ABSTRACT This study evaluates the performance of a counter‐rotating dual rotor wind turbine (CR‐DRWT) with 2 m2 rotor radius equipped with a double rotational armature in an open jet wind tunnel. With only one similar‐sized design previously assessed in a wind tunnel, this study offers valuable validation material for the literature. Through wind tunnel testing, the CR‐DRWT confirmed earlier findings in literature and achieved a 15% to 50% increase in power output and a 10% increase in efficiency (CP) compared to a single rotor configuration at higher wind speeds (> 7 m/s). Though these gains were not observed at lower wind speeds (4–7 m/s). The simplified mechanics of a double rotational armature show promise for SWTs, as financial viability depends on reducing LCOE through efficiency improvements that maximize energy capture. The design's maximum CP values were below those achieved in previous field tests at larger scale highlighting potential for improvement for smaller sized turbines. To further explore the aerodynamics of CR‐DRWT's, computational fluid dynamics (CFD) simulations are recommended, as they could provide insights into optimizing flow dynamics around CR‐DRWT's. Finally, the study emphasizes the need for precise pitch angle and rotational speed measurements to improve the value of future measurements.

This article discusses results from an international contest, open for university student teams (bachelor and master), involving the design, construction, and testing of small wind turbines in a large wind tunnel. The wind tunnel has an outlet of 2.85 x 2.85 m allowing a maximum rotor swept area of 2 m 2 without significant tunnel effects. Both horizontal and vertical axis wind turbines are part of the competition. The turbines are evaluated by an external jury of industry experts based on criteria such as Annual Energy Production, cut-in wind speed, innovations, design, and sustainability. Although the contest has been initiated in 2013 with an educational focus, it has also evolved into a valuable database for scientific purposes by providing a decade worth of performance measurements for roughly 9-10 various turbine concepts each year. The collected data may serve as a unique validation resource for assessing the accuracy of design codes in modelling diverse turbine concepts thanks to detailed design reports with model descriptions accompanying each turbine (such turbine descriptions are often considered confidential for field measurements). The paper aims to explore the scientific value of this database by comparing calculations with measurements, offering explanations where possible, and reporting intriguing findings on unconventional concepts’ performance. Even though not all observations could be explained fully they provide food for thought. Recommendations are provided for both students to enhance their designs and for contest organizers to elevate the scientific value of the measurements in future contests.

Within an international collaboration framework, the accuracy of rotor aerodynamic models used for design load calculations of wind turbines is being assessed. Where the use of high-fidelity computation fluid dynamics (CFD) and mid-fidelity free-vortex wake (FVW) models has become commonplace within the wind energy community, these still fail to meet the requirements in terms of execution time and computational cost needed for design load calculations. The fast but engineering fidelity blade-element/momentum (BEM) method can therefore still be considered the industry workhorse for design load simulations. At the same time, upscaling of wind turbine rotors makes inflow non-uniformities (e.g. shear, veer, turbulence) more important. The objective of this work is to assess model accuracy in non-uniform inflow conditions, which violate several BEM assumptions. Thereto a comparison in turbulent inflow has been executed including a wide variety of codes, focusing on the DanAero field measurements, where a 2.3-MW turbine was equipped with, among other sensors, a pressure measurement apparatus. The results indicate that, although average load patterns are in good agreement, this does not hold for the unsteady loads that drive fatigue damage and aero-elastic stability. A simplified comparison round in vertical shear was initiated to investigate the observed differences in a more controlled manner. A consistent offset in load amplitude was observed between CFD and free-vortex codes on the one hand and BEM-type codes on the other hand. To shed more light on the observations, dedicated efforts are ongoing to pinpoint the cause for these differences, in the end leading to guidelines for an improved BEM implementation.

