Results/Status as of December 2011

The project started in June 2008. In October 2011, the project is officially ended but the IEA Executive Committee approved an extension of the project with three years. In the extension of Mexnext it will not only be Mexico measurements which are analysed but a wide database of aerodynamic measurements on wind turbines (both in the wind tunnel as well as in the field) will be considered.

The main results from the first Phase of Mexnext are described in the references [1] to [19]. Furthermore a public final report is completed in draft and it will be made available soon.

The final report discusses  a large variety of subjects, eg. investigations on measurement quality and tunnel effects. Apart from some outliers, generally speaking the data quality was found to be good. Furthermore the measured rotating airfoil characteristics were presented in different ways using different angle of attack methods (or even without angle of attack methods) and the rotating characteristics were compared with 2D measurements. PIV measurements in the wake have been processed and analysed, see below and a comparison was made between the observations from NASA-Ames and Mexico experiment. Moreover the effects from airfoil imperfections were estimated, effects from Reynolds number and Mach number were discussed etc.It is worthwhile mentioning that Kari and INTA even built scaled down models of the Mexico rotor and measured the performance of it in their own wind tunnel sin comparison to the performance of the ‘real’ Mexico turbine (see the figure below). The results from some comparisons are presented in [19].

Furthermore comparisons are made between calculational and measured results. The first comparisons were made for non-yawed conditions at three different tip speed ratios: A low tip speed ratio (i.e. stalled conditions), the design tip speed ratio and a high tip speed ratio (i.e. turbulent wake state). Most interesting in this comparison is the fact that the comparison is not only made with load measurements but also with the underlying flow field measurements which drive these loads. This comparison has led to a striking observation. In the figures below the axial velocity decay at 82% span is presented for design conditions. Also indicated are the results from a cylindrical vortex wake method which is compatible to the momentum theory. The axial force coefficient in the cylindrical vortex sheet method was set to 0.89 which was beforehand the expected value in view of the fact that these measurements are performed at design conditions.
 
Apart from some  discrepancies near the rotor plane (which are explained in [10])  the measured velocity decay seems to agree reasonably well with the momentum theory if  C_Dax is set to the expected value of 0.89 but it was then very striking to note that the measured C_Dax at the design tip speed ratio of 6.66 obtained with two independant measurement techniques (pressure measurements and balance measurements) and for two tip speeds, is only 0.72, see the figure below.  Such low C_Dax yields much lower axial induced velocities and hence a higher velocity level.
The first logical thought would be that the C_Dax measurements are incorrect but this is difficult to believe in view of the fact that these values are determined with  two fully independent measurement techniques, the results of which agree very well at all datapoints at different tunnel speeds and different rotational speeds.
  
Another explanation for the anomalies could be tunnel effects. An extensive investigation on the impact of these effects has taken place within the Mexnext project on basis of CFD calculations.  Until now the impact of these effects seems to be  small at design conditions, and the limited effect which has been
found points in the  opposite direction: When the induction in the tunnel is the same to the induction in the  free stream,  the C_Dax is higher in the tunnel situation. This implies that the free stream C_Dax would even be lower than 0.72.

As such an explanation for the discrepancy between measurements and momentum theory has not been found yet but the logical consequence is that none of the calculations from the Mexnext group can predict both the velocities AND loads in a correct way: The  calculations overpredict the velocity (underpredict the induction) and/or overpredict the loads.
This is clearly visible in the figures below which show results from several CFD calculations for the normal force  distribution along the blade and the radial and axial traverses of the axial  elocity in comparison with  the measurements. As a matter of fact almost all codes overpredict both the loads and the velocities. Nevertheless the results of some codes approach the measurements very closely but this is only true for either the velocities or the loads. Despite these discrepancies it is very encouraging to see the good qualitative agreement between calculations and measurements also in terms of flow details. This was even found in yawed conditions. 

Another interesting result is the vorticity which was found in the PIV measurements around a blade position of 60% span, see the velocity contours below, which show velocity fluctations at the inner part of the PIV range (i.e. at r ~ 1.2 m) between x= 0 and x=2.5 m, i.e. shortly behind the rotor.  This vorticity is attributed to the change in airfoils from the DU91-W2-250 airfoil to the RISO A1-21 airfoil. Although the blade was designed to have a gradual transition between these two airfoils, this seemed to have been unsuccesfull from which it can be learnt that  transition of airfoils might be an item which deserves more attention.

All graphs with comparisons between calculations and measurements are included in the final report which is available in draft and which will be made public in the beginning of 2012. A request for high resolution graphs should be forwarded to the Operating Agent (http://www.mexnext.org/contact).

With regard to availability of the measurement data, it can be mentioned that a selection of data is made public. It concerns the measurements which have been used as basis for the comparison with calculational results. Also the information on the experimental set-up and the Mexico rotor can be made available under the condition that an NDA is signed on the confidentiality of one of the airfoils on the rotor. A request for the measurement data and the data of the Mexico rotor should be forwarded to the Operating Agent (http://www.mexnext.org/contact).

