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The Science of Making Torque from Wind (TORQUE 2018)

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Room: BL.27 Building
Topic: WWT. Poster Session
Form of presentation: Poster
Duration: 75 minutes

Authors:
Jörn Nathan, Christian Masson, Louis Dufresne

Abstract:
The Actuator Line Method exists for more than a decade and has become awell established choice for simulating wind rotors in computational fluid dynamics. Numerousimplementations exist and are used in the wind energy research community. A crucial parameterof this method is the distribution width used to distribute the blade forces in the domain inorder to attain numerical stability but also to apply the force term over a length which makesphysically sense. Several work has been done on the subject in general terms and also withfocus on the (NEW) MEXICO experiment. In this work a base case for the (NEW) MEXICOexperiment is evaluated and then the optimal distribution width is found by setting the globaltorque as target variable.

Authors:
Nils Gerke, Inga Reinwardt, Peter Dalhoff, Matthias C. Dehn, Wolfgang Moser

Abstract:
The wake induced turbulences calculated in the planning phase of a wind farm have a great influence on layout and sector management of the farm. Thus, it is important to validate the used model against turbine measurements of a representative turbine inside the calculated wind farm. The widely used engineering turbulence models are mostly calibrated with outdated wind turbines. Hence it is questionable if these models are still producing results that are applicable for modern multi megawatt wind turbines. In this analysis the most common engineering turbulence models will be compared with Supervisory Control and Data Acquisition (SCADA) data from operating onshore wind farms in the 2 – 3 MW class. The analysis includes effects based on turbine distances, the orography, topography as well as multiple wake situations. The investigated models are S. Frandsen, G.C. Larsen and D.C. Quarton, which all appear to overestimate the wake added turbulence. The model closest to the measurements is the model of G.C. Larsen.

Authors:
Inga Reinwardt, Nils Gerke, Peter Dalhoff, Dirk Steudel, Wolfgang Moser

Abstract:
When planing new wind farms, an accurate prediction of power output and turbine loads is highly relevant. For this reason, it is necessary to predict wind speed and turbulence inside a wind farm as exactly as possible in an acceptable period of time. In order to do this, simple analytical wake and turbulence models are used in industrial applications. This analysis compares current wake and turbulence models, such as the Sten Frandsen model, an engineering wake and turbulence model developed by G. Chr. Larsen, and the dynamic wake meandering model to measurements from onshore wind farms. Special attention is given to the dynamic wake meandering model. The validation of the mentioned wake and turbulence models is based on turbine load, power and wind measurements.

Authors:
Alessandro Bianchini, Simone Bigalli, Francesco Balduzzi

Abstract:
The analysis of wind turbine wakes and their interaction with other machines installed in the same array or park has become a key element in the current wind energy research. If in case of siting of large rotors for energy production the use of high-fidelity CFD simulations is well established yet, there is still a lack of knowledge in the analysis of proper wind turbine siting for small wind turbines, which are typically installed in quite complex environments. The present study provides the description and validation of a hybrid BEM-CFD model for the analysis of wind turbine performance and wake structure. With respect to similar existing methods, the proposed one includes a specific correction of turbulence parameters able to make it compatible for use in combination with the standard settings of the turbulence models needed to properly describe the wind profile in the urban environment. The model was then used to carry out a demonstrative sensitivity analysis on the proper siting of a small wind turbine in the rooftop of a typical tall building in a densely built environment.

