Introduction

Overview of Mexico rotor in DNW

In the past the accuracy of wind turbine design models has been assessed in several validation projects. They all showed that the modeling of a wind turbine response (i.e. the power or the loads) is subject to large uncertainties. These uncertainties mainly find their origin in the aerodynamic modeling, where several phenomena, like 3D geometric and rotational effects, instationary effects, yaw effects, stall, tower effects etc, contribute to unknown responses in particular at off-design conditions. These unknown responses make it very difficult to design cost-effective and reliable wind turbines. Turbines behave unexpectedly, experiencing 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 availability of high quality measurements is considered to be the most important pre-requisite to gain insight into these uncertainties and to validate and improve aerodynamic wind turbine models. However, conventional experimental programs on wind turbines generally do not provide sufficient information for this purpose, since they only measure the integrated, total (blade or rotor) loads. These loads consist of an aerodynamic and a mass induced component and they are integrated over a certain spanwise length. In the late 80’s and the 90’s it was realized that more direct aerodynamic information was needed in order to improve the aerodynamic modelling. For this reason several institutes initiated experimental programs in which pressure distribution and the resulting normal and tangential forces at different radial positions were measured. Under the auspices of the IEA Wind, many of these measurements were stored into a database in Task XIV and Task XVIII. The results of these measurements turned out to be very useful and important new insights on e.g. 3D stall effects, tip effects and yaw were formed. However, the measurements were taken on turbines in the free atmosphere, where the uncertainty due to the instationary, inhomogeneous and uncontrolled wind conditions formed an important problem (as it is in all field measurements).
This problem was overcome in NREL’s  NASA-Ames wind tunnel experiment which was carried out in 2000. In this experiment a heavily instrumented rotor with a diameter of 10 meter was placed in the world’s largest wind tunnel, i.e. the NASA-Ames (24.4x36.6 m2) wind tunnel. As such measurements were performed at stationary and homogeneous conditions. The huge size of the wind tunnel allowed a rotor diameter of 10 m, with little blockage effects. Obviously this rotor diameter is still (much) smaller than the diameter of the nowadays commercial wind turbines, but nevertheless the blade Reynolds number (in the order of 1 Million) is sufficiently high to make the aerodynamic phenomena at least to some extend representative for modern wind turbines. NREL made the measurements from this experiment available to other institutes and they were analysed within IEA Wind Task XX. This Task was finished in december 2007.

The IEA Wind Task MEXNEX(T), like the above mentioned Tasks XIV, XVIII and XX organised under the auspices of IEA Wind can be considered as the successor of IEA Wind Task XX. It focussed on the wind tunnel measurements which became available in december 2006 within the EU project Mexico. 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 largest European wind tunnel, the LLF facility of the German Dutch Wind Tunnel, DNW with a size of 9.5 x 9.5 m2. Within the Mexico project it was not only pressure and load data which were measured but in addition detailed flow field data were taken with the Particle Image Velocimetry (PIV) technique.

In the IEA Wind Task MEXNEX(T), the accessibility of data was facilitated and a thorough analysis of the data has taken place. This included an assessment of the measurement uncertainties and a validation of different categories of aerodynamic models (rotor aerodynamics + near wake models, where the latter type of models form part of wind farm models as well). 

The first phase of the project ended on June 1st, 2011, but in October 2011 an extension of the project (Mexnext-II) was approved by the IEA Executive Committee. Within this extension unexplored aerodynamic measurements on wind turbines (both in the wind tunnel as well as in the field) will be analysed from a wide variety of sources.  Thereto an inventory of promising experiments has been made. On basis of this inventory the folllow up activities are currently being defined. These follow-up activities are largely similar to the activities as carried out in Mexnext-I, i.e. a thorough analysis of the data has taken place. This included an assessment of the measurement uncertainties and a validation of different categories of aerodynamic models.

Moroever within Mexnext-II a second set of measurements is planned to be performed on the Mexico rotor in the LLF facility of the German Dutch Wind Tunnel, DNW. Thése 'New Mexico' measurements are sponsored by the ESWIRP program and the recently started project Innwind.