The MEXICO project: The Database and Results of Data Processing and Interpretation
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The MEXICO project: The Database and Results of Data Processing and Interpretation Herman Snel, Gerard Schepers (ECN), Arn é Siccama (NRG). Introduction. MEXICO project = M odel EX periments I n Co ntrolled Conditions (European Union project, Framework Programme 5)

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The MEXICO project: The Database and Results of Data Processing and Interpretation Herman Snel, Gerard Schepers (ECN), Arné Siccama (NRG)

Introduction Processing and Interpretation

  • MEXICO project = Model EXperimentsIn Controlled Conditions

  • (European Union project, Framework Programme 5)

  • Main objective: create a database of detailed aerodynamic measurements on a realistic wind turbine model, in a large high quality wind tunnel. Complementary to the NREL NASA Ames measurements

  • The database is to be used for aerodynamic model evaluation, validation and improvement, from BEM to CFD

  • The programme ran from 2001 to the end of 2006:

  • Dec 2006: a six day measurement campaign in the LLF of DNW (9.5 x 9.5 m2) with a 3 bladed model of 4.5 m diameter, leading to 100 GB of very useful data.

Overview Processing and Interpretation

  • Participants

  • Model and instrumentation

  • Flow field measurements, PIV

  • The measurement matrix and the data base

  • PIV quantitative flow field analyses

  • Comparison with CFD (Fluent)

  • Conclusions

Participants and main tasks
Participants and main tasks Processing and Interpretation

  • ECN (NL): coordinator, model design and experiment coordination

  • Delft University of Technology (NL): 2D profile measurements, model data acquisition

  • NLR, NL: tunnel data acquisition and experiment coordination

  • RISOE National Laboratories: CFD and experimental matrix

  • Technical University if Denmark, DTU: CFD, tunnel effects

  • CRES (GR) CFD, tunnel effects

  • NTUA (GR) CFD, tunnel effects

  • FOI/FFA (S) flow visualization

  • Israel Institute of Technology Technion: model construction

  • National Renewable Energy Laboratories NREL (USA): invited participant

  • Subcontractor: DNW, wind tunnel facilities

The model and the instrumentation
The model and the instrumentation Processing and Interpretation

  • Three bladed rotor (NREL was 2 bladed) with a diameter of 4.5 m and a design tip speed ratio of 6.7. Tip speed for most of the measurements 100 m/s for a higher Re number (7.07 Hz)

  • Profiles: DU91-W2-250, RISOE A1-21 and NACA 64 418

  • Total of 148 Kulite pressure sensors distributed over 5 sections, at 25, 35, 60, 85 and 95% radial position, in the three blades.

  • Two strain gauge bridges at the root of each of the blades.

  • Total forces and moments at the six component balance of DNW

  • Speed and pitch variable

  • Model data effective sampling frequency 5.5 kHz

  • Balance and tunnel data averaged over run (5 seconds, 35 revolutions)

Blade layout
Blade layout Processing and Interpretation

DU 91 W2 250

Risø A1-21

2.25 m

Kulite instrumented sections:

25% and 35 % DU 91 W2 250

60% Riso A1-21

82% and 92% NACA 64 418

NACA 64-418


Flow field measurements stereo piv

Zero rotor azimuth: blade 1 vertically upward Processing and Interpretation


PIV planes at 270 degrees azimuth

PIV traverse tower with two cameras, aimed at horizontal PIV sheet of 35*42 cm2 in horizontal symmetry plane of the rotor

Seeding (tiny soap bubbles) injected in settling chamber.

Sheet is illuminated by laser flashes at 200 nanosecond interval and photographed.

Sheet is subdivided into ‘interrogation windows’ (79*93, 4.3*4.3mm2). Velocity vector is the vector giving maximum correlation between these two shots.

Flow field measurements, stereo PIV

Photo: Gerard Schepers

The measurement matrix a pressures and loads
The measurement matrix. A) pressures and loads Processing and Interpretation

  • Tip speed ratios varying from 3.3 to 10, at many tip angles

  • Yaw angles 0, ±15, ±30 and ±45 degrees

  • Rotor parked condition with blade angles varying from -5.3 to 90 degrees.

