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Wednesday, 12 March 2014
09:00 - 10:30 Aerodynamics and rotor design
Science & Research  


Room: Llevant
Session description

The session is oriented to show recent computational and experimental findings on aerodynamic phenomena in horizontal (HAWT) and vertical access wind turbines (VAWT), as well as on new developments on system identification techniques related to the aeroelastic behaviour of wind turbine rotors and new aerodynamic design trends for very large wind turbines.

Learning objectives:

Delegates will learn about:

  • recent computational and experimental findings on aerodynamic phenomena in HAWT and VAWT
  • new developments on system identification techniques related to the aeroelastic behaviour of wind turbine rotors
  • innovative design trends for the aerodynamics of very large wind turbines
Lead Session Chair:
Alvaro Cuerva, Universidad Politécnica de Madrid, Spain

Co-chair(s):
Sandrine Aubrun, Univ. Orléans, PRISME Laboratory, France
Koen Boorsma ECN, The Netherlands
Co-authors:
Koen Boorsma (1) F P Gerard Schepers (1) Sugoi Gomez Iradi (1) Helge Aagaard Madsen (3) Niels Sørensen (3) Wen Zhong Shen (3) Christoph Schulz (4) Scott Schreck (5)
(1) ECN, Petten, The Netherlands (2) CENER, Pamplona, Spain (3) DTU, Lyngby, Denmark (4) University of Stuttgart, Stuttgart, Germany (5) NREL, Denver, United States

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Mr. Boorsma has been working in wind energy for more than 6 years as researcher at the Energy research Center of the Netherlands (ECN) in Petten. He obtained his MSc in Aerospace Engineering from Delft University of Technology, after which he successfully pursued his PhD at the University of Southampton in the field of experimental Aero-acoustics. His
research is focused on aerodynamic and acoustic modeling and load analysis in combination with experimental measurements. He has been involved in various research and industrial projects involving field and wind tunnel measurements as well as model improvement and development of new turbine concepts.

Abstract

Mexnext-II: the latest result on experimental wind turbine aerodynamics

Introduction

The subject of aerodynamics is extremely important in wind energy. It does not only determine the energy production of a wind turbine, but it also determines the loads, stability, and the noise of a wind turbine and not to forget the wake behind the turbine and the consequent wind farm losses. As such a thorough understanding of the detailed wind turbine aerodynamics is of crucial importance. The present paper describes the latest results from Mexnext, a large joint international project in which 18 parties from 9 countries cooperate in understanding the wind turbine aerodynamic behavior using advanced aerodynamic measurements.

Approach

The approach which is followed is described in a work plan of IEA Task 29 Mexnext-II. An important component of all IEA tasks is cooperation which is also true in Mexnext since 18 partners from 9 different countries cooperate, see www.mexnext.org and [1].
The goal of Mexnext is to validate and improve aerodynamic models through detailed aerodynamic measurements. Thereto an inventory was carried out in order to find useful aerodynamic measurements. Core experiments form the NREL Phase VI measurements as performed in 2000 and the Mexico measurements as performed in 2006. In the NREL Phase VI experiment a 10 meter diameter turbine is placed in the large NASA-Ames wind tunnel, which included measurement of pressure distributions along the blade [2].
In the Mexico project a 4.5 meter diameter turbine was placed in the Large Low-Speed Facility (LLF) of German Dutch Wind tunnel DNW. The pressure distributions were measured where the underlying flow field was mapped with PIV [3]. Part of Mexnext is the preparation and execution of the ‘New-Mexico’ experiments, i.e. new measurements on the Mexico test rig which will again be placed in the LLF. The aim of New-Mexico is to complement the original Mexico database taking into account the lessons learnt from the first experiment to assure an even higher quality data set.
Apart from the Mexico and NREL Phase VI experiments (old and new) useful measurements are searched from other sources. Such measurements are for example taken in Japan (Mie University), China (This includes the measurements taken by FFA in the 1990’s on a 5 meter diameter rotor in the large CARDC wind tunnel [4]) but also older measurements from IEA Task 18 [5].
Several subjects have been defined which are investigated with these measurements: Standstill aerodynamics, Reynolds number effects, the determination of the angle of attack in wind turbine aerodynamics, near wake aerodynamics, tip and 3D effects, boundary layer transition, yaw and unsteady effects.
Last but not least: An important part of the analysis forms the comparison of the high quality measurement data with calculations from a large variety of codes available in the project.