Authors:
Javier Sanz Rodrigo, Roberto Chávez Arroyo, Pawel Gancarski, Matias Avila, Jordi Garcons, Arnau Folch, Dalibor Cavar, Dries

Abstract:
A summary of the initial results of the “NEWA Meso-Micro Challenge for Wind Resource Assessment” is presented. The objective of this activity, conducted in the context of the New European Wind Atlas (NEWA) project, is to establish a process for the evaluation of meso-micro methodologies in the context of wind resource and wind turbine site suitability assessment.  A hierarchy of methodologies that rely on coupling mesoscale and microscale models is evaluated as a tradeoff between accuracy and computational cost in terms of relevant wind conditions for wind turbine siting such as annual energy production, turbulence intensity, etc. Besides integrated annual quantities, these metrics are analyzed in terms of atmospheric boundary-layer drivers at wind climate (mesoscale tendencies) and site characteristics (atmospheric stability). This provides more granularity in the characterization of errors for the identification of knowledge-gaps in the model-chain. This first phase of the meso-micro challenge analyzes Cabauw (onshore) and Fino-1 (offshore) tall met mast, both in horizontally-homogeneous  conditions, to focus the assessment on mesoscale-to-microscale downscaling methods rather than on site complexity. A second phase of the challenge will add sites in heterogeneous terrain conditions from the NEWA database of experiments.

Authors:
Alexander Meyer Forsting, Niels Troldborg, Paul van der Laan

Abstract:
Sheared inflow introduces significant periodic load variations to wind turbine blades, but has only limited impact on the mean wake deficit. Following these findings, the wind speed reduction upstream of the turbine – referred to as the induction zone – might also show little difference to uniform inflow. Using the local free-stream velocity to normalise the upstream flow-filed, should then render uniform and sheared inflow induced velocity profiles indiscernible, hinting towards wind shear acting solely as a linear addition. This has great implications in BEM methods for determining the velocity at the blades and also for near-rotor lidar measurements. The latter being applied in for power/loads assessment and turbine control. LES simulations with an actuator line representation of the rotor confirm the linearity assumption for moderate wind shear.

Authors:
Jiangang Wang, Chengyu Wang, Omar David Castaneda Rodriguez, Filippo Campagnolo, Carlo L. Bottasso

Abstract:
Offshore floating wind turbines are attracting an ever increasing interest for deep-water applications. Among the many different research aspects of floating offshore technology, understanding wake interactions of multiple floating wind turbines under complex motions is a particularly challenging task, which requires both suitable high-fidelity numerical models and relevant experimental observations. In this work, we first present a framework for the large-eddy simulation of floating wind turbines. Then, the simulation model is validated with the help of wind tunnel experimental data. Measurements were obtained in a neutrally stratified atmospheric boundary layer for very closely spaced wind turbine models, whose pitching motion was prescribed to simulate various wave-wind conditions. The pitching motion of the wind turbines induces significant vertical meandering of the wakes, exhibiting a strong interaction with downstream turbines. The simulation results are compared to the experimental measurements in terms of inflow conditions and wind turbine response parameters, showing a good matching.

Authors:
Yasir Shkara, Ralf Schelenz, Georg Jacobs

Abstract:
The trend of the current development of modern HAWTs is to increase rotor diameter while keeping the rated wind speed constant, thereby taller tower structures are required. With the increase of the tower height, tower base is getting bigger and transportation problems appear. Nowadays the tower dimensions are almost reached roads limits. To optimize tower dimensions there is the need to get a better understanding of what loads act on the tower. In addition to the rotor and nacelle weight, rotor thrust, the rotor-tower interaction of upwind HAWT has a significant effect on the tower loading as the components become bigger. A CFD model has been built to simulate the blade-tower interaction of a 3MW upwind horizontal axis wind turbine. Results showed that there is an obvious reduction of the pressure on the tower as the blade passes it. The tower suffers pressure fluctuation known as 3P oscillations for three-bladed rotor where the wake occurs three times per revolution. Furthermore, the mentioned interaction resulted in a change of tower shedding vortices that leads to not only stream-wise fatigue loads but side loads as well on the tower.

