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Challenges in High Reynolds Number

Flows for Large Offshore Wind Turbines

Özlem Ceyhan and Herman Snel

EFMC10, Copenhagen, september 2014

Presented by Gerard Schepers

Outline

• Consequences of upscaling in rotor aerodynamics

• Unavailability of high Reynolds numbers measurements for wind turbine

airfoils

• Possible effects of high Re number

• Computed high Reynolds number effects on airfoils

• Need for validation

• AVATAR measurements in DNW-HDG high pressure tunnel

• Conclusions

• Final Remarks

How large is a 20MW wind

turbine blade? Very!!!

A ‘small’ 73.5 meter blade

Consequences of Upscaling on

Aerodynamics

00,0E+00

4,0E+06

8,0E+06

12,0E+06

16,0E+06

20,0E+06

24,0E+06

0,0 0,2 0,4 0,6 0,8 1,0 1,2

Re

yno

lds

Nu

mb

er

r/R

20MW

5MW

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

0,0 0,2 0,4 0,6 0,8 1,0 1,2

Ch

ord

[m]

r/R

5MW (126m diameter;

UPWIND Reference)

20 MW (252m diameter)

UPWIND project:

Upscaling from 5MW reference turbine to 20MW wind turbine with Classical Similarity Rules:

•Tip speed is constant

• Rotational speed is therefore inversely proportional to rotor diameter growth

•Local velocities along the blade stay the same.

• Dimensions of the blades are scaled linearly

•The only change in the aerodynamics is the increase in the local Reynolds numbers!

ν

Uc=Re

U= local velocity

c = chord length

υ = kinematic viscosity

U=11.5m/s (rated wind speed)

Availability of Cl, Cd and Cm data of the wind

turbine airfoils for high Reynolds numbers

Up to Re=6x106, for NACA airfoils

up to Re=9x109 (*)Up to Re=3x106

Availability of the wind tunnel test data;

The effects of very high Reynolds numbers?

(*) Some tests are available for high Re numbers at low Mach numbers of thin NACA profiles coming from 1940’s1- Loftin, K.L.,Jr., Bursnall, W.J., “Effects of Variations in Reynolds Number Between 3.0x106 and 25x106 upon the Aerodynamic Characteristics of a number of NACA 6-

Series Airfoil Sections”, NACA-TN-1773, 1948

High Reynolds numbers on aircrafts

0

10

20

30

40

50

60

70

80

90

0 0,2 0,4 0,6 0,8 1 1,2

Rey

nold

s N

umbe

r [m

illio

ns]

Mach Number

o A380

o o B747C17o A350

o B777A340 oo B787

o B737A320o

Take off andLanding

10-20 MW

Wind Turbines

⇒ Transport aircraft airfoils are for transonic

speed (Ma about 0.8). wind turbine airfoils

are for low subsonic speed (Ma about 0.3).

⇒ Wind turbine airfoils are thicker.

⇒ During the take off and landing, flaps

and/slots are extracted.

⇒ High Reynolds data are absolutely needed

for large WT designs. MEASUREMENTS!!

Source : http://www.etw.de; reproduced.Source of the image: http://en.wikipedia.org/

High Reynolds number effects on airfoils

Reynolds Number Effects: Background

Two basic (and perhaps contradictory) effects, depending on airfoil

shape

1) Generally thinner boundary layer as a result of higher Re number and less decambering, but:

2) Earlier laminar to turbulent boundary layer transition, which tends to thicken the boundary layer. The transition position is very difficult to predict but depends on the pressure distribution and shear and hence on the airfoil shape!

Hence, prediction methods must be validated by measurements

FP7-ENERGY-2013-1/ n° 608396 9

Intermezzo: The EU FP7 project Avatar

Motivation:• We simply don’t know if present aerodynamic models are good enough to

design 10MW+ turbines

– “No mature industry will ever design a MEuro machine with

unvalidated tools” (M. Stettner, GE Global Research)

• 10MW+ rotors violate assumptions in current aerodynamic tools, e.g.:

– Reynolds number effects,

– Compressibility effects

– Flow transition and separation,

– (More) flexible blades

• Hence 10MW+ designs fall outside the validated range of current state

of the art tools.