In these full scale aerodynamic test program local aerodynamic quantities (forces, inflow velocities, inflow angles) are measured at several radial positions along the blade.The aerodynamic experiments at ECN were performed in two different phases: The non-rotating phase (1993-1994). In this phase the blade was mounted vertically on a non-rotating facility in the free stream. This test aimed at acquiring data at free stream conditions without rotational effects. Furthermore the instrumentation could be tested relatively easy in order to prepare for the next phase. The rotating phase (1995-1997). In this phase the blade was mounted on the HAT-25 experimental wind turbine and data were acquired under rotation. Furthermore some non-rotating measurements have been repeated. In this report the measurements from the rotating phase are reported only. The HAT-25 wind turbine is located at ECN near Petten in the Netherlands. The prevailing wind direction is from South-West, at which most of the measurements are taken. In this direction, the terrain upstream of the turbine is obscured by dunes over around 600 m. As a result, the turbulence intensities at the site may depend strongly on the conditions. Generally, the values of the turbulence intensities are between 5% and 20%.Time series measurements are supplied, in which the angle of attack ranges from negative values to deep stall values. Also measurements at yaw misalignment and at stand still have been supplied. Also sectional profile coefficients under rotating and 2D conditions are stored. The SourceDatabase.zip archive contains original database with data file formats and conventions harmonized (across all IEA Task 18: Enhanced Field Rotor Aerodynamics experiments) in order to make the full database easy accessible. The rotating phase measurements have been merged into two files: "rottim_prof_merged.csv" (containing profile, SCADA and SHM data) and "rottim_p_merged.csv" (containing aerodynamic pressure measurements) to facilitate import of all the data.

This article reviews the aerodynamic measurement programs on wind turbines that have been performed in the last decades. It is largely based on results from four projects carried out under auspices of the International Energy Agency (IEA), which are denoted as IEA Tasks 14, 18, 20, and 29. The aim of these projects was to collect and analyze aerodynamic measurements on five field facilities (IEA Tasks 14 and 18), the National Renewable Energy Laboratory Phase VI turbine placed in the large NASA Ames wind tunnel (IEA Task 20), and the Mexico turbine placed in the Large Low Speed Facility of the German Dutch Wind tunnel DNW (IEA Task 29). Other experimental programs with an important aerodynamic content are touched upon as well. Research areas for which these measurements have led to important progress are identified. The progress is illustrated with analyses on these experiments. It is shown that detailed aerodynamic measurements are an absolute necessity for the validation and improvement of wind turbine design codes, where it is also concluded that the amount of measurement data which has been produced until now is still far too limited. This article is categorized under: Wind Power > Science and Materials

Besides classical horizontal and vertical axis wind turbines, alternative ways to harvest wind energy are currently being explored. One promising concept is represented by airborne wind energy systems. Different airborne wind energy concepts have been ideated and investigated over the past few decades. The work reported here focuses on the concept being developed by Ampyx Power that basically generates power using a tethered airplane which drives a generator on the ground. The aim of the work reported here is to develop and compare design calculations for such a power plane. In this study a comparison was made between steady and unsteady RANS CFD calculations from SU2 and OpenFOAM. Subsequently, using the data from the CFD calculation two different power prediction models were developed and compared, one using 2D CFD data with the free vortex code AWSM and the other using the same data with a nonlinear lifting line approach to model the power plane in its figure eight flying trajectory. Overall, comparisons show that for all practical angles of attack, as well as a range of flap angles, that 2D CFD results agree extremely well between both solvers. The 3D CFD results show <20% differences on the computed total forces, despite achieving good agreement on the relative force contributions across the power plane. A very good agreement has also been established between the power prediction models.