Authors:
Mark Richmond, Athanasios Kolios, Taka Nishino, Lin Wang, Vipin Sukumara Pillai

Abstract:
In this paper, a method for stochastic analysis of an offshore wind farm using computational fluid dynamics is proposed. An existing offshore wind farm is modelled using a steady-state solver at several deterministic input ranges and an approximation model is trained on these results. The approximation model is then used in a Monte-Carlo analysis to build joint probability distributions for values of interest within the wind farm. Machine learning methods are also employed to automate identification and quantification of wake flow within the wind turbine array. The results are compared with the real values of data obtained from the existing wind farm to quantify the accuracy of the predictions. It is shown that this method works well for the relatively simple problem which it is applied to and has potential to be used in more

Authors:
Stefano Macrì, Olivier Coupiac, Annie Leroy, Sandrine Aubrun

Abstract:
This work focuses on the dynamic analysis of a modelled wind turbine wake during yaw manoeuvres. Indeed, in the context of wind farm control, misalignment of wind turbines is envisaged as a solution to reduce wind turbine wake interactions, by skewing the wake trajectory. To optimize the control strategies, the aerodynamic response of the wake to this type of yaw manoeuvres, as well as the global load response of the rotor disc of the downstream wind turbine, must be quantified. As a first approach, the identification of the overall system is performed through wind tunnel experiments, using a rotor model based on the actuator disc concept. A misalignment scenario of the upstream wind turbine model is imposed and the wind turbine wake deflection is dynamically captured and measured by the use of Particle Imaging Velocimetry.

Authors:
Lun Ma, Takafumi Nishino

Abstract:
An extended theoretical model, which is based on a ‘two-scale coupled’ momentum conservation argument, is proposed to estimate aerodynamic effects of support structures on the performance of very large wind farms. A key implication of this extended model is that the parameter , where  and  are the rotor swept area and support-structure frontal projection area respectively and  is an effective support-structure drag coefficient, plays an important role in the design of very large wind farms. In particular, the optimal farm density is strongly influenced by the normalised support-structure drag. To validate this extended model, several CFD simulations are currently in progress and will be reported in the final paper.

Authors:
Ilmas Bayati, Marco Belloli, Luca Bernini, Koen Boorsma, Marco Caboni, Marion Cormier, Robert Mikkelsen, Marcin Serdeczny, Thorsten Lutz, Alberto Zasso

Abstract:
UNAFLOW(UNsteady Aerodynamics for Floating Wind) is a joint EU-IRPWINDfounded experiment on wind turbine rotor unsteady aerodynamics. It brings together fourdierent academic contributors: Energy research Centre of the Netherlands (ECN), DTU WindEnergy, University of Stuttgart (USTUTT) and Politecnico di Milano (POLIMI). It bringsin knowledge both in numerical modelling and in experimental tests design, allowing directnumerical and experimental comparison.The experimental tests carried out for UNAFLOW are of the same type of the ones carriedout during the ongoing EU H2020 project LIFES50+ [1], regarding both the unsteady behaviourof the 2d blade section and the entire turbine rotor, although with improved setup and widertest matrix.The project partners are already currently jointly collaborating in the AVATAR project [2],developing and validating numerical models of dierent accuracy level. The numerical modelsused in the UNALFOW project range from engineering tool (eg. BEM) to high delity CFDmethods. Numerical simulations are used both in the design of experiment phase and in theresults analysis allowing for an in depth understanding of the experimental ndings throughadvanced modelling approach.The UNAFLOW project, together with a new understanding of the unsteady behaviour ofthe turbine rotor aerodynamics, will provide also an open database to be shared among thescientic community for future analysis and new models validation.

Authors:
Joseph Cheuk Yi Lee, M J Fields, Julie K Lundquist, Monte Lunacek

Abstract:
While the Gaussian distribution and nonrobust and nonresistant statistics such as standard deviation are often used to describe the variability of wind resources, such methods oversimplify the underlying variability. To examine the applicability of Gaussian distribution, we quantify the variability as well as the distribution characteristics of wind speeds in different time scales in North America over 38 years. The non-zero skewness and kurtosis values demonstrate the distributions in the United States are mostly non-Gaussian, for either monthly mean or annual mean wind speeds. Moreover, distributions of annual mean wind speeds differ from Gaussian more than those of monthly mean data. Additionally, the choice of spread metric also changes the portrayal of wind-speed variability. In particular, using standard deviation to quantify long-term variability can lead to misleading interpretations, in contrast to estimating variability via Robust Coefficient of Variation (RCoV), a normalized, robust, and resistant spread metric. In conclusion, we recommend RCoV to compute wind-speed variability.

