, in person.

Following the call for three-page abstracts, 435 submissions were made and, after a two-stage peer review process by over 100 reviewers, 300 full papers were accepted for publication in the proceedings. The conference consisted of three plenary sessions, 28 parallel oral sessions and two poster sessions. Oral presenters had 15 minutes to present their work followed by 5 minutes of questions. All poster presenters were allowed a recorded one-minute pitch which could be accessed in advance of the live 90-minute poster sessions.
A new wake detection methodology to capture wind turbine wakes using adaptive mesh refinement and Large Eddy Simulation
Authors: Vigny U. , Benard P. , Tene Hedje P. , Houtin-Mongrolle F. , Bricteux L. , Zeoli S.
Published: June 2022

The development of turbulent vortical wakes released downstream of wind turbines is crucial as it presents many technological implications for wind farm design and exploitation. The numerical prediction of these wakes constitutes a challenging problem as they involve the shedding of fine vortical structures, their instabilities, and interactions with a turbulent ambient flow. A Large Eddy Simulation (LES) approach allows capturing such flow phenomena, which implies a suitable mesh. Adaptive Mesh Refinement (AMR) is used to refine the mesh in the wind turbine wake to limit the computational cost. A methodology is developed to define and capture the wake envelope adequately. Three main parts of this methodology can be identified: The wind turbine wake detection, the target cell size required and adaptation frequency. The target cell size needed to properly capture the wind turbine wake is investigated in previous work [1], while this paper focuses on wind turbine wake detection. A strategy based on a progress variable with a source term in the rotor region is used to capture the wake. This variable is transported by the flow and thus defines the wake envelope. AMR is used to refine the mesh within this region. To validate the method, a comparison between an adaptive mesh case and a reference mesh case has been performed on a single rotor and a two aligned rotor configuration. For both, the wind turbine wake tracking method is effective. The progress variable is transported correctly and leads to a well-defined wake area. The mesh is refined adequately within it. The physical comparison between cases showed similar results, while the performance comparison showed a computational cost reduction of 30% in the single turbine configuration and 50% in the two turbines configuration. Therefore, our methodology coupled with adaptive mesh refinement can adequately capture wind turbine wake, define an accurate wake envelope and decrease the computational cost for the same physical precision.
Large-scale benchmarking of wake models for offshore wind farms
Authors: Nygaard N G , Poulsen L , Svensson E , Pedersen J G
Published: June 2022

We present a systematic framework for the validation and uncertainty quantification of wind farm wake models. The methodology is based on a new definition of the freestream wind speed. We apply the framework on data from 19 offshore wind farms. Our results show that the new wake model TurbOPark is overall unbiased and that the wake model error at each specific site does not depend on the mean turbine spacing. The Park model underestimates the wake loss unless a slower wake expansion than typically used is assumed. In either case, the Park model tends to underestimate the wake loss more for increasing distances between the turbines. We estimate the wake model uncertainty as less than 10% of the wake loss for the considered models.
A one-year long turbulence simulation using a WRF-LES based modeling system at Østerild
Authors: Peña Alfredo , Mirocha Jeffrey D. , Hahmann Andrea N.
Published: June 2022

Here we show an intercomparison between turbulence simulations and measurements performed at a 250-m tall meteorological mast located at the wind turbine test center of Østerild in northern Denmark. The Weather Research and Forecasting model is used to perform the simulations. We use the model’s nesting capability to downscale the forcing reanalysis down to sub-kilometer domains, which are run in a large-eddy simulation fashion, thus being able to partly resolve turbulence. We find that the simulations are able to reproduce accurately the wind climatology of the site when compared to the observations at the mast. The wind power densities derived from the time series of 10-min mean wind speeds from simulations and observations differ by less than 2%. The observed turbulence conditions, in the form of histograms of the wind speed standard deviation, seem fairly well reproduced by the simulations, although there seems to be a tendency of lower simulated turbulence levels. This is also partly reflected when studying the behavior of the turbulence intensity with increasing mean wind speeds. Spectral analysis on the mean wind speed time series reveals that the simulations are able to reproduce the energy content of the observations rather well, but we find a slight larger variability of simulated winds at frequencies larger than 1 h.
A wall-modeled approach accounting for wave stress in Large Eddy Simulations of offshore wind farms
Authors: Aiyer A K , Deike L , Mueller M E
Published: June 2022

In the context of offshore wind farms, accurate predictions of surface fluxes in the marine atmospheric boundary layer are critical for Large Eddy Simulations (LES) of airflow over waves. The effect of the waves on the airflow is often modeled by prescribing the roughness length in the framework of Monin-Obukhov similarity theory. However, such approaches lack generalizability over different wave conditions due to reliance on model coefficients tuned to specific datasets. Wave phase-resolving simulations on the other hand have higher accuracy but also a higher computational cost. In this work, a sea surface-based hydrodynamic drag model is applied to model the pressure-based surface drag felt by the wind due to the waves. An offshore wind farm configuration is simulated using a wall-modeled LES, with the effect of the waves represented using the wave-drag model and the wind turbines represented by an actuator disk model. The approach is validated with data from high fidelity wave-resolving Large Eddy Simulations. The effect of the waves and the farm configuration on the mean velocity profiles power production, and kinetic energy budget are quantified.
Rotor-wake interactions in a wind turbine row: a multi-physics investigation with large eddy simulation
Authors: Gremmo Simone , Muller Etienne , Houtin-Mongrolle Félix , Duboc Bastien , Bénard Pierre
Published: June 2022

