The MEXICO project: The Database and Results of Data Processing and Interpretation

Herman Snel, Gerard Schepers (ECN), Arné Siccama (NRG)

2

Introduction

MEXICO project = Model EXperiments In 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.

3

Overview

o Participants

o Model and instrumentation

o Flow field measurements, PIV

o The measurement matrix and the data base

o PIV quantitative flow field analyses

o Comparison with CFD (Fluent)

o Conclusions

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

o 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)

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

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

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

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

o Speed and pitch variable

o Model data effective sampling frequency 5.5 kHz

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

6

DU 91 W2 250

2.25 m

-NACA 64-418

Risø A1-21

Blade layout

Kulite instrumented sections:

25% and 35 % DU 91 W2 250

60% Riso A1-21

82% and 92% NACA 64 418

7

Flow field measurements, stereo PIV

Photo: Gerard Schepers

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.

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°)

PIV Windows Run 10, Priority 3

0

500

1000

1500

2000

2500

3000

-5000 -3000 -1000 1000 3000 5000X [mm]

Y [

mm

]

Priority 3 Priority 3 extra Rotor Plane

PIV Windows Run 10, Priority 1

0

500

1000

1500

2000

2500

3000

-5000 -3000 -1000 1000 3000 5000

X [mm]

Y [m

m]

Priority 1.1 Priority 1.2 Rotor Plane

PIV Windows Run 10

0

500

1000

1500

2000

2500

3000

-5000 -3000 -1000 1000 3000 5000X [mm]

Y [

mm

]

Priority 1.1 Priority 1.2 Priority 2 Priority 4 Rotor Plane

10

Results of inflow and wake traverses

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-1.5 -1 -0.5 0 0.5 1 1.5

ua

x/V

tun

ne

l [

-]

x/D [-]

= 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

0

2

4

6

8

10

12

14

16

18

1 1.5 2 2.5 3

radial position [m]

Ax

ial v

elo

cit

y [

m/s

]

Psi = 120 deg

edge PIV sheet

Psi = 0 deg

Shows good repeatability and coherence between different PIV sheets

Blade tip position at 2.25 m

PIV

Win

do

ws R

un

10

0 500

1000

1500

2000

2500

3000

-5000-3000

-10001000

30005000

X [m

m]

Y [mm]

Priority 1.1

Priority 1.2

Priority 2

Priority 4

Rotor P

lane

14

Up-flow and down-flow effect of blades, yaw = 0

1

1.2

1.4

1.6

1.8

2

2.2

2.4

0 2 4 6 8 10 12 14

radia

l posi

tion [m

]

axial velocity [m/s]

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

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

vo

rte

x y

po

sit

ion

[m

]

x [m]

Tip vortex trajectories, axial flow

= 10 = 6.67

= 4.17

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

1 2 3 4 5 6 7 8

vo

rte

x x

po

sit

ion

[m

]

time [1 unit = 0.047 s]

= 4.17

= 6.67

= 10

Trajectories for 3 tip speed ratios

Vortex position against time: transportation speed constant!

PIV Windows Run 10, Priority 3

0

500

1000

1500

2000

2500

3000

-5000 -3000 -1000 1000 3000 5000X [mm]

Y [

mm

]

Priority 3 Priority 3 extra Rotor Plane

16

Vortex trajectories for 30 degrees yaw

-2

-1

0

1

2

3

-2 -1 0 1 2 3 4 5 6

y [

m]

x [m]

Rotor plane position, seen from above

Flow direction

Vw

Rotor plane

Wake skew angle

Vwaketunnel 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

tunnel model

blade

21

Velocity components compared for axial traverse

-10

-5

0

5

10

15

20

-6 -4 -2 0 2 4 6

x [m]

velo

city

[m

/s] vx exp

vy expvz expvx Fluentvy Fluentvz Fluent

22

Comparison of wake expansion and radial traverse

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

distance to rotor [m]

dist

ance

to a

xis

[m]

experimentFluent

-10

-5

0

5

10

15

20

0.0 0.5 1.0 1.5 2.0 2.5 3.0

radial coordinate [m]

velo

city

[m/s

]

vx exp

vy exp

vz expvx Fluent

vy Fluent

vz Fluent

Calculated expansion much lower than measured !!??

Radial traverse at 30 cm behind rotor qualitatively good.

23

Absolute pressure distributions at 5 radial stations compared

-2500

-2000

-1500

-1000

-500

0

500

1000

0 10 20 30 40 50 60 70 80 90 100

koorde (-)

druk

(P

a)

exp 25cfd fijn

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

0 10 20 30 40 50 60 70 80 90 100

koorde (-)

druk

(P

a)

exp 35

cfd fijn

-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

0 10 20 30 40 50 60 70 80 90 100

koorde (-)

druk

(P

a)

exp 60cfd fijn

-10000

-8000

-6000

-4000

-2000

0

2000

4000

6000

0 10 20 30 40 50 60 70 80 90 100

koorde (-)

dru

k (P

a)

exp 82

cfd fijn

-12000

-10000

-8000

-6000

-4000

-2000

0

2000

4000

6000

8000

0 10 20 30 40 50 60 70 80 90 100

koorde (-)

druk

(P

a)

exp 92cfd fijn

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

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

o Free vortex wake models

o Dynamic stall models (in yaw)

o General inflow modelling

o 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