Authors:
Björn Roscher, Georg Jacobs, Alexander Werkmeister, Yasir Shkara, Ralf Schelenz, Laura Stössel

Abstract:
Due to the increased interest in wind energy more and more wind farms are constructed. In order to use the landscape effectively, wind turbine are placed closer up to 2.5 diameter distance. This packing leads to a highly turbulent inflow, resulting in greater loads. Multiple methods are available to determine the effect of such inflow situations. Each method has their benefit as well as their disadvantage. The paper will compare the hub loads of turbine standing in the wake. Two methods will be used the IEC 61400 model and a CFD simulation. Both output will be fed towards a multiple body simulation to determine the change in loadings and the damage equivalent load.

Authors:
Carl Michael Schwarz, Sebastian Ehrich, Raquel Martín, Joachim Peinke

Abstract:
Fatigue loads related to a statistical feature of wind are investigated: Super-Gaussian distributed wind velocity increments as a result of intermittency of turbulence and their impact on the longevity of wind turbines are in the focus. Two types of synthetic wind fields are fed to a common Blade-Element Momentum theory based wind turbine model. A special effort has been made in the generation of these fields in order to isolate the effect of the increment statistics. Besides these intended differences other statistical features like mean wind speed, turbulence intensity and spectral properties are equivalent. Based on these wind field characteristics, we are able to show differences in the fatigue loads.

Authors:
Mengqi Zhang, Richard J.A.M. Stevens

Abstract:
Vertical staggering of turbines can lead to an increase of power production in the entrance region of a wind farm. This is because the downstream turbines are consequently outside of the wake flows of preceding ones. In this work we compare the performance of streamwise aligned vertically staggered wind farms in which the even turbine rows are elevated and the odd turbine rows are lowered with a configuration in which the even turbine rows are lowered and the odd rows are elevated. We perform large eddy simulations of both these wind farm layouts and calculate their power production. It is found that the power output in the entrance region of the wind farm is significantly higher when the odd turbine rows are elevated compared to the case where the even turbine rows are elevated. The reason is that the first high turbine row can fully benefit from the strong winds at a high elevation when the odd turbine rows are elevated, while this is not the case when the even turbine rows are elevated.

Authors:
Francesco Castellani, Matteo Mana, Davide Astolfi

Abstract:
The comprehension of short-term wind and wind turbine power fluctuations has been recently obtaining a certain attention, for scientific and technology issues as well. This work is an experimental study of wind and power fluctuations at on onshore wind farm in Italy. Four wind turbines having 2 MW of rated power each are studied through time-resolved data. The sampling frequency is up to 0.65 Hz.  This wind farm has been selected because it is characterized by mid wakes and far wakes and therefore it is possible to connect the fluctuations dynamics with the wake regime. The behavior of each wind turbine power curve at short time scales is studied and the inertia of the wind turbine, with respect to the wind fluctuations, is highlighted through the observation of the hysteresis in the power curve. Subsequently, the distribution of the wind and power variations is studied on several time scales and different features of the distributions are observed, for downstream wind turbines with respect to upstream ones. This can be considered a data-based approach to the Langevin power curve and it can be useful for connecting the modelling of turbulence and wind turbine power curve to different wake regimes.

