bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimisation of diamond quantum processors
Authors: Chen YunHeng , Stearn Sophie , Vella Scott , Horsley Andrew , Doherty Marcus W
Published: September 2020

Diamond quantum processors consisting of a nitrogen-vacancy centre and surrounding nuclear spins have been the key to significant advancements in room-temperature quantum computing, quantum sensing and microscopy. The optimisation of these processors is crucial for the development of large-scale diamond quantum computers and the next generation of enhanced quantum sensors and microscopes. Here, we present a full model of multi-qubit diamond quantum processors and develop a semi-analytical method for designing gate pulses. This method optimises gate speed and fidelity in the presence of random control errors and is readily compatible with feedback optimisation routines. We theoretically demonstrate infidelities approaching ∼10−5 for single-qubit gates and established evidence that this can also be achieved for a two-qubit CZ gate. Consequently, our method reduces the effects of control errors below the errors introduced by hyperfine field misalignment and the unavoidable decoherence that is intrinsic to the processors. Having developed this optimal control, we simulated the performance of a diamond quantum processor by computing quantum Fourier transforms. We find that the simulated diamond quantum processor is able to achieve fast operations with low error probability.
The mass of a dark matter WIMP derived from the Hubble constant conflict
Authors: Baruch John
Published: September 2020

There continues to be good reason to believe that dark matter particles, which only “feel” the gravitational force, influence the local and distant Universe, despite drawing a complete blank in the search for such a particle. The expansion rate of the Universe is defined by the Hubble constant h. Measurements of the Hubble constant at different wavelengths produce different results, differing well beyond their errors. Here it is shown that the two precise but different values for the Hubble constant can be used to derive the mass of a weakly interacting massive particle (WIMP). An approximate mass of 1022 eV is determined with indications of why, so far, it has not been found and what is required to get positive confirmation of its presence. This result also indicates that the Hubble constant is the sum of more than one contribution with suggestions for experimental tests to determine, more precisely, the level of these contributions.
The gravitational wave emission of double white dwarf coalescences
Authors: Zou Ze-Cheng , Zhou Xiao-Long , Huang Yong-Feng
Published: September 2020

Type Ia Supernovae (SNe Ia) are widely used as standard candles to probe the Universe. However, how these fierce explosions are produced itself is still a highly debated issue. There are mainly two popular models for SNe Ia: the double-degenerate scenario and the single-degenerate scenario. The double-degenerate scenario suggests that SNe Ia are produced by the coalescence of two degenerate white dwarfs, while the single-degenerate scenario suggests that the continuous accretion of a single degenerate white dwarf from its normal stellar companion will finally lead to a disastrous explosion when it is over-massive, resulting in an SN Ia. The rapid development of the gravitational wave astronomy sheds new light on the nature of SNe Ia. In this study, we calculate the gravitational wave emissions of double white dwarf coalescences and compare them with the sensitivities of several upcoming detectors. It is found that the gravitational wave emissions from double white dwarf mergers in the local universe are strong enough to be detected by LISA. We argue that LISA-like gravitational wave detectors sensitive in the frequency range of 0.01 — 0.1 Hz will be a powerful tool to test the double-degenerate model of SNe Ia, and also to probe the Universe.
A survey of time series forecasting from stochastic method to soft computing
Authors: Hendikawati P , Subanar  , Abdurakhman  , Tarno
Published: September 2020

Forecasting is a part of statistical modelling that is widely used in various fields because of its benefits in decision-making. The purpose of forecasting is to predict the future values of certain variables that vary with time using its previous values. Forecasting is related to the formation of models and methods that can be used to produce a good forecast. This research is a survey paper research that used a systematic mapping study and systematic literature review. Generally, time series research uses linear time series models, specifically the autoregressive integrated moving average model that has long been used because it has good forecasting accuracy. The successfulness of the Box–Jenkins methodology is based on the reality that various models can imitate the behaviour of various types of series, usually without requiring many parameters to be estimated in the final choice of the model. However, the assumption of stationarity that must be met makes this method inflexible to use. With the advances in computers, forecasting methods from stochastic models to soft computing continue to develop and extend. Soft computing for forecasting can provide more accurate results than traditional methods. Moreover, soft computing has many advantages in terms of the amount of data that can be analysed and the time- and cost-effectiveness of the process.
Identification the preliminary concept of geometry through Prambanan temple artifacts
Authors: Andriani A , Marsigit
Published: September 2020