Main body of abstract

The paper presents a selection of Mexnext results. It starts with a convincing example showing the importance of detailed aerodynamic measurements. The example shows a good agreement between calculated and measured rotor axial force. Hence, without any further information on the local aerodynamic loads this would indicate a good quality of the prediction code. However the availability of measured local aerodynamic loads made it possible to assess the agreement on a local level which showed a very poor performance of the codes, see also . It was then found that the good agreement in rotor axial force is misleading and caused by compensating errors only.
Thereafter the preparation for the New Mexico experiment is described. The first Mexico measurements led to various insights on the added value of CFD, on modeling improvements (e.g. tip effects, 3D-effects, yawed flow) and it led to an enhanced understanding of the 3D-flowfield around wind turbine, see [1]. Still the analysis of the first Mexico measurements led to some open questions which needs to be answered in a new experiment: Mexico was the first experiment in which the detailed pressure distribution as well as the underlying flow field was measured but the relation between the two opposed the momentum relations (at least at design conditions). Consequently the relation between velocities and load needs further attention. Moreover some results suffered from data uncertainties and faulty instrumentation and some parts of the database (e.g. the measurements on dynamic inflow and standstill) were considered incomplete.
A further motivation for New Mexico are the enhanced PIV capabilities from DNW which became available recent years, where in particular the resolution and/or the size of the PIV sheets is increased leading to a much more complete mapping of the flow field. In addition the acoustic noise sources will be measured with an acoustic array in order to establish the acoustic-aerodynamic link.
In preparation for this test the blades will be placed in the TUDelft Low Speed Tunnel. In addition to checking the status of the blades, this test also enables the measurement of the quasi 2D airfoil characteristics which will serve as reference for the rotating measurements.

In a first phase of Mexnext several comparisons were made between measurements and calculations. In order to understand some of the differences it was considered important to know the actual blade shape and to compare it with the specified shape. Thereto the blade shape has been scanned and at first sight the deviations were small. Nevertheless a CFD investigation from the Mexnext partners CENER and University of Stuttgart showed non-negligible differences, see . Although the difference is at first sight seemingly small, the results give an indication of uncertainties due to geometry differences between design and actual blade geometry.

It is however not only the Mexico measurements which are used in the comparison but a main aim of Mexnext-II is also to consider other experiments. Figure shows a comparison between the local loads along the blade of a 5.35 meter diameter rotor which was placed in the large Chinese CARDC tunnel at the end of the 1980’s [4]. A discrepancy is found at the root (indicating an underpredicted stall delay effect, since the angle of attack is approximately 30 degrees at the root). Moreover discrepancies are found at the tip (which indicates the need for a better tip correction model).

In addition a calculational round is defined on a NREL Phase VI experiment at a rotational speed of 90 rpm. Even though the NREL Phase VI experiment has been used in many validation cases already, the rotor speed was usually only 72 rpm where the cases at 90 rpm remained unexplored. In figure a comparison is made between calculated and measured loads along the blade taken at such large rotor speed from some lifting line codes in the project. Apart from generally good agreement, the results again show some discrepancies near the tip region indicating the need for further improvements.


Conclusion

In the Mexnext project very detailed aerodynamic measurements are used. These measurements provide local aerodynamic loads along the blade where in addition the underlying flow field which drives these loads is mapped in some experiments. It is proven that such very detailed information is needed to better understand the aerodynamic behaviour of a wind turbine. With this better understanding it is possible to improve wind turbine design codes which eventually leads to more reliable and more efficient wind turbines. Furthermore the detailed measurements form essential and unique validation material for wind turbine design code by which the the validity of these codes can be assessed and which enables the identification of areas where improvement is needed. Much progress has been made in the project where measurements are used from a large variety of sources, mainly wind tunnel measurements but also field measurements. The paper puts emphasis on a comparison between calculations and hardly explored measurements from the famous NREL Phase VI experiment at a relatively high rotational speed. Moreover results from the Mexico measurements are shown and the differences are assessed between the measured and specified blade geometry including the effect of these differences on the loads. It will be shown that much more work is needed. Thereto the so called New Mexico experiment is performed, i.e. a second experiment on the existing Mexico test rig in order to fill the missing gaps and to take into account the lessons learnt from the first Mexico experiment by which an even higher quality data set can be assured than the first data set.


Learning objectives
The most important lessons learnt from this abstract focus on the shortcomings which have been found on current aerodynamic modeling techniques. Moreover it is explained which advanced aerodynamic tests are carried out and how advanced aerodynamic testing on wind turbines should be performed in order to get the most value out of it. Amongst others, attention is paid to load uncertainties due to differences between design and manufactured blade geometry.


References
[1] J.G. Schepers and K. Boorsma et al : Final Results form Mexnext-I: Analysis of detailed aerodynamic measurements on a 4.5 m diameter rotor placed in the large German Dutch Wind Tunnel DNW, Presented at The Science of Making Torque From Wind, Oldenburg, 9-11 oktober 2012
[2] M.M. Hand et al, Unsteady Aerodynamics Experiment Phase VI Wind Tunnel Test Configurations and Available Data Campaigns, NREL/TP-500-29955, National Renewable Energy Laboratory, NREL, 2001
[3] J.G. Schepers and H. Snel, Mexico, Model experiments in controlled conditions, ECN-E-07-042, Energy Research Center of the Netherlands, 2007

[4] G. Ronsten, Geometry and Installation in Wind Tunnels of a Stork 5.0 WPX Wind Turbine Blade Equipped with Pressure Taps, FFAP-A--1006, The Aeronautical Research Institute of Sweden, FFA, 1994

5] J.G. Schepers et al, Final report of IEA Annex XVIII' Enhanced Field Rotor Aerodynamics Database, ECN-C-02-016, Energy Research Centre of the Netherlands, ECN, 2002