Authors:
Patrick Letzgus

Abstract:
Within the project WINSENT a wind energy test site will be installed in complexterrain in Southern Germany. On such complex orographies various inflow conditions influencethe power gain and the blade loads of wind turbines. For example, forest vegetation modifies thewind direction and velocity, in addition to inclination angles caused by the complex escarpment.The scope of the present work is to characterize the influence of a forested area in complextopographies on the local flow field by means of CFD. The results will be compared to inflowconditions of wind turbines in complex terrain without consideration of forests. Objective is toanalyze the degree of influence a forested hill introduces to the flow field on top of the terrainnear the designated wind turbine position and to observe flow separation due to the vegetation.

Authors:
Linlin Tian, Ning Zhao

Abstract:
Large-eddy simulation (LES) technique combined with the actuator line model (ALM) is employed to investigate the characteristics of wind turbine wakes under both neutral and convective atmospheric boundary layer (ABL) stability regimes. Turbulence properties of the incoming wind that due to thermal stratification are collected by the precursor ABL simulation, and then are used to initialize the flow field upstream of a single wind turbine. For the neutral case, comparison with the wind tunnel experiments shows an acceptable agreement in the wake velocity distribution and turbulence characteristics, which proves the accuracy of the employed numerical methods to some extent. Besides, numerical results are compared between two atmospheric conditions. Through the evaluations of the velocity deficit and turbulent intensity in the wake area, it is shown that the ABL stability has a significant effect on the wake development; and particularly, a more rapid wake recovery is observed in the convective condition. These results suggest that atmospheric thermal stability is crucial to be taken into account in the prediction of wake effects and in the design of wind farms.

Authors:
Jens Nørkær Sørensen, Robert F. Mikkelsen, Valery L. Okulov, Ivan Kabardin, Igor Naumov, Ivan Litvinov

Abstract:
The present work shows the results of a series of experimental investigations of the wake development behind a model rotor subject to upstream disturbances created either by another rotor or by a disc. The aim of the work is to clarify similarities and differences in the wake of a wind turbine subject to different inflow disturbances, and in particular to see if there is any difference in the rotor wake resulting from a upstream disturbance created by a rotor and one created by an immobile disk. The background for the study is an on-going discussion if disks can replace rotors in laboratory experiments. In the paper, we will also show new experimental data that support our main conclusion, which is that strong differences exist between the near wakes characteristics of a rotor and a disc.

Authors:
Matthieu Duponcheel, Caroline Leroi, Stephanie Zeoli, Gregoire Winckelmans, Laurent Bricteux, Emmanuel De Jaeger, Philippe Chatelain

Abstract:
The study of wind turbines requires a multidisciplinary approach that covers the electrical, mechanical and aerodynamic aspects. Most wind turbine studies focus their modelization effort on a single aspect and rely on simplified models for the other ones.The present paper aims at a complete modelization and investigation of the energy conversion chain of small wind turbines (up to 100kW), going from the turbulent wind resource to the electrical power injected into the grid. For that purpose, the following computational chain is set up: a synthetic turbulence is used as inflow to a Vortex Particle-Mesh method which computes the wind turbine coarse scale aerodynamics taking into account the rotor dynamics; the aero-mechanical results are used to feed an electrical simulation and the consistency of both simulations is ensured by using a common Maximum Power Point Tracking algorithm. We apply this methodology to compare the behavior of horizontal- and vertical-axis machines in a turbulent wind with two different turbulent intensities (low TI= 8.3% and high TI=16%).

Authors:
Sandrine Aubrun, Eulalio Torres Garcia, Matthieu Boquet, Olivier Coupiac, Nicolas Girard

Abstract:
Within the frame of the French project ANR SMARTEOLE, a 7-month measurement campaign had been set-up in the north of France to study the wake behaviour of 2 wind turbines spaced from 3.5D, with a set-up using scanning LiDAR scenarii that capture the wakes of both wind turbines for different degrees of interactions. Wind turbines are Senvion MM82 with a hub height of 80m and a rotor diameter of D = 82m. A scanning lidar Windcube 200S from Leosphere is located at 1.5km on the east side of the wind turbines of interest in order to capture their both wakes. 3 PPI (Plan Position Indicator) and 1 RHI (Range Height Indicator) have been programed. Data are classified according to stability, wind direction and wind speed at hub height. Ensemble-averaged velocity fields, obtained with an acceptable statistical convergence, are then studied in order to quantify the influence of the different parameters. The present paper will focus on the effects of the thermal stability of the atmosphere on the wake recovery and interactions