Student preliminary concepts are useful for knowing how much knowledge he already has. One activity that can be used to find out preliminary concepts is to ask students to describe an object. Observation skills are needed by the student to construct mathematical knowledge. This research was conducted at Prambanan Temple. The purpose of this study was to determine the preliminary concept of geometry for junior high school students. Students are invited directly to the location, then asked to describe mathematical characteristics. The method used in this paper is qualitative reseacrh in which the data analysis uses descriptive qualitative. The data collection is in the form of tests and interviews of 3 students of Junior High School Penelitian ini bertempat di Candi Prambanan dengan rentang waktu 2 minggu. The result of the research is that the students can describe the character of geometry in Prambanan Temple such as rectangle, square, triangle, circle, cube, cuboid, and cylinder which becomes student’s preliminary concepts of geometry.
Mathematical Reasoning and Communication in TGT Learning Model with PQ4R Strategy
Authors: Primadani A I , Mardiyana  , Triyanto
Published: September 2020

This study aims to determine the effect of the TGT learning model with the PQ4R strategy on students’ reasoning abilities and mathematical communication skills. This study is a quasi-experimental design. The study population was all students of eight-grade in SMP Negeri 1 Gemolong, Indonesia. While the research sample was students of class VIII F and VIII G. Data collection techniques were the observation and test methods. Data collection techniques with observation and test methods. Analysis of the data used is one-way Manova. In the statistical test, it was found that the reasoning ability had a value of F = 18,165 while the communication skills had a value of F = 8,139, both of which had a significance of less than α = 0.05 so that H0 was rejected. Therefore, the results show that (1) the TGT learning model with the PQ4R strategy is better than the direct learning model of students’ mathematical reasoning abilities. (2) The TGT learning model with the PQ4R strategy is also better than the direct learning model for students’ mathematical communication skills.
Are students having trouble solving problems polyhedron?
Authors: Arifah A N , Retnawati H
Published: September 2020

The ability to solve problems is not only needed in school learning, but also to solve problems in daily life. One of the problems that cannot be separated from daily life is related to space objects such as tables, cabinets, bottles, balls, houses, and others. Mathematics is a lesson’s school that addresses these problems, especially regarding (to) polyhedrons. The purpose of this study is to describe the difficulties of students in solving problems related to polyhedrons. The difficulties are classified based on Newman’s Error Procedure (NEP). The research method used is a case study. The respondents of this study were middle school students of class IX (ninth grade of middle school). Data collection is done through tests and interviews. Validity is measured based on the Aiken index with a score of 0.82 which mean it is very valid. The difficulties are analyzed descriptively by observing the students’ errors in answering test questions. The results showed that about 8% of students had difficulty in reading error, 13% of students had difficulty in comprehension error, 21% of students had difficulty in transformation error, 49% of students had difficulty process skills error, and 71% of students had difficulty in encode error. It is known several factors that cause these errors so that some solutions can be obtained by the teacher to overcome these errors.
Analysis of metacognition ability in solving environmental mathematics problems
Authors: Mumu Jeinne , Tanujaya Benidiktus
Published: September 2020

This study aims to analyse the metacognition abilities of students in solving environmental mathematical problems. The method used is the descriptive qualitative method. The research instruments used were ability tests, questionnaires, and interview guidelines. The subjects included 18 students of Mathematics Education. The students were chosen randomly from the group of students’ interests in algebra. The results showed that there were three groups of students based on metacognition abilities, aware, reflective, and strategic learner. The first group is aware learner who are knows about some of the kinds of thinking they do. The second group is strategic learner, who are knows the strategies to help them learning something. The last group is reflective learner who are able to reflect upon their thinking in progress, ponder their strategies and revise them. These results prove that environmental education can be integrated in mathematics through the use of environmental mathematics problems. Integrating environmental education and mathematics can improve students’ metacognitive abilities as well as increase students’ awareness of environmental problems.
Students’ cognitive style in mathematical thinking process
Authors: Izzatin M , Waluyo S B , Rochmad  , Wardono
Published: September 2020

The process of mathematical thinking in learning mathematics is very important. One internal factor that affects it is cognitive style. The purpose of this study is to describe how each student’s cognitive style performs mathematical thinking processes. The types of cognitive styles studied are field dependent (FD) and field independent (FI). This research is a literature study by examining journals related to cognitive style and students’ mathematical thinking abilities. The results of journals review show that students with FI cognitive style analyze and orient analytically to process information and are able to process information using mathematical notation and their own language, and are able to model problems using image visualization. Students with FD cognitive styles adopt a global orientation to understand and process information. In solving mathematical problems, students with FI cognitive style were better than students with FD cognitive style.
Enhancing students’ self-efficacy through metacognitive strategies in learning mathematics
Authors: Amal M F , Mahmudi Ali
Published: September 2020

