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

2 Introduction MEXICO project = M odel EX periments I n Co ntrolled 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 m 2 ) with a 3 bladed model of 4.5 m diameter, leading to 100 GB of very useful data.

3 Overview oParticipants oModel and instrumentation oFlow field measurements, PIV oThe measurement matrix and the data base oPIV quantitative flow field analyses oComparison with CFD (Fluent) oConclusions

4 Participants and main tasks  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

5 The model and the instrumentation oThree 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) oProfiles: DU91-W2-250, RISOE A1-21 and NACA oTotal of 148 Kulite pressure sensors distributed over 5 sections, at 25, 35, 60, 85 and 95% radial position, in the three blades. oTwo strain gauge bridges at the root of each of the blades. oTotal forces and moments at the six component balance of DNW oSpeed and pitch variable oModel data effective sampling frequency 5.5 kHz oBalance and tunnel data averaged over run (5 seconds, 35 revolutions)

6 DU 91 W m - NACA Ris ø A1-21 Blade layout Kulite instrumented sections: 25% and 35 % DU 91 W %Riso A % and 92%NACA

7 Flow field measurements, stereo PIV Photo: Gerard Schepers PIV traverse tower with two cameras, aimed at horizontal PIV sheet of 35*42 cm 2 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*4.3mm 2 ). Velocity vector is the vector giving maximum correlation between these two shots. PIV planes at 270 degrees azimuth Zero rotor azimuth: blade 1 vertically upward 

8 The measurement matrix. A) pressures and loads 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

9 The measurement matrix B) PIV 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°)

10 Results of inflow and wake traverses = 4.17 = 6.7 = 10 Cylindrical vortex wake model 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

11 The moment of truth The first intelligible pressure distribution appears in the quick look system, during the measurements

12 Attached and stalled flow, PIV images just behind rotor, at 82% span. Attached flow = 6.7: Thin viscous wake, left by passing blade Flow direction Stalled flow = 4.17 : Much thicker blade wake and ‘trailing vortex’ at location of large jump in bound vorticity, explains chaotic behaviour in velocity decay To blade tip

13 Axial velocity in radial traverse in rotor plane, for 0 and 120 azimuth Shows good repeatability and coherence between different PIV sheets Blade tip position at 2.25 m

14 Up-flow and down-flow effect of blades, yaw = 0 az = 40° az = 20 ° az = 40° az = 20 ° Measured Inflow for blade just below and just above PIV sheet. PIV sheet always at 270 ° azimuth position Blade tip position Difference of approximately 5 m/s, 1/3 of free tunnel speed !

15 Tip vortex trajectories, axial flow = 10 = 6.67 = 4.17 = 6.67 = 10 Trajectories for 3 tip speed ratios Vortex position against time: transportation speed constant!

16 Vortex trajectories for 30 degrees yaw Rotor plane position, seen from above Flow direction  VwVw  Rotor plane Wake skew angle  V wake tunnel axis

17 PIV images of tip vortex trajectories for 30 degrees yaw

18 Example of tip vortex in yawed flow Blade tip position Flow direction Vortex roll up inward of tip position

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

20 Some grid details, tunnel environment included! 1/3 of the region covered, with symmetry boundary conditions 5.3 M cells, including tunnel environment tunnelmodel blade

21 Velocity components compared for axial traverse

22 Comparison of wake expansion and radial traverse Calculated expansion much lower than measured !!?? Radial traverse at 30 cm behind rotor qualitatively good.

23 Absolute pressure distributions at 5 radial stations compared

24 Computed surface streamlines 3D stall is observed in computations, most likely not present in tip area

25 Conclusions A very large amount of very valuable data is available, to validate oAxi symmetric and yawed flow models, including turbulent wake state oFree vortex wake models oDynamic stall models (in yaw) oGeneral inflow modelling oCFD 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 5 th Framework program and by National Agencies