18-9-2014

FP7-ENERGY-2013-1/ n° 608396 10

Avatar: Main objective

To bring the aerodynamic and fluid-structure models to a

next level and calibrate them for all relevant aspects of

large (10MW+) wind turbines

18-9-2014

-0.5

0

0.5

1

1.5

2

-10 -5 0 5 10 15 20

Cl

AoA

Re=7mil. Clean

Re=7mil. Rough

Re=20mil. Clean

Re=20mil. Rough

-0.5

0

0.5

1

1.5

2

-5 0 5 10 15 20

Cl

AoA

Re=7mil. Clean

Re=7mil. Rough

Re=20mil. Clean

Re=20mil. Rough

High Reynolds number effects on Cl and Cd of thick

airfoils (predicted by RFOIL, eN method for transition )

DU91-W2-250

DU97-W-300

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

Cl

Cd

Re=7mil. Clean

Re=7mil. Rough

Re=20mil. Clean

Re=20mil. Rough

-1

-0.5

0

0.5

1

1.5

2

0 0.01 0.02 0.03 0.04 0.05 0.06

Cl

Cd

Re=7mil. Clean

Re=7mil. Rough

Re=20mil. Clean

Re=20mil. Rough

NB: only usablemeasurements found in literature for high Re are for a 18% thick symmetrical airfoil, Re = 20M.

AVATAR: Comparison results ECN/other partners

Large differences in computed 2D Polars show the need for model improvement at

high Reynolds numbers

Courtesy: N Sorensen

Need for validation by measurement:

the AVATAR project

• A high Re effect assessment was done using RFOIL

• Different partners in the AVATAR project use 2D CFD with existing

transition models

– Differences are large

• Results must be validated to be used with confidence in designs.

• There is a possibility that the high Re effects enable thicker airfoils without

drag penalties, so that weight increase can be limited for the very large

blades and gravitatonal loads can be reduced.

• The AVATAR project includes measurements in the DNW-HDG high

pressure wind tunnel, from Re = 3M upto 18M, DU00-W-212 section. In

order to check Re influence, lower Re measurements (upto 6M) will be

performed in LM wind tunnel.

AVATAR measurements

Measurements are scheduled to start this very week,

with the following special features:

• Concurrent measurement of the wind tunnel

turbulent SPECTRUM with a hot film at entrance

• Estimation of transition location using embedded

Kulite pressure sensors as high frequency

microphones

• Pressure distribution measurements (90 sensors)

• Wake rake for drag determination, sidewise

traversabel

• Flourescent oil flow visualization

• Profile: DU00-W-212, c=15 cm

• Due to the growing sizes of the rotors, higher Reynolds numbers (up to 25

million) are introduced.

• There is a lack of measurement data at high Reynolds numbers of the thick

wind turbine airfoils at incompressible conditions.

• RFOIL is used in order to estimate the effects of Reynolds numbers.

• Within the AVATAR project, measurements will be performed in the DNW-

HDG wind tunnel in Göttingen, to validate or correct the desing calculations

• Results are forthcoming, measurements starting this week.

Conclusions

• Reynolds number is reduced with slender blades. It is increased again with higher

tip speed operations.

• Those effects should already be included in the existing or the next generation (5-

10 MW) wind turbines!

• More detailed design work is necessary to be performed in order to design the

best airfoils with right thickness. (stall, dynamic effects, stability etc.)

• There are only a few atmospheric wind tunnels in the world that are able to reach

such high Re numbers in incompressible flow - lower than Mach=0.3. In this

tunnels, the chord length of the model should be extremely large! Therefore the

number of facilities to be used for this purpose is quite limited.

Final Remarks

This project has received funding from the European Union’s Seventh Programme for research, technological development and

demonstration under grand agreement No FP7-ENERGY-2013-1/n° 608396 .

Thank you for your attention

Questions...

ceyhan@ecn.nl