Student activities in the learning process in the classroom have not yet determined the effectiveness in thinking and learning. Students and teachers must learn in the right collaboration, one of which applies a learning strategy. Metacognitive strategies are learning strategies that support students in improving their cognitive abilities. The cognitive development of students is very important because it is often used with children’s intelligence and children’s intelligence is always directly proportional to self-efficacy. In addition to cognitive, affective domains are also important to stimulate students’ minds to improve their academic abilities. The question is that students’ self-efficacy, self-efficacy depends on the individual’s confidence in his ability to complete the task. In this paper, the authors argue that metacognitive strategies can be applied in mathematics learning to improve students’ self-efficacy in two ways. First, the orientation of the two variables are similar, although not completely as metacognitive activities are able to train students’ thinking processes and their relevance with self-efficacy ie students who have a high sense of self-efficacy will affect their cognitive processes, for example students who are taught using metacognitive strategies will have a positive impact on metacognition and self-efficacy abilities. Second, learning with metacognitive strategies based on strengthening self-efficacy students will obtain high academic achievements in several cognitive aspects, such as reasoning and communication skills, problem solving skill.
Unconstrained optimization based fractional order derivative for data classification
Authors: Hapsari Dian Puspita , Utoyo Imam , Purnami Santi Wulan
Published: September 2020

Data classification has several problems one of which is a large amount of data that will reduce computing time. The Fractional gradient descent method is an unconstrained optimization algorithm to train classifiers with support vector machines that have convex problems. Compared to the classic integer-order model, a model built with fractional calculus has a significant advantage to accelerate computing time. In this research it is to conduct a qualitative literature review in order to investigate the current state of these new optimization method fractional derivatives can be implemented in the classifier algorithm.
Statistics literacy: what, why and how?
Authors: Hidayati N A , Waluya S B , Rochmad  , Wardono
Published: September 2020

21st Century learning directs human resources to become literate societies, where human resources can obtain information, organize, facilitate, communicate in various contexts. Statistical literacy and mathematical literacy are among the developed literacy. Statistics Literacy and mathematical literacy might be like literacy about the same discipline, but is that the case? What exactly is statistical literacy? How is the mindset designed to be built in statistical literacy? How to calculate the statistical literacy of a person? Why has it become an important competency for society today? This paper will try to explain it.
Pressure based active load control of a blade in dynamic stall conditions
Authors: Barrio J. Fernández , Mertens C. , Ragni D. , Schiacchitano A. , Oudheusden B. van
Published: September 2020

A proportionate controller is investigated experimentally for unsteady load alleviation purposes on a 2D wing model with a trailing-edge flap. The controller acts on the velocity of the flaps, and pressure sensors are used to detect the unsteady loads, which are generated by actuating the wing model in a sinusoidal motion. Two different regimes are considered: attached flow and dynamic stall. The influence of actuation frequency and controller time lag is also studied. A reduction of 87.5% in the standard deviation of the lift is obtained for a frequency of 0.2Hz and time lag in the control system of 12ms for attached flow conditions. The reduction of the standard deviation of the lift deteriorates for increased frequency and time lag. The proposed controller is also able to reduce the loads during dynamic stall, although the reduction is smaller, close to 40%, and can negatively affect the aerodynamic damping of the model. The flap actuation is also shown to delay the onset of dynamic stall, by increasing the static stall angle with respect to the case without flap deflection.
Response sensitivity of a semisubmersible floating offshore wind turbine to different wind spectral models
Authors: Putri R M , Obhrai C , Jakobsen J B
Published: September 2020

Previous research on the OC3 spar floating offshore wind turbine (FOWT) has shown the sensitivity of the yaw and side-side modes’ load and motion responses to different atmospheric conditions. Using the same baseline turbine of the OC3 spar wind turbine for a semisubmersible floater (OC4), this study investigates the load and motion responses of such offshore wind turbine for neutral and unstable atmospheric conditions. The effect of different levels of wind spatial coherence associated with two different wind spectral models for neutral conditions (Kaimal and Mann) are studied for the same turbulence intensity levels. An increase of 18% in the tower torsional moment fatigue damage equivalent load (DEL) is observed for the wind inflow with the weakest coherence (Mann spectral model), compared to the DELs under turbulent wind inflow with the highest coherence (Kaimal spectral model). Unstable atmospheric conditions are also simulated based on the Pointed-Blunt spectral model derived from FINO1 wind measurement. The yaw mode of the semisubmersible wind turbine is found to be the response component most affected by the variation in atmospheric stability conditions. A 28% higher fatigue DEL for the tower torsional moment is observed for very unstable atmosphere than the DELs under neutral atmospheric conditions.
A Novel Frequency Regulation Control Method for Deloaded Wind Turbines
Authors: Karimpour Mostafa , Schmid Robert , Tan Ying
Published: September 2020