This work presents an investigation on different methods for the calculation of the angle of attack and the underlying induced velocity on wind turbine blades using data obtained from three-dimensional Computational Fluid Dynamics (CFD). Several methods are examined and their advantages, as well as shortcomings, are presented. The investigations are performed for two 10MW reference wind turbines under axial inflow conditions, namely the turbines designed in the EU AVATAR and INNWIND.EU projects. The results show that the evaluated methods are in good agreement with each other at the mid-span, though some deviations are observed at the root and tip regions of the blades. This indicates that CFD results can be used for the calibration of induction modeling for Blade Element Momentum (BEM) tools. Moreover, using any of the proposed methods, it is possible to obtain airfoil characteristics for lift and drag coefficients as a function of the angle of attack.

This paper presents the most recent results from the EU project AVATAR in which aerodynamic models are improved and validated for wind turbines on a scale of 10 MW and more. Measurements on a DU 00-W-212 airfoil are presented which have been taken in the pressurized DNW-HDG wind tunnel up to a Reynolds number of 15 Million. These measurements are compared with measurements in the LM wind tunnel for Reynolds numbers of 3 and 6 Million and with calculational results. In the analysis of results special attention is paid to high Reynolds numbers effects. CFD calculations on airfoil performance showed an unexpected large scatter which eventually was reduced by paying even more attention to grid independency and domain size in relation to grid topology. Moreover calculations are presented on flow devices (leading and trailing edge flaps and vortex generators). Finally results are shown between results from 3D rotor models where a comparison is made between results from vortex wake methods and BEM methods at yawed conditions.

This work is aimed to investigate the effects of elastic blade deformations on the aerodynamics of large wind turbine rotors subjected to yawed inflow. Due to the increasing rotor size and advanced light weight blade designs, significant blade deflections can be observed on a regular basis for modern wind turbines. However, especially for complex flow situations like yawed inflow, the role of blade deformations is still not completely understood. In this paper, numerical simulations are conducted on the DTU 10 MW reference wind turbine to gain a better understanding of the involved phenomena. Results are obtained by two numerical methods of different fidelity. First, by the aero-elastic simulation tool FAST, which is based on the low fidelity Blade Element Momentum Theory (BEM) and makes use of common skewed wake correction models. Secondly, by a high-fidelity framework which couples the open-source Computational Fluid Dynamics (CFD) toolbox OpenFOAM with the in-house geometrically non-linear beam solver BeamFOAM. The evaluation of the results is based on the analysis of azimuthal variations of the sectional forces along the blade span and reveals a generally good agreement between the used numerical approach in terms of force amplitudes and variation phases. However, especially for cases of larger yaw angles, the BEM models clearly over-predict the variation amplitudes of the forces up to 40% in the outer blade part.

As part of the AVATAR and Mexnext projects, this study compares several methods used to derive lifting line variables from CFD simulations of the MEXICO rotor in yawed inflow. The results from six partners within the AVATAR/Mexnext consortium using five different methods of extraction were compared. Overall comparison of the induced velocities at the mid and tip parts of blade shows fairly good agreement between the tested methods, where the derived angle of attack differs within 1, within the linear range this accounts to <10% uncertainty on the aerodynamic forces. The presented comparison shows inadequate agreement between the methods for application towards the root.

The aim of this work is to investigate the capabilities of current engineering tools based on Blade Element Momentum (BEM) and free vortex wake codes for the prediction of key aerodynamic parameters of wind turbines in yawed flow. Axial induction factor and aerodynamic loads of three wind turbines (NREL VI, AVATAR and INNWIND.EU) were investigated using wind tunnel measurements and numerical simulations for 0 and 30 degrees of yaw. Results indicated that for axial conditions there is a good agreement between all codes in terms of mean values of aerodynamic parameters, however in yawed flow significant deviations were observed. This was due to unsteady phenomena such as advancing & retreating and skewed wake effect. These deviations were more visible in aerodynamic parameters in comparison to the rotor azimuthal angle for the sections at the root and tip where the skewed wake effect plays a major role.

