Mexnext: Analysis of
Wind Tunnel Measurements
OPERATING AGENT ORGANIZATION:
Energieonderzoek Centrum Nederland (ECN)
OA Representative:
Gerard SCHEPERS
ECN
Westerduinweg 3
Postbus 1
1755 ZG PETTEN
The Netherlands
Tel: +31 224 56 48 94
Fax: +31 224 56 32 14
Email: Gerald Schepers
Web Site: Task 29 Mexnext: Analysis of Wind Tunnel Measurements
The accuracy of wind turbine design models has been assessed in several validation projects. They all show that the modeling of a wind turbine response (e.g. the power or the loads) is subject to large uncertainties. These uncertainties mainly find their origin in the aerodynamic modeling where several phenomena such as 3-D geometric and rotational effects, instationary effects, yaw effects, tower effects, and stall, amongst others contribute to unknown responses, particularly at off-design conditions. These unknown responses make it very difficult to design cost-effective and reliable wind turbines. Turbines behave unexpectedly; they experience instabilities, power overshoots, or higher loads than expected. Alternatively, the loads may be lower than expected which implies an over dimensioned (and costly) design.
The problem of measurements taken in free atmosphere was overcome when the National Renewable Energy Laboratory (NREL) carried out a wind tunnel experiment in the National Aeronautics and Space Administration (NASA)-Ames wind tunnel in 2000 in the United States (2). In this experiment, a heavily instrumented rotor with a 10 m diameter was placed in the 24.4-m by 36.6-m) wind tunnel and measured with few blockage effects. Although this rotor diameter is still much smaller than the diameter of modern commercial wind turbines, the blade Reynolds number (in the order of 1 million) was sufficiently high to make the aerodynamic phenomena, at least to some extent, representative of modern wind turbines. NREL made the measurements from this experiment available to other institutes and they were analyzed within IEA Wind Task 20 HAWT Aerodynamics and Models from Wind Tunnel Measurements, completed in 2007.
IEA Wind Task 29 Mexnext is the successor of IEA Wind Task 20. It will use the wind tunnel measurements from the EU project Model Experiments in Controlled Conditions (MExICo) that became available in December 2006. In this project, detailed aerodynamic measurements were carried out on a wind turbine model with a diameter of 4.5 m, which was placed in the 9.5 m2 LLF facility of the German Dutch Wind Tunnel (DNW). Within the MEXICO project, pressure surface data were measured at five radial positions (25%, 35%, 60%, 82%, and 92% span) together with blade root bending moments and tower bottom moments from a tunnel balance from DNW (Figure 1). Perhaps the most important feature of the measurements is the extensive flow field mapping from the stereo Particle Image Velocimetry (PIV) technique.
Although the size of the wind turbine rotor used is smaller, the MEXICO experiments were designed to be complementary with the NREL measurements at NASA-Ames. The most important difference between the two experiments is that the MEXICO project includes extensive flow field measurements, simultaneous with the pressure and load measurements. Also, the MEXICO model was three bladed, whereas the NREL model used at NASA-Ames was two bladed. Furthermore, the majority of the NREL measurements concern (the very important) stalled flow, while the entire operational envelope was covered in the MEXICO measurements. Finally, the MEXICO measurements made use of fast Kulite pressure transducers, which measure absolute pressures, whereas differential pressures were measured in the NREL experiment (both techniques have pros and cons).
The MEXICO database is still in a rather rudimentary form and only limited analyses have been carried out. This is the case because the amount of data is vast and the time needed to analyse all data is extremely long for a single country. A cooperative research task under IEA Wind is an efficient way to organize the analysis of the MEXICO data. Added value also lies in the fact that the task will serve as a forum for discussion and interpretation of the results. This will generate more value from the data than the summed value from the individual projects.
In the IEA Wind Task 29 Mexnext, the data will be accessible and a thorough analysis will take place. This includes an assessment of the measurement uncertainties and a validation of different categories of aerodynamic models (rotor aerodynamics and near wake models, where the latter type of models form part of wind farm models as well). The insights will be compared with the knowledge that was gained from IEA Wind Task 20 on the NASA-Ames experiment and from other experiments such as wind tunnel measurements from the Technical University of Delft.
The time line of the project leads to production of the final report in 2011. For more information, please visit the Task 29 Web site.