The recent proliferation of converter-based power generating systems used within renewable energy sources has increased the need to consider how renewable energy sources, including wind turbines, may participate in ancillary services to ensure the security of the power grid. In this paper, we discuss a modern control methodology known as exact output regulation (EOR) and investigate its application to the problem of wind turbine provision of grid frequency support services. The EOR method assumes that both wind preview information from LIDAR measurements and the power demand curve from the grid are available in the design of the turbine control law. Our simulation study will compare the performance of EOR with a baseline controller, for the tracking of a time-varying power demand curve. Our work focuses on region 3 wind signals, however it has the capability to extend to other wind speed region. The simulations will employ the FAST simulator for a 5MW wind turbine, with wind signals generated by TurbSim. The simulations show that the LIDAR-enhanced EOR method has the potential to substantially improve power demand tracking performance, relative to a baseline controller.
Multi-Sectional Lift Actuation for Wind Turbine Load Alleviation
Authors: Liu Chang , Gupta Abhineet , Rotea Mario
Published: September 2020

The development of large rotors can be limited by the ability to manage dynamic loads. Active Flow Control (AFC) actuators have been proposed to reduce these dynamic loads. This paper aims to study and quantify the advantages of using multiple AFC actuators compared to using a single AFC actuator for each blade. For this study, AFC actuators are idealized and modeled as instantaneous changes to the local lift coefficient at the corresponding blade sections. We refer to this idealized actuators as sectional lift actuators (SLA). A comparison between a single SLA and two SLAs per blade is made using feedforward and feedback control. Blade loads in above-rated wind conditions are used for the comparative analysis, which is conducted using the DTU-10MW reference turbine model.
Revisiting disturbance accommodating control for wind turbines
Authors: Sinner Michael , Pao Lucy Y.
Published: September 2020

Disturbance accommodating control has received considerable interest in the wind turbine research community for its ability to explicitly account for disturbances in the incoming wind field. Early work was based around estimating the disturbance from feedback information, while more recent research into disturbance accommodating control (as well as other feedforward control laws) has considered disturbance measurements produced by lidar. This work compares the two methods (estimating and measuring the disturbance) while keeping all other aspects of the controller the same. By doing so, we shed light on the performance improvements that can be attained using preview disturbance measurements of the wind.
Dynamic Flow Modelling for Model-Predictive Wind Farm Control
Authors: van den Broek Maarten J. , Wingerden Jan-Willem van
Published: September 2020

We aim to improve wind farm control for power output by building on the results from WFSim for the development of a dynamic wind farm model. This model will be part of a closed-loop, economic model-predictive control approach for wind farms. It is constructed from first principles using open-source tools to be suitable for adjoint-based optimisation of turbine yaw angles.

In a steady-state inflow configuration with two turbines, the new control model matches power expectations from high fidelity simulations in SOWFA to within 15 %. Under time-varying wind directions, it shows time delays in wake direction as inflow changes propagate through the farm with the wind speed, although the dynamics still differ from the SOWFA reference. The model runs flow simulations for a wind farm with a 3 x 3 array of turbines at a real-time order of magnitude on a regular laptop computer.

The new control model shows dynamic flow behaviour as wake changes propagate through the wind farm. Some further adjustments are necessary to accurately model three-dimensional flow in two dimensions. With more validation of the wake dynamics, it will be suitable for application in a new closed-loop wind farm controller.
Expert Elicitation on Wind Farm Control
Authors: van Wingerden J. W. , Fleming P. A. , Göçmen T. , Eguinoa I. , Doekemeijer B. M. , Dykes K. , Lawson M. , Simley E. , King J. , Astrain D. , Iribas M. , Bottasso C. L. , Meyers J. , Raach S. , Kölle K. , Giebel G.
Published: September 2020

Wind farm control is an active and growing field of research in which the control actions of individual turbines in a farm are coordinated, accounting for inter-turbine aerodynamic interaction, to improve the overall performance of the wind farm and to reduce costs. The primary objectives of wind farm control include increasing power production, reducing turbine loads, and providing electricity grid support services. Additional objectives include improving reliability or reducing external impacts to the environment and communities. In 2019, a European research project (FarmConners) was started with the main goal of providing an overview of the state-of-the-art in wind farm control, identifying consensus of research findings, data sets, and best practices, providing a summary of the main research challenges, and establishing a roadmap on how to address these challenges. Complementary to the FarmConners project, an IEA Wind Topical Expert Meeting (TEM) and two rounds of surveys among experts were performed. From these events we can clearly identify an interest in more public validation campaigns. Additionally, a deeper understanding of the mechanical loads and the uncertainties concerning the effectiveness of wind farm control are considered two major research gaps.
Increasing wind farm efficiency by yaw control: beyond ideal studies towards a realistic assessment
Authors: Ciri U , Rotea M A , Leonardi S
Published: September 2020

