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. This is not surprising since the subject of aerodynamics is known to be very complicated in view of the fact that the so called Navier Stokes equations  (i.e. the equations which describe every flow and aerodynamic problem) cannot be solved in an exact way  (as a matter of fact fluid dynamics and so aerodynamics forms one of the 7 millennium price problems, see http://www.claymath.org/millennium-problems/millennium-prize-problems).  For wind turbine aerodynamics several phenomena such as 3-D geometric and rotational effects, instationary effects, yaw effects, stall, and tower effects, form even additional complications, particularly at off-design conditions. These uncertainties become very prominent for large wind turbines, see e.g. http://www.eera-avatar.eu/

The 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 span wide 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. The first three phases of this task focused on the wind tunnel measurements which became available in December 2006 within the EU project Mexico and in the later New Mexico project which was carried out in June/July 2014. In these projects 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.

The connection between IEA Wind Task 29 and the (New) Mexico experiments made that the phases I to III of IEA Task 29 were often denoted as Mexnext. The first phase (Mexnext-I) started in 2008 and it ended on December 31st, 2011. It validated and improved aerodynamic models making use of the Mexico experiment. Then the second phase (Mexnext-II) analysed aerodynamic measurements on wind turbines (both in the wind tunnel as well as in the field) from a wide variety of sources which were unexplored hitherto. Moreover in Mexnext-II a second set of measurements was performed on the Mexico rotor in the LLF facility of the German Dutch Wind Tunnel, DNW. The second phase started on January 1st 2012 and ended on December 31st 2014. It led to the third phase: Mexnext-III which ran from January 1st 2015 until December 31st 2017. The main aim of Mexnext-III was to analyse the New Mexico measurements, not forgetting other interesting experiments. After finishing the third phase of Mexnext the emphasis of Task 29 moved to field measurements again in the fourth phase of IEA Task 29 which started on January 1st 2018 and which will end on December 31st 2020. Although the basic goal of Phase IV, at first sight looks similar to the goal from the previous phases I, II and III, i.e. improvement of aerodynamic models for wind turbine design codes, the goal is now reached by means of full scale field measurements from the Danish Danaero experiment. This is a step closer to the operation of wind turbines in real life conditions since the first phases wind tunnel measurements were used.

The fact that Phase IV relies on Danaero Field measurements makes the name Mexnext not applicable anymore.