Large Eddy Simulation (LES) is growing in importance to support the development, validation and tuning of efficient and reliable engineering models. Such models are essential for the wind energy industry to predict the performance of wind turbines, especially when rotor-wake interactions are involved. In this paper, we present high-fidelity numerical results obtained with a new aero-servo-elastic solver relying on LES and the actuator line method. Two sub-configurations of the Westermost Rough wind farm are investigated: an isolated wind turbine and a seven-turbine row. For validation purposes, field data are compared to time-averaged numerical results for the rotation speed, blade pitch angle, output power, and blade loads. A very good agreement between the results and field data is obtained for the isolated turbine. For the seven-turbine row, some discrepancies are observed. Those can primarily be explained by the inherent difficulty of translating both the actual wind conditions and the influence of the neighboring row into boundary conditions.
Microscale simulations of extreme events in complex terrain driven by mesoscalar budget components
Authors: Avila Matias , Lehmkuhl O. , Navarro J. , González-Rouco J.F. , Paredes D. , Diaz-Marta G. , Owen H.
Published: June 2022

In this work, we apply a downscaling strategy to analyze extreme weather events that may impact wind farm operation. The coupling applies mesoscale momentum budget components (tendencies) from the WRF model as forcing terms to the governing microscale equations. Our study focuses on flow over complex terrain during specific days to reproduce extreme weather events that produced wind turbine damage. The interaction of the meso- and micro-scale features are relevant in the simulation of extreme conditions. The simulation results are compared with observations from nacelle anemometers of the wind turbines in two different wind farms by analyzing time series and wind profiles.

The microscale code Alya, developed at the Barcelona Supercomputing Center (BSC), is closed with URANS and LES closures to solve the momentum and energy equations. Both closures use the same mesoscalar to microscalar coupling methodology and are used in this work to simulate the wind flow. We present the implementation of the mesoscalar coupling to the microscale solver when using URANS and LES closures.

We show that the coupling via tendencies has excellent potential for understanding transient events under extreme weather conditions in very complex terrain. The wind industry can use such simulations to enhance forensic analysis in cases of wind turbine accidents or any other event that may impact turbine operation, such as high turbulence phenomena. We test the ability of the meso- to microscale coupling model to reproduce extreme events with regard to quantities of interest in wind energy.

Simulation results using URANS and LES closures agree reasonably well with observations. In some scenarios, the LES provides results that are closer to measurements. LES models have the advantage of providing wind gusts. We compare the accuracy and performance (CPU-time) of the URANS vs. LES approaches.
Microscale modelling of wind turbines in the New York offshore lease area
Authors: Barthelmie R.J. , Larsen G.C. , Mølgaard Pedersen M. , Pryor S.C.
Published: June 2022

Simulations of the New York offshore wind energy lease area are undertaken within the PyWake program using both NOJ wake parameterization and the Fuga model. Seven different wind farm layouts are simulated to evaluate how the potential annual energy production (AEP) and capacity factor (CF) are impacted by the installed capacity (IC) density and the rotation of the wind turbines compared to the control. The control simulations use the east-west, north-south turbine layout with equidistant spacing of 1.85 km (7.7D) that will be used for the Vineyard Wind offshore wind farm south of Massachusetts. Both CF and AEP have an almost linear relationship with IC for the IC densities of 2.2 to 8.1 MWkm−2. Modelled CF are lower using Fuga (48.9% to 53.9%) than from NOJ (51.5% to 54.4%). The rotated turbine layouts have similar CF to the control because the wind direction distribution is dominated by peaks in both the southwest and northwest directions.
Effects of freestream turbulence on the wake growth rate of a model wind turbine and a porous disc
Authors: Öztürk Buğrahan , Hassanein Abdelrahman , Tuğrul Akpolat M , Abdulrahim Anas , Perçin Mustafa , Uzol Oğuz
Published: June 2022