Recent studies have demonstrated promising results for yaw control to mitigate wake interactions and optimize wind farm efficiency. These studies have been carried out for a single ‘ideal’ operating condition, typically assuming a fixed wind direction and speed. In a real scenario, the wind speed and direction change continuously over time, including cases in which yaw control does not provide any improvement. Thus, results from idealized studies cannot be generalized to estimate annual energy production (AEP) improvements in real scenarios with sufficient accuracy. In this paper, we provide a method to estimate the impact of yaw control on annual energy production, based on high-fidelity simulations. The stochastic procedure leverages high-fidelity numerical simulations to obtain a surrogate model with generalized Polynomial Chaos (gPC). For a single wind direction improvements of the order ∼ 10% are obtained; which is typical of an ideal estimate. On the other hand, the AEP gain is about 3% for the entire wind farm under realistic variable wind directions. The magnitude of the AEP gain is site-dependent. The proposed methodology provides an accurate tool useful for evaluating the value proposition of yaw control for wake steering.
Does the use of load-reducing IPC on a wake-steering turbine affect wake behavior?
Authors: Wang C , Campagnolo F , Bottasso C L
Published: September 2020

This paper aims at quantifying and explaining the effects of Individual Pitch Control (IPC) on the wake of a wake-steering wind turbine. As the machine is intentionally misaligned with respect to the wind, IPC can be used to mitigate the resulting extra loading. However, while IPC reduces loads, it also affects the wake, which influences the power of downstream turbines. The question is therefore whether the IPC activity has any other appreciable effect at the wind farm level, in addition to its original turbine-level load reduction goal. In this work, experiments and CFD simulations of scaled wind turbines in a boundary layer wind tunnel are considered. The CFD model is first validated with measurements, and then used to show subtle changes in the wake, power and loads caused by IPC. It is observed that IPC does indeed have some non-negligible effects, and that these effects differ for positive and negative yawing.
Determining Remaining Lifetime of Wind Turbine Gearbox Using a Health Status Indicator Signal
Authors: Lázaro Roberto , Yürüșen Nurseda Y. , Melero Julio J.
Published: September 2020

Wind turbine component’s failure prognosis allows wind farm owners to apply predictive maintenance techniques to their fleets. This permits optimal scheduling of the maintenance actions considering the best time to stop the turbines and perform those actions. Determining the health status of a turbine’s component typically requires verifying a wide number of variables that should be monitored simultaneously. The scope of this study is the investigation and the selection of an effective combination of variables and smoothing and forecasting methodologies for obtaining a wind turbine gearbox health status indicator, in order to interpret clearly the remaining lifetime of the gearbox. The proposed methodology is based on Gaussian Mixture Copula Model (GMCM) models combined with the smoothing treatment and the forecasting model to define the health index of the wind turbine gearbox. Then, the resulting index is tested using various warning and critical thresholds. These thresholds should be chosen adequately in order to indicate appropriate inspection visit and preventive maintenance intervention date. Then, the best combination found, for the studied cases, was 50% and 70% for warning and critical respectively. This combination ensures that the developed procedure is capable of providing long enough time window for maintenance decision making.
Model predictive control of wind turbine fatigue via online rainflow-counting on stress history and prediction
Authors: Loew S , Obradovic D , Bottasso C L
Published: September 2020

The standard fatigue estimation procedure is implemented in Model Predictive Control via externalization of the Rainflow algorithm from the optimization problem. Additionally, stress history is considered in a consistent manner by employing a so-called stress residue. The formulation is implemented in the state-of-the-art MPC framework acados and tested in closed-loop with the 5MW onshore turbine in OpenFAST. Simulation results indicate that the new formulation outperforms conventional PID- and MPC-controllers over the entire wind regime, and that the consideration of stress history is highly beneficial.
Vertical wake deflection for offshore floating wind turbines by differential ballast control
Authors: Nanos Emmanouil M , Letizia Stefano , Clemente Daniel J Barreiro , Wang Chengyu , Rotea Mario , Iungo Valerio I , Bottasso Carlo L
Published: September 2020

This paper discusses the idea of vertical wake deflection for offshore wind turbines using active ballast control. First, the concept of active ballast control for changing the attitude of an offshore wind energy semi-submersible platform is presented and briefly explored. Next, the influence of vertical wake deflection on the wake behavior and power production of a cluster of two wind turbines is studied through wind tunnel experiments and LES simulations. Results show that vertical deflection changes substantially the wake characteristics and affects the inflow at a downstream wind turbine in terms of available kinetic energy and turbulence intensity. For the most favorable case considered here, the cluster power was increased by approximately 8%.
Feedforward control for wave disturbance rejection on floating offshore wind turbines
Authors: Al M. , Fontanella A. , van der Hoek D. , Liu Y. , Belloli M. , van Wingerden J. W.
Published: September 2020

Floating offshore wind turbines allow wind energy to be harvested in deep waters. However, additional dynamics and structural loads may result when the floating platform is being excited by wind and waves. In this work, the conventional wind turbine controller is complemented with a novel linear feedforward controller based on wave measurements. The objective of the feedforward controller is to attenuate rotor speed variations caused by wave forcing. To design this controller, a linear model is developed that describes the system response to incident waves. The performance of the feedback-feedforward controller is assessed by a high-fidelity numerical tool using the DTU 10MW turbine and the INNWIND.EU TripleSpar platform as references. Simulations in the presence of irregular waves and turbulent wind show that the feedforward controller effectively compensates the wave-induced rotor oscillations. The novel controller is able to reduce the rotor speed variance by 26%. As a result, the remaining rotor speed variance is only 4% higher compared to operation in still water.
Biomimetic individual pitch control for load alleviation
Authors: Coquelet M , Bricteux L , Lejeune M , Chatelain P
Published: September 2020