To validate and reduce the large uncertainty associated with rotor aerodynamic and acoustic models, there is a need for detailed force, noise and surrounding flow velocity measurements on wind turbines under controlled conditions. However, high quality wind tunnel campaigns on horizontal axis wind turbine models are scarce due to the large wind tunnel size needed and consequently high associated costs. To serve this purpose an experiment using the Mexico turbine was set-up in the large low speed facility of the DNW wind tunnel. An overview of the experiments is given including a selection of results. A comparison of calculations to measurements for design conditions shows a satisfactory agreement.In summary, after years of preparation, ECN and partners have performed very successful aerodynamic experiments in the largest wind tunnel in Europe. The comprehensive high quality database that has been obtained will be used in the international Mexnext consortium to further develop wind energy aerodynamic and acoustic modeling.

The present paper regards axial and angular momentum considerations combining detailed loads from pressure sensors and the flow field mapped with particle image velocimetry (PIV) techniques. For this end, the study implements important results leaning on experimental data from wind tunnel measurements of the New MEXICO project. The measurements, taken on a fully instrumented rotor, were carried out in the German Dutch Wind tunnel Organisation (DNW) testing the MEXICO rotor in the open section. The work revisits the so-called momentum theory, showing that the integral thrust and torque measured on the rotor correspond with an extent of 0.7 and 2.4% respectively to the momentum balance of the global flow field using the general momentum equations. Likewise, the sectional forces combined with the local induced velocities are found to plausibly obey the annular streamtube theory, albeit some limitations in the axial momentum become more apparent at high inductions after a=0.3. Finally, azimuth induced velocities are measured and compared to predictions from models of Glauert and Burton et al., showing close-matching forecasts for blade spans above 25%.

In this report, the details of an investigation into the effect of low induction rotor (LIR) wind turbines on the Levelised Cost of Electricity (LCoE) in a 1GW offshore wind farm is outlined. The 10 MW INNWIND.EU conventional wind turbine and its low induction variant, the 10 MW AVATAR wind turbine, are considered in a variety of 10x10 layout configurations. The Annual Energy Production (AEP) and cost of electrical infrastructure were determined using two in-house ECN software tools, namely FarmFlow and EEFarm II. Combining this information with a generalised cost model, the LCoE from these layouts were determined. The optimum LCoE for the AVATAR wind farm was determined to be 92.15 e/MWh while for the INNWIND.EU wind farm it was 93.85 e/MWh. Although the low induction wind farm offered a marginally lower LCoE, it should not be considered as definitive due to simple nature of the cost model used. The results do indicate that the AVATAR wind farms require less space to achieve this similar cost performace, with a higher optimal wind farm power density (WFPD) of 3.7 MW/km2 compared to 3 MW/km2 for the INNWIND.EU based wind farm.

2D wind tunnel tests at high Reynolds numbers have been done within the EU FP7 AVATAR project (AdVanced Aerodynamic Tools of lArge Rotors) on the DU00-W-212 airfoil and at two different test facilities: the DNW High Pressure Wind Tunnel in Gottingen (HDG) and the LM Wind Power in-house wind tunnel. Two conditions of Reynolds numbers have been performed in both tests: 3 and 6 million. The Mach number and turbulence intensity values are similar in both wind tunnels at the 3 million Reynolds number test, while they are significantly different at 6 million Reynolds number. The paper presents a comparison of the data obtained from the two wind tunnels, showing good repeatability at 3 million Reynolds number and differences at 6 million Reynolds number that are consistent with the different Mach number and turbulence intensity values.