1. J. G. Schepers and H. Snel: ‘Model Experiments in Controlled Conditions, Final report’, ECN-E-07-042, Energy Research Center of the Netherlands, ECN, February 2007, http://www.ecn.nl/docs/library/report/2007/e07042.pdf
2. H. Snel, J.G. Schepers, B. Montgomerie: ‘The MEXICO project (Model Experiments in Controlled Conditions): The database and first results of data processing and interpretation’, The Science of Making Torque from the Wind, 28–31 August 2007, Technical University of Denmark, http://iopscience.iop.org/1742-6596/75/1/012014/pdf?ejredirect=iopsciencetrial
3. Kay, A. Investigating the Unsteady Aerodynamics associated with a horizontal axis wind turbine, with reference to the recent measurements gathered during the Mexico project. TU master project report
4. L. Pascal Analysis of Mexico measurements,  ECN-Wind Memo-09-010, January 2009
5. H. Snel, J.G. Schepers and A. Siccama, Mexico, the database and results of data processing and analysis 47th AIAA Aerospace Sciences meeting, Orlando, USA, January 2009
6. A. Bechmann and N. Sørensen CFD simulation of the Mexico rotor wake, European Wind Energy Conference, March 2009, Marseille France
7. A.K. Kuczaj Virtual Blade Model Simulations of the Mexico experiment, NRG-12810/09.97106, NRG Petten, The Netherlands
8. Daniel Micallef, Menno Kloosterman, Carlos Ferreira, Tonio Sant , Gerard van Bussel Validating BEM, Direct and Inverse Free Wake Models with the MEXICO experiment, 48th AIAA Aerospace Sciences meeting, Orlando, USA, January 2010
9. S Breton, C Sibuet, C Masson Using the Actuator Surface Method to Model the Three-Bladed MEXICO Wind Turbine 48th AIAA Aerospace Sciences meeting, January 2010
10. J.G. Schepers, L. Pascal and H. Snel. First results from Mexnext: Analysis of detailed aerodynamic measurements on a 4.5 m diameter rotor placed in the large German Dutch Wind Tunnel DNW. European Wind Energy Conference, EWEC, April 2010, Warsaw Poland
11. Wen Zhong Shen et al Validation of the Actuator Line / Navier Stokes technique using Mexico measurements, 'The Science of Making Torque from the Wind', June 2010
12. Yang Hua et al  Determination of Aerofoil Data and Angle of Attack on the Mexico Rotor using Experimental Data  'The Science of Making Torque from the Wind', June 2010
13. S. Schreck et al Rotational Augmentation Disparities in the UAE Phase VI and MEXICO Experiments 'The Science of Making Torque from the Wind', June 201
14. S. Gomez-Iradi and X. Munduate: A CFD Investigation of the Influence of Trip-Tape on the MEXICO Wind Turbine Blade Sections 'The Science of Making Torque from the Wind', June 201
15. B. Stoevesandt et al ''OpenFOAM:RANS-Simulation of a wind turbine and verification " 'The Science of Making Torque from the Wind', June 2011
16. S. Breton, C. Sibuet, C. Masson, Analysis of the inflow conditions of the MEXICO Rotor : comparison between measurements and numerical simulations, 'The Science of Making Torque from the Wind', June 201
17. J.G. Schepers, K. Boorsma, H. Snel,  IEA Task 29 Mexnext: Analysis of wind tunnel measurements from the EU project Mexico,  'The Science of Making Torque from the Wind', June 2010
18. T. Lutz, Near Wake studies of the Mexico Rotor 'EWEA Annual Event', March 2011
19. J.G. Schepers, K. Boorsma, C. Kim, T Cho,  Results from Mexnext: Analysis of detailed aerodynamic measurements on a 4.5 m diameter rotor placed in the large German Dutch Wind Tunnel DNW,  'EWEA Annual Event', March 2011
20.  R. Pereira, J.G. Schepers, KM. Pavel,  VAlidation of the Beddoes Leishman Dynamic Stall model for Horizontal Axis Wind Turbines using Mexco data,  49^th AIAA Aerospace Sciences Meeting Orlando USA, January 2011
21.  S.K. Guntur, C. Bak and N.N. Sorensen,  Analysis of 3D stall models for wind turbine blades using data from the Mexico experiment,  13th International conference on Wind Engineering, ICWE, Amsterdam Holland, July 2011
22. D. Micallef et al, The relevance of spanwise flows for yawed horizontal-axis wind turbines,  13th International conference on Wind Engineering, ICWE, Amsterdam Holland, July 2011
23. Réthoré, P.-E., Sørensen, N.N., Zahle, F., Bechmann, A., Madsen, H.A., "CFD model of the MEXICO wind tunnel", 'EWEA Annual Event', March 2011
24.  Réthoré, P.-E., Sørensen, N.N., Zahle, F., Bechmann, A., Madsen, H.A., "MEXICO Wind Tunnel and Wind Turbine modelled in CFD". AIAA Conference. Honolulu, Hawaii, USA. June 2011
25. K. Meister "Grid dependency studies on tip vortex preservation in wind turbine CFD simulations". Wake Conference. Gotland University, Sweden. June 2011
26. W.Z. Shen "Actuator Line / Navier Stokes Computations for Flows past the Yawed MEXICO Rotor". Wake Conference. Gotland University, Sweden. June 2011
27. N. Sorensen "Near Wake Predictions Behind the MEXICO Rotor in Axial and Yawed Flow Conditions". Wake Conference. Gotland University, Sweden. June 2011
28. R. Szasz "LES of the near wake of the MEXICO wind turbine". Wake Conference. Gotland University, Sweden. June 2011
29. S. Breton "Numerical Analysis of the Vorticity Structure of the MEXICO Rotor in the Near Wake". Wake Conference. Gotland University, Sweden. June 2011
30. K. Nillsen  "The wake behind the Mexico rotor". Wake Conference. Gotland University, Sweden. June 2011
31. Yang Hua et al  "Extraction of airfoil data using PIV and pressure measurements” Journal of Wind Energy, 14, 2011, pages 539-556 .