Authors:
Marion Cormier, Marco Caboni, Thorsten Lutz, Koen Boorsma, Ewald Krämer

Abstract:
The unsteady aerodynamics of a scale model floating horizontal-axis offshore wind turbine is numerically investigated. Different aerodynamic models are used to analyze the complex unsteady flow, namely models based on unsteady Reynold-averaged Navier-Stokes, free vortex wake and blade element momentum theory. The wind turbine base is subjected to imposed sinusoidal surge oscillations to reproduce the motions of the floating platform, which are added separately in order to catch their individual effects on the flow field. The aerodynamic response of the rotor and the wake unsteadiness are investigated, while an evaluation of the numerical models is performed with help of wind tunnel data.

Authors:
Antoine Vergaerde

Abstract:
Vertical-axis wind turbines (VAWTs) have received a renewed interest in  the wind energy research community, mainly for off-shore applications.  One advantage is that installing a pair of counterrotating VAWTs on the  same floating platform, would result in a zero-net torque. In this article, we report on wind tunnel experiments to study the  mechanical power output of a reference VAWT scale model, tested individually  and in a closely-spaced pair of VAWTs. The power output of the individual  VAWT is compared with a pair of VAWTs in multiple configurations. A net  power increase in the power coefficient of the pair of up to  \SI{11.4}{\percent} compared with two individual rotors has been observed.

Authors:
Sarah Ollier, Simon Watson

Abstract:
Atmospheric Gravity Waves (AGWs) frequently occur around near coastal offshore wind farms. Yet our understanding of how they interact with individual turbines and whole farm energy output is limited. This research uses computational fluid dynamics modelling to investigate the impact of coastal, topographically forced AGWs on offshore wind farm power output. Preliminary results show that a topographically forced AGW results in substantial variation in windspeed across the farm. There was a subsequent variation in power output across the farm where the wind farm spans a distance of around one AGW wavelength.

Authors:
Martín Draper, Andrés Guggeri, Bruno López, Álvaro Díaz, Filippo Campagnolo, Gabriel Usera

Abstract:
Wind energy has become a mature technology. Nevertheless there are still open challenges to further continue its development. Among them, wind farm control to maximize the power production of a whole wind farm has received special attention in the past few years. Several studies have been performed in order to increase the power production of  a group of wind turbines, taking into account different approaches, from simplified and reduced order physical models to statistical and more complex models. In order to validate the proposed algorithms two alternatives have been used: physical experimental campaigns in wind tunnels and numerical simulation with high-fidelity models. The aim of the present paper is to assess the capability of a validated numerical framework, under different inflow conditions and wind turbine setups, to simulate accurately the flow over a group of wind turbines operating under non-optimal conditions focused on maximizing the overall power production. A state-of-the-art wind tunnel campaign is chosen as validation case, that consists of three model wind turbines. The paper is centered on the power computation of each yaw setting, particularly in finding the optimum yaw setting. In addition to this, a comparison of the wind fields between each configuration will be performed.

Authors:
Andreu Carbo Molina

Abstract:
Small Darrieus Vertical-Axis Wind Turbines (VAWTs) have become a relevant research topic within the wind energy community, since they are thought to perform better than horizontal-axis rotors in the complex and highly-turbulent flow of the urban environment. Indeed, a preliminary wind tunnel test campaign on a H-Darrieus VAWT showed a significant increase of the performance for high turbulence levels. The present study then analyses in details the near wake of the turbine in the same turbulent conditions, allowing one to understand better the reason of this power increase, and how to take advantage of it. Near-wake measurements are benchmarked with a CFD simulation of the whole wake, in order to match the measured conditions with the detachment of flow structures observed in CFD measurements. This could provide a deeper and useful insight on the reasons why VAWTs perform better in turbulent environments, and how to exploit them for urban wind harvesting.