1 Project Manager, Wind Energy, Westerduinweg 3, Member 2 Consultant, Wind Energy, Westerduinweg 3 3 Project Manager, Wind Energy, Westerduinweg 3 4 Senior Engineer, GE Global Research Center, Freisinger Landstrasse 50, Senior Member 5 Principal Engineer, GE Wind Energy, Holsterfeld 16 6 RES Director, Wind Energy Department 19 th km Marathonos Ave 7 Mechanical Engineer, Wind Energy Department, 19 th km Marathonos Ave 8 Assistant Professor, Aerodynamics and Wind Energy, Kluyerweg 1 9 Professor, Wind Energy, Frederiksborgvej 399, Senior Member 10 Researcher, Ammerländer Heerstr. 136 11 Head of department, Turbinesimulation, Softwaredevelopment and Aerodynamics, Ammerländer Heerstr. 136 12 Group Manager, Aircraft aerodynamics, Pfaffenwaldring 21 13 Researcher, Wind Energy, C/Ciudad de la Innovación 7 D ow nl oa de d by B ib lio te ca D ip ar tim en to I ng eg ne ri a on N ov em be r 30 , 2 01 5 | h ttp :// ar c. ai aa .o rg | D O I: 1 0. 25 14 /6 .2 01 504 97 33rd Wind Energy Symposium 5-9 January 2015, Kissimmee, Florida AIAA 2015-0497

Continued inquiry into rotor and blade aerodynamics remains crucial for achieving accurate, reliable prediction of wind turbine power performance under yawed conditions. To exploit key advantages conferred by controlled inflow conditions, EU-JOULE DATA Project and UAE Phase VI experimental data were used to characterize rotor power production under yawed conditions. Anomalies in rotor power variation with yaw error were observed, and the underlying fluid dynamic interactions were isolated. Unlike currently recognized influences caused by angled inflow and skewed wake, which may be considered potential flow interactions, these anomalies were linked to pronounced viscous and unsteady effects.

A new integrated design tool for optimization of offshore wind farm clusters is under development in the European Energy Research Alliance - Design Tools for Offshore wind farm Cluster project (EERA DTOC). The project builds on already established design tools from the project partners and possibly third-party models. Wake models have been benchmarked on the Horns Rev and, currently, on the Lilgrund wind farm test cases. Dedicated experiments from 'BARD Offshore 1' wind farm will using scanning lidars will produce new data for the validation of wake models. Furthermore, the project includes power plant interconnection and energy yield models all interrelated with a simplified cost model for the evaluation of layout scenarios. The overall aim is to produce an efficient, easy to use and flexible tool - to facilitate the optimised design of individual and clusters of offshore wind farms. A demonstration phase at the end of the project will assess the value of the integrated design tool with the help of potential end-users from industry.This abstracts summarizes the objectives and preliminary results of work package 3. In order to provide an accurate value of the expected net energy yield, the offshore wind resource assessment process has been reviewed as well as the sources of uncertainty associated to each step.Methodologies for the assessment of offshore gross annual energy production are analyzed based on the Fino 1 test case. Measured data and virtual data from Numerical Weather Prediction models have been used to calculate long term wind speed, wind profile and gross energy. (C) 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

A selection of results from IEA Annex 29 Mexnext on analysis of wind tunnel measurements is presented. A convincing example illustrates the importance of detailed aerodynamic measurements. The influence of MEXICO blade shape deviations between design and manufactured geometry was assessed by scanning the blade geometry and performing comparative CFD simulations with this geometry. Generally speaking the differences between the results for design and scanned geometry do not justify the differences observed between experiments and computations A comparison between calculations and unexplored measurements from the famous NREL UAE PHASE VI experiment at a relatively high rotational speed is performed. It was found that as long as prescribed airfoil data is used, a good agreement exists between lifting line code results. The tip effect remains difficult to predict, although it is questioned in how far the limited blade aspect ratio is representative for large commercial wind turbines. CFD RANS simulations generally perform better in this respect, although separated flow features remain a challenge for these models as well. Preparations for a second experiment on the existing MEXICO test rig are discussed. New configurations will be tested and new apparatus including an acoustic array will be used, by which an even higher quality data set can be assured than the first data set. A standstill test of the MEXICO blades in the Delft Low speed tunnel allowed to determine an appropriate roughness configuration and to make sure that the blades and their data acquisition are in good shape for the New MEXICO campaign.