Individual pitch control has shown great capability of alleviating the oscillating loads experienced by wind turbine blades due to wind shear, atmospheric turbulence, yaw misalignement or wake impingement. This work presents a bio-inspired structure for individual pitch control where neural oscillators produce basic rhythmic patterns of the pitch angles, while a deep neural network modulates them according to the environmental conditions. This mimics, respectively, the central patterns generators present in the spinal chord of animals and their cortex. The mimicry further applies to the neural network as it is trained with reinforcement learning, a method inspired by the trial and error way of animal learning. Large eddy simulations of the reference NREL 5MW wind turbine using this biomimetic controller show that the neural network learns how to reduce fatigue loads by producing smooth pitching commands.
Rejecting wake-rotor overlapping load disturbances: An extension to active power control of wind farms
Authors: Vali M , Petrović V , Steinfeld G , Kühn M
Published: September 2020

This paper studies an extension to the active power control of wind farms for further structural load alleviation of waked wind turbines. We demonstrate that the structural fatigue loading of a downstream turbine, which its rotor overlaps with wakes of its upwind turbine, can be significantly alleviated, while the wind farm power production follows a power reference signal. A load variations model is proposed, based on the multi-rotor concept, to sense the variations of the wind velocity and the associated loading across the rotor area of a single wind turbine. Then, an individual rotor control is proposed to reject the rotor-wake overlapping load disturbances about an operating condition, commanded with the closed-loop APC at the wind farm level. The applicability and key features of the controller are discussed with a wind farm example consisting of 2×2 turbines. A large-eddy simulation model is employed for resolving the turbulent flow, the wake structures and its interaction with an atmospheric boundary layer. The effectiveness of the proposed load disturbance rejection is evaluated using the damage equivalent load of the tower base fore-aft bending and torsional moments of the individual wind turbines.
Wake redirection for active power control: a realistic case study
Authors: Kretschmer Matthias , Raach Steffen , Taubmann Julian , Ruck Nico , Cheng Po Wen
Published: September 2020

This paper presents a realistic case study of how wake redirection can be used to provide ancillary grid service. Wake redirection is a wind farm control approach in which the wake of a wind turbine is redirected by imposing a yaw misalignment to minimise the flow interactions with downwind wind turbines. The increase in power is controlled in our approach to meet a requested power increase. The methodology is implemented in FAST.Farm, a mid-fidelity wind farm simulation tool. In a case study with 12 wind turbines the applicability of the methodology is analysed. Hereby, provision of active power control was achieved in some cases. Structural loads were investigated at tower base, blade root and drivetrain of the turbines, indicating that the concept tends to increase the fatigue damage compared to a baseline case where all turbines are operated with zero yaw misalignment.
Experimental Analysis of a Scaled-Down Vestas Four-Rotor Wind Turbine
Authors: Filsoof Oliver Tierdad , Yde Anders , Bøttcher Peter , Zhang Xuping
Published: September 2020

In this work, a 1/30 scaled-down Vestas four-rotor demonstrator wind turbine is established as an experimental setup. The experimental setup is used to analyze how the structural natural frequencies are affected by the rotor speed for a multi-rotor wind turbine. To have comparable dynamic behavior between the Vestas turbine and the experimental setup, Froude scaling is used. For standstill tests, a metal brushing technique is employed to excite the setup, and when rotating an automated impulse mechanism provide impulses to the rotor blades. Modal testing experiments are conducted, and testing results show that standstill frequencies between the Vestas turbine and experimental setup coincided. The experiment demonstrates that by using the automated impulse mechanism from 40 rpm to 110 rpm, edgewise whirling modes are excited and proportional to the rotor speed. The setup experiences large vibrations if the rotor speed continues increasing beyond 110 rpm using the impulse mechanism. Switching to the metal brushing technique achieves better results, and makes it possible to see that both the flapwise and edgewise whirling modes are proportional to the rotor speed.
Reference Design and Simulation Framework of a Multi-Megawatt Airborne Wind Energy System
Authors: Eijkelhof Dylan , Rapp Sebastian , Fasel Urban , Gaunaa Mac , Schmehl Roland
Published: September 2020