This study presents the results of an experimental investigation that focuses on quantifying the differences between the spreading rates of a model wind turbine wake and a porous disc wake at different freestream turbulence intensity levels. Two-dimensional two-component particle image velocimetry (2D2C PIV) measurements are performed within the wakes of a model wind turbine and a porous disc (up to 7D downstream) of the same diameter and a matching thrust coefficient. The wind turbine is operated at a Tip Speed Ratio (TSR) of 2 in order to have matching thrust coefficient conditions for a consistent wake comparison. The results show that the mean wake flow field (both near and far wake) is significantly different for the wind turbine compared to the porous disc even if they are operating at similar, high or low, freestream turbulence levels. The wake of the wind turbine recovers much faster than that of a porous disc with a matching thrust coefficient especially in the far wake region at both low and high freestream turbulence levels. On the other hand, the data shows that the far wake of the turbine operating at low freestream turbulence is very similar to that of the disc operating at high freestream turbulence. This suggests caution and stresses the importance in choosing the freestream turbulence intensity level when using porous discs to represent wind turbines in wind tunnel studies.
How does the rotational direction of an upwind turbine affect its downwind neighbour?
Authors: Englberger Antonia , Wrba Linus , Dörnbrack Andreas , Lundquist Julie K.
Published: June 2022

Wind-turbine blades rotate in clockwise direction looking downstream on the rotor. During daytime conditions of the atmospheric boundary layer, the rotational direction has no influence on the turbine wakes. In stably stratified conditions occurring during night, the atmospheric inflow is often characterized by a veering inflow describing a clockwise wind direction change with height in the Northern Hemisphere. A changing wind direction with height interacting with the rotor impacts its wake characteristics (wake elongation, width and deflection). We investigate the impact on the turbine performance (streamwise velocity for power, turbulence kinetic energy for loading) of a downwind turbine considering the four possible combinations of rotational directions of two 5 MW NREL rotors by means of large-eddy simulations. A counterclockwise rotating upwind turbine results in a 4.1% increase of the rotor averaged inflow velocity at the downwind rotor in comparison to a common clockwise rotating upwind turbine rotor. In case of two counterclockwise rotating rotors, the increase is 4.5%. This increase in streamwise velocity is accompanied by a 3.7% increase in rotor averaged turbulence kinetic energy. The performance difference of the downwind rotor (+4.8% increase of cumulative power of both wind turbines, if the upwind rotor rotates counterclockwise) results from the rotational direction dependent amplification or weakening of the spanwise and the vertical wind components, which is the result of the superposition of veering inflow and upwind rotor rotation.
A flexible actuator curve model for aeroelastic simulations of wind turbines in atmospheric boundary layers
Authors: Trigaux François , Chatelain Philippe , Winckelmans Grégoire
Published: June 2022

The current trend of wind turbine upscaling has led to the use of long and slender blades prone to large structural deformations. In the present study, the impact of the aeroelastic effects is assessed for the NREL-5MW wind turbine in a turbulent wind. To this end, an actuator curve method coupled to a one-dimensional finite-element structural solver is implemented in a fourth-order finite difference code that can perform large eddy simulation (LES) of realistic winds. The approach is computationally affordable compared to blade-resolved simulations and hence long time series can be computed. This, combined to the ability of the LES to capture the relevant scales of the unsteadiness wind, leads to a better estimation of the fluctuating loads and power of the turbine. The results are here shown for one and two wind turbines operating in a neutrally stable atmospheric boundary layer. It appears that the blade mostly deforms according to its first bending mode. The rotation of the blade in the sheared atmospheric flow is responsible for large amplitude deformations, but the turbulence also plays a role in causing deformations at higher frequencies with a smaller amplitude. The azimuthal distributions of loads and of power are significantly affected when the aeroelasticity is considered.
Recalibration of a wind atlas using local wind measurements
Authors: Duc Thomas , Guilbert Marie , Péréniguez Léo , Prat Gasull Arnau , Buffard Benoit , Dugouchet Nathan
Published: June 2022

Wind atlases are very useful for getting a first insight of the wind resource at a given site before local measurement is available. The more accurate a wind atlas, the lower the uncertainty in the wind resource and the lower the risk of investing into a new wind project. A new method for improving the accuracy of wind atlases is presented in this article. Using kriging, a statistical technique applied to georeferenced data, existing atlases are combined with past wind measurement campaigns in order to reduce the error in wind speed prediction on a given territory. This methodology being only based on existing datasets, it is quite inexpensive compared to the costs required for building a new atlas from scratch. It is validated considering two different wind atlases, the Global Wind Atlas (GWA) and the AROME-based atlas, which are recalibrated thanks to 55 local wind measurement campaigns available in the north-eastern region of France. The results show a reduction in the root-mean-square error (RMSE) in the wind speed prediction for both atlases. This reduction is particularly significant for the GWA whose initial bias is very strong: its RMSE is decreased by 68%. For the AROME-based atlas, the reduction of RMSE is lower (∼ 28%) but still noticeable. Even with a low number of wind measurement campaigns used for the recalibration, there are still benefits to applying the kriging process. For now, this methodology has only been tested and validated in simple terrain on the wind speed variable, and further research would be needed in order to investigate how it could also be extended to complex terrain.
Wind turbine LIDAR-assisted control: Power improvement, wind coherence and loads reduction
Authors: Meng Fanzhong , Hou Lio Wai , Larsen Gunner Chr.
Published: June 2022

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

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

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

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

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

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

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

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

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

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

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.