www.ecn.nl

High Reynolds Number Effects on

20MW Wind Turbine Rotor Design

Özlem Ceyhan 11.10.2012

“The science of making torque from wind” conference, 9-11 October 2012, Oldenburg, Germany

Outline

• Consequences of upscaling in rotor aerodynamics

• Availability of high Reynolds numbers measurements for wind turbine airfoils

• High Reynolds number effects on airfoils

• High Reynolds number effects on rotor design

– Geometry

– Cp results

– AEP

– Modified designs (thicker airfoils)

• Conclusions

• Final Remarks

How large is a 20MW wind

turbine 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

• Dimensions of the blades are scaled linearly • Local velocities along the blade stay the same.

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

UcRe

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’s 1- 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 B747 C17 o A350

o B777 A340 o o B787

o B737 A320 o

Take off and Landing

10-20 MW Wind Turbines

Transport aircraft airfoils are for transonic, wind turbine airfoils for subsonic speeds. Wind turbine airfoils are thicker. During the take off and landing, flaps and/slots are extracted.

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

High Reynolds number effects on airfoils

Solution of BL equations for a flat plate suggests for laminar flows,

For turbulent flows,

Reynolds Number Effects: Background

2/1Re

91.4

x

x

5/1Re

37.0

x

x

δ : Boundary layer thickness (99% velocity thickness)

For airfoils;

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

-1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 1.6

Cd

Cl

Re=9 mil.

Re=20mil.

Test Re=9mil

Test Re=20mil

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 5 10 15 20

Cl

Angle of Attack

Test Re=20mil.

Test Re=9mil.

rfoil Re=9mil

rfoil Re=20mil

0.002

0.004

0.006

0.008

0.01

-0.8 -0.4 0 0.4 0.8

Cd

Cl

Re=9 mil.

Re=20mil.

Test Re=9mil

Test Re=20mil

High Reynolds number effects on Cl and

Cd performace of (thick) airfoils

NACA 633018

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Xtr

Cl

Suction Side Re=9 mil

Pressure Side Re=9 mil

Suction Side Re=20mil

Pressure side Re=20mil

-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

performance of thick airfoils (RFOIL predicted)

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

High Reynolds number effects on 20MW wind

turbine rotor design:

Airfoil Data for high Re numbers Rotor Design

BEM coupled with gradient based optimization method.

• Prandtl tip loss corrections • Turbulent wake state corrections • Corrections for 3D effects due to rotation • Airfoil data is read from an airfoil database • Golden search optimization algorithm • Cost function is maximizing the annual yield for the given wind conditions

High Reynolds number effects on rotor

design: Methodology

Existing airfoil Cl and Cd data is corrected for high Reynolds number effects by using RFOIL.

testRFOILlRFOILlRFOILl

RFOILltestll

CCC

CCC

Re,Re,

Re,Re,

,,,

,

A new airfoil database is generated by using max. Re number of 25 million.

rgrrcUPUpyield ,,,,max

High Reynolds number effects on rotor

design: Geometry

0

10

20

30

40

50

60

25

29

33

39

47

55

63

71

79

87

95

103

111

115

119

123

126

Blade Radius [m]

% C

ho

rd R

ed

ucti

on Design with UPWIND Airfoils (7 mil)

Design With 25mil Re Numbers

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

0 20 40 60 80 100 120 140

Ch

ord

[m]

Radius [m]

Classical Upscaled WT

Design With 25mil Re Numbers

-2.00

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

0 20 40 60 80 100 120 140

Twis

t an

gle

[°]

Radius [m]

Classical Upscaled WT

Design With 25mil Re Numbers

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

0 20 40 60 80 100 120 140

Ab

solu

te T

hic

kne

ss [m

]

Radius [m]

Classical Upscaled WT

Design With 25mil Re Numbers

Performance: On and Off Design

Conditions

0,40

0,41

0,42

0,43

0,44

0,45

0,46

0,47

0,48

0,49

0,50

5,00 5,50 6,00 6,50 7,00 7,50 8,00 8,50 9,00 9,50 10,00 10,50 11,00 11,50 12,00 12,50 13,00

CP

Lambda

Design with Re=25x106 airfoils

pitch=0.0

pitch=0.5

pitch=-0.5

pitch=1.0

pitch=-1.0

0,40

0,41

0,42

0,43

0,44

0,45

0,46

0,47

0,48

0,49

0,50

5,00 5,50 6,00 6,50 7,00 7,50 8,00 8,50 9,00 9,50 10,00 10,50 11,00 11,50 12,00

CP

Lambda

Classical Upscaled - Re=7x106 airfoils

pitch=0.0

pitch=0.5

pitch=-0.5

pitch=1.0

pitch=-1.0

Performance in terms of AEP: Clean

and Rough Conditions

Significant improvement in operating in rough surface conditions. Can we translate this into an improvement in the rotor design?

Cle

an

Ro

ugh

-5% C

lean

Ro

ugh

Cle

an

Ro

ugh

-1.2%

Significant improvement in operating in

rough surface conditions:

How can we translate this improvement

into a better rotor design?

Significant improvement in operating in

rough surface conditions:

How can we translate this improvement

into a better rotor design?

Use of thick airfoils at the tip

Thick airfoils at the tip: 3 design

examples

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

0 20 40 60 80 100 120 140

Ab

solu

te T

hic

kne

ss [

m]

Radius [m]

Classical Upscaled WT

Design With 25mil Re Numbers

Modified design with DU 21% A/f at the tip

Modified design with DU 25% A/f at the tip

Modified design with DU 30% A/f at the tip

-2.00

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

0 20 40 60 80 100 120 140

Twis

t an

gle

[°]

Radius [m]

Classical Upscaled WT

Design With 25mil Re Numbers

Modified design with DU 21% A/f at the tip

Modified design with DU 25% A/f at the tip

Modified design with DU 30% A/f at the tip

0.15

0.20

0.25

0.30

0.35

0.40

20 40 60 80 100 120 140

Re

lati

veTh

ickn

ess

[-]

Radius [m]

Original Airfoil Distribution

Modified design with DU 21% A/f at the tip

Modified design with DU 25% A/f at the tip

Modified design with DU 30% A/f at the tip

Thick airfoils at the tip: Results in terms

of AEP

Cle

an

Ro

ugh

5%

Cle

an

Ro

ugh

Cle

an

Cle

an

Cle

an

Ro

ugh