Authors:
Paula Doubrawa, Luis A Martínez-Tossas, Thomas G Herges, Patrick Moriarty, Matthew J Churchfield

Abstract:
Comprehensive full-scale wake measurements are difficult to obtain. As a result, research in wind farm aerodynamics is often based on smaller scale measurements and numerical experiments. It is therefore crucial for the scientific community to understand how results from different scales compare and translate to full-scale scenarios. In this work, we compare mean and dynamic wake properties from three different wind turbines (representative of research scale, typical onshore full-scale, and modern, offshore full-scale) subjected to similar inflow. Analyses are based on four actuator line, large-eddy simulations under unstable atmospheric conditions so that the dynamic characteristics of the wakes can be quantified in addition to their mean behavior. Three separate analyses are conducted, in which the effect of hub height on the wakes is either removed, isolated, or considered along with the effects of other differences between the turbines such as rotor size, aerodynamic properties, and control strategies.

Authors:
Eliot Quon

Abstract:
We investigate the feasibility of introducing synthetic turbulence into finite-domain large-eddy simulations (LES) of the wind plant operating environment. This effort is motivated by the need for a robust mesoscale-to-microscale coupling strategy in which a microscale (wind plant) simulation is driven by mesoscale data without any resolved microscale turbulence. A neutrally stratified atmospheric boundary layer was simulated in an LES with 10-m grid spacing. We show how such a fully developed turbulence field may be reproduced with spectral enrichment starting from an under-resolved coarse LES field (with 20-m and 40-m grid spacing). The velocity spectra of the under-resolved fields are enriched by superimposing a fluctuating velocity field calculated by two turbulence simulators: TurbSim and Gabor Kinematic Simulation. Both forms of enrichment accurately simulated the autospectra of all three velocity components at high wavenumbers, with agreement between the enriched fields and the full-resolution LES observed at 400 m from the inflow boundary. In contrast, the spectra of the unenriched fields reached the same fully developed state at four times the downstream distance.

Authors:
Pieter Bauweraerts, Johan Meyers

Abstract:
The turbulent nature of the atmospheric boundary layer leads to wind-turbine power fluctuations and fatigue loading. The possibility to perform faster than real-time turbulent flow field simulations opens up applications in the forecasting and potentially actively controlling of turbulent structures. In this article a fully developed pressure-driven boundary layer is used as a case study to evaluate the potential of forecasting. As a forecast model for the large scale turbulent structures, we use a large eddy simulation (LES) code run on a relatively coarse grid, to decrease the model evaluation-time. The initialization of the forecasts is done by a 4D-Var approach. For the optimization problem, we use a L-BFGS approach combined with an adjoint LES simulation for the gradient calculations. For this study we use virtual LIDAR based measurements that are taken from a fine grid LES simulation, which is also used as a reference to benchmark the state estimation and the flow field predictions of the coarse LES.

Authors:
Tanmoy Chatterjee, Nihanth Wagmi Cherukuru, Yulia T. Peet, Ronald J. Calhoun

Abstract:
The wake interactions in the downstream turbines and the power generated by the wind turbines in massive wind farms depend significantly on turbulent inflow wind conditions from atmospheric boundary layer (ABL). Most of the computational and laboratory studies assume constant mean wind speed and wind direction neglecting large scale geophysical effects In the current paper we perform a high Reynolds number large eddy simulation study (LES) of the offshore Alpha Ventus wind farm in North Sea. Actuator line modeling has been used for the rotating turbines. The wind farm is driven by a neutral atmospheric boundary layer coupling realistic wind flux and veering from LIDAR field experiments. An extensive comparison of the LES results with the field data has been performed which would elucidate more on the capability of LES inflow methodology in capturing realistic large scale structures that are observed in wind farm flows.

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