In this paper, we present the design and computational model of a representative multi-megawatt airborne wind energy (AWE) system, together with a simulation framework that accounts for the flight dynamics of the fixed-wing aircraft and the sagging of the tether, combining this with flight control and optimisation strategies to derive the power curve of the system. The computational model is based on a point mass approximation of the aircraft, a discretisation of the tether by five elastic segments and a rotational degree of freedom of the winch. The aircraft has a wing surface area of 150 m2 and is operated in pumping cycles, alternating between crosswind flight manoeuvres during reel out of the tether, and rapid decent towards the ground station during reel in. To maximise the net cycle power, we keep the design parameters of the aircraft constant, while tuning the operational and controller parameters for different wind speeds and given contraints. We find that the presented design can generate a net cycle power of up to 3.8 megawatts.
Flight Anomaly Detection for Airborne Wind Energy Systems
Authors: Salma Volkan , Schmehl Roland
Published: September 2020

Airborne wind energy (AWE) systems use tethered flying devices to harvest wind energy beyond the height range accessible to tower-based turbines. AWE systems can produce the electric energy with a lower cost by operating in high altitudes where the wind regime is more stable and stronger. For the commercialization of AWE, system reliability and safety have become crucially important. To reach required availability and safety levels, we adapted an fault detection, isolation and recovery (FDIR) architecture from space industry. This work focuses on, “flight anomaly detection” layer of the FDIR. Tests verifies that proposed architecture is capable of detecting flight anomalies without generating false alarms.
An hybrid approach for the comparison of VAWT and HAWT performances for floating offshore wind turbines
Authors: Matoug C , Augier B , Paillard B , Maurice G , Sicot C , Barre S
Published: September 2020

This article describes the experimental comparison of the DTU 10MW HAWT with the WindQuest 10MW VAWT scaled models when both fitted on the Nautilus-10 semi-submersible. For the campaign in Ifremer Wind&Wave tank, different rotor representations are tested from inertia only to hybrid testing with propellers and a Software in the Loop. The hybrid testing uses a tabulated approach for thrust computation and is discussed in this article. The different thrust reproduction methods are compared to OpenFAST simulations calibrated during the Lifes50+ project and used as reference points. When quantifying the scale model’s fidelity to Open Fast, cross-correlation was increased up to 14% with the use of the SiL. Thanks to the validation of hydrodynamic simulations based on experimental responses of the floater, it is shown that the VAWT rotor can be upscaled to 13MW for the same platform.
Characterizing turbine inflow during a stability transition using dual-Doppler radar measurements
Authors: Hirth Brian D. , Schroeder John L.
Published: September 2020

Measurements from two specialized Doppler radars are used to construct a dual-Doppler synthesis of the boundary layer wind flow during an unstable-to-stable transition. Three rotor sweep concepts are inserted into the flow to evaluate turbine inflow variability across the rotor sweeps. One-second turbine inflow forecasts are constructed using an advanced advection correction technique at each rotor grid point allowing for detailed statistical analysis of shear, veer, and turbulence intensity across the three rotor sweeps.
OC6 Phase I: Investigating the underprediction of low-frequency hydrodynamic loads and responses of a floating wind turbine
Authors: Robertson A N , Gueydon S , Bachynski E , Wang L , Jonkman J , Alarcón D , Amet E , Beardsell A , Bonnet P , Boudet B , Brun C , Chen Z , Féron M , Forbush D , Galinos C , Galvan J , Gilbert P , Gómez J , Harnois V , Haudin F , Hu Z , Dreff J Le , Leimeister M , Lemmer F , Li H , Mckinnon G , Mendikoa I , Moghtadaei A , Netzband S , Oh S , Pegalajar-Jurado A , Nguyen M Q , Ruehl K , Schünemann P , Shi W , Shin H , Si Y , Surmont F , Trubat P , Qwist J , Wohlfahrt-Laymann S
Published: September 2020

Phase I of the OC6 project is focused on examining why offshore wind design tools underpredict the response (loads/motion) of the OC5-DeepCwind semisubmersible at its surge and pitch natural frequencies. Previous investigations showed that the underprediction was primarily related to nonlinear hydrodynamic loading, so two new validation campaigns were performed to separately examine the different hydrodynamic load components. In this paper, we validate a variety of tools against this new test data, focusing on the ability to accurately model the low-frequency loads on a semisubmersible floater when held fixed under wave excitation and when forced to oscillate in the surge direction. However, it is observed that models providing better load predictions in these two scenarios do not necessarily produce a more accurate motion response in a moored configuration.
Long-range Doppler lidar measurements of wind turbine wakes and their interaction with turbulent atmospheric boundary-layer flow at Perdigao 2017
Authors: Wildmann Norman , Gerz Thomas , Lundquist Julie K.
Published: September 2020

As part of the Perdigão 2017 campaign, vertical RHI (range-height indicator) scans with long-range pulsed Doppler wind lidars were performed aligned with the main wind direction and a wind turbine (WT) located on a mountain ridge. The measurements are used to not only retrieve flow velocities, but also their variance and - by using the turbulent broadening of the Doppler spectrum - also turbulent kinetic energy (TKE) dissipation rate. The study shows that turbulence in the WT wake is dependent on the turbulence of the inflow, but also on atmospheric stability. In stable atmospheric conditions, wakes could be analyzed up to five rotor diameters downstream (D) and showed the maximum turbulence in the wake at 2-3 D, whereas in unstable conditions, the maximum was found at 2 D and the wake could not be detected further than 3 D. A clear dependency of wake turbulence enhancement on inflow turbulence intensity is found, which levels out to no further enhancement at turbulence intensities of 30%.
Experimental test of variable loads on a vertical-axis wind turbine
Authors: Brandetti L , Tavernier D De , Leblanc B , Ferreira C
Published: September 2020

The paper presents an experimental study of applying variable loads on a vertical-axis wind turbine (VAWT). The experiment is conducted in an open-jet wind tunnel on a two-bladed Darrieus VAWT equipped with active individual blade pitch control. Variable loads are achieved by dynamically changing the pitch angle of the individual blades and by keeping the wind speed of the tunnel constant. The blade loads are measured using strain gages and the flow velocity is measured upwind and downwind of the rotor using a hotwire. Dynamic inflow phenomena are clearly visible both in the turbine loads and in the velocity field. A time delay based upon the flow convection in the wake is identified. It results that the induction of the turbine can be controlled by changing the pitch of the blades. The experimental database allows to validate a new dynamic inflow model for VAWT and will be made publicly available for research purposes.
Wind tunnel testing of a wind turbine in complex terrain
Authors: Nanos Emmanouil M , Yilmazlar Kutay , Zanotti Alex , Croce Alessandro , Bottasso Carlo L
Published: September 2020

The paper describes the development of a scaled model of complex terrain, suitable for terrain-wind turbine interaction wind tunnel studies, taking into account flow similarity criteria. The size and the geometry of the experimental model of the complex terrain were refined using results of CFD simulations in order to achieve the best possible flow similarity and avoid edge effects arising from the finite (relative to the rotor size) terrain geometry. Moreover, Particle Image Velocimetry was used to survey the flow field on a longitudinal plane along the terrain center line. Flow measurements with and without a wind turbine model enabled to quantitatively evaluate the speed up produced by the terrain in the region of the wind turbine and the effect of the terrain on the wake characteristics of the wind turbine model.
Bayesian based estimation of turbulent flow fields from lidar observations in a conventionally neutral atmospheric boundary layer
Authors: Bauweraerts Pieter , Meyers Johan
Published: September 2020

In this paper, we consider the reconstruction of 3D turbulent flow fields from a time series of lidar data in a conventionally neutral atmospheric boundary layer (CNBL). For the reconstruction we use the maximum a posteriori estimate of the flow field. This corresponds to an optimization problem, with a cost function that has two contributions; a first term originating from the prior belief on the probability of having a certain turbulent flow field without any observations. Flow field fluctuations are assumed normally distributed and thus statistically fully determined by the mean and two-point covariance of the velocity field. The second term, is related to the likelyhood of the observations, influenced by model and measurement uncertainties. The two-point covariance is computed and found to be significantly altered by the Coriolis force, breaking up longer streamwise velocity streaks and veering spanwise structures by ∼ 45° with respect to the mean flow direction. For the reconstruction, we consider two different scanning modes, a plan position indicator (PPI) mode and a trajectory which is based on a Lissajous curve. For the PPI scanning mode we find that the mean squared error of the reconstructed velocity field is around 10% of the background variance in the scanning plane, and quickly increases outside this region. The Lissajous curve on the other hand attains an average error of 40% over the scanning region, which spans almost the whole BL height.
High-Torque Oscillating Wind Energy Generator
Authors: Garzozi A , Dunaevich L , Greenblatt D
Published: September 2020

This research investigates a novel concept in small-scale wind-energy generation. Instead of rotating turbine blades, the present concept produces reciprocating motion that is achieved by alternately activating boundary layer control (slot blowing) on opposite sides of a circular cylinder. This is commonly called the Coandă-effect, and here we deploy it periodically on a spring-stabilized vertical cylinder, to produce reciprocating motion. This
Co
andă-based
Re
ciprocating (CoRe) system converts wind energy to mechanical work. Compared to wind turbines and windmills, the CoRe system generates higher torque and is therefore well suited to directly driving positive displacement pumps for drip-irrigation or reverse osmosis water desalination. This new approach necessitates the investment of energy in order to extract energy from the wind and therefore a major challenge is to maximize the output/input ratio in order to render the concept viable. Previous preliminary wind tunnel tests conducted on the CoRe system produced gross and net power coefficients of 10% and 3% respectively. However, the blowing slot design has a decisive impact on output power. On the basis of data presented in the literature and our computational analysis, the CoRe system produces significantly greater torque than all types of wind turbines with net power coefficients over 10%.