  • ‘Data points’ taken during 5 sec = 35 revolutions

  • Additionally:

  • Pitch angles ramps from -2.3° to 5° and back

  • Rotational speed ramps from tip speed of 100 m/s to 76 m/s and back

The measurement matrix b piv
The measurement matrix B) PIV Processing and Interpretation

  • Particle Image Velocimetry (PIV) was carried out simultaneously with (repeated) pressure and load measurement, showing good repeatability. Tip speed ratios of 4.17, 6.7 and 10

  • Three types:

  • In rotor plane, at 6 different azimuth angles between blades (flow between blades !)

  • Inflow and wake traverses at 2 radial stations (61% and 82%)

  • Tip vortex tracking

  • 30 to 100 ‘takes’ at 2.4 Hz (phase locked)

  • Both for axi-symmetric and yawed flow (plus and minus 30°)

Results of inflow and wake traverses
Results of inflow and wake traverses Processing and Interpretation

All data shown for tip speed of 100 m/s and -2.3° tip angle, zero yaw

Tunnel speeds of 10 m/s, 15 m/s and 24 m/s

l = 4.17

l = 6.7

Cylindrical vortex wake model

l = 10

The moment of truth
The moment of truth Processing and Interpretation

The first intelligible pressure distribution appears in the quick look system, during the measurements

Attached and stalled flow piv images just behind rotor at 82 span
Attached and stalled flow, PIV images just behind rotor, at 82% span.

Flow direction

To blade tip

Attached flow l = 6.7:

Thin viscous wake, left by passing blade

Stalled flow l = 4.17:

Much thicker blade wake and ‘trailing vortex’ at location of large jump in bound vorticity, explains chaotic behaviour in velocity decay

Axial velocity in radial traverse in rotor plane for 0 and 120 azimuth

Blade tip position at 2.25 m 82% span.

Axial velocity in radial traverse in rotor plane, for 0 and 120 azimuth

Shows good repeatability and coherence between different PIV sheets

Up flow and down flow effect of blades yaw 0
Up-flow and down-flow effect of blades, yaw = 0 82% span.

Measured Inflow for blade just below and just above PIV sheet. PIV sheet always at 270 ° azimuth position

Blade tip position

az = 40°

az = 20 °

az = 40°

az = 20 °

Difference of approximately 5 m/s, 1/3 of free tunnel speed !

Tip vortex trajectories axial flow

Trajectories for 3 tip speed ratios 82% span.

Tip vortex trajectories, axial flow

l = 10

l =6.67

l = 4.17

l = 4.17

l = 6.67

l = 10

Vortex position against time: transportation speed constant!

Vortex trajectories for 30 degrees yaw

V 82% span.wake


tunnel axis



Rotor plane

Wake skew angle c

Vortex trajectories for 30 degrees yaw

Flow direction

Rotor plane position, seen from above

Example of tip vortex in yawed flow
Example of tip vortex in yawed flow 82% span.

Vortex roll up inward of tip position

Blade tip position

Flow direction

Comparison with Fluent calculations for tip speed ratio of 6.7, tip angle of -2.3° and zero yaw(Design conditions)

Some grid details tunnel environment included

tunnel 6.7, tip angle of -2.3° and zero yaw


Some grid details, tunnel environment included!


1/3 of the region covered, with symmetry boundary conditions

5.3 M cells, including tunnel environment

Velocity components compared for axial traverse
Velocity components compared for axial traverse 6.7, tip angle of -2.3° and zero yaw

Comparison of wake expansion and radial traverse
Comparison of wake expansion and radial traverse 6.7, tip angle of -2.3° and zero yaw

Calculated expansion much lower than measured !!??

Radial traverse at 30 cm behind rotor qualitatively good.

Computed surface streamlines
Computed surface streamlines compared

3D stall is observed in computations, most likely not present in tip area


  • A very large amount of very valuable data is available, to validate

  • Axi symmetric and yawed flow models, including turbulent wake state

  • Free vortex wake models

  • Dynamic stall models (in yaw)

  • General inflow modelling

  • CFD blade flow and near wake flow

  • Many years of work ahead, first ideas give hints towards improvements of BEM methods

  • An IEA Annex (has been / is being) set-up to coordinate this work (Gerard Schepers)

  • Acknowledgements

  • Financial support by EC 5th Framework program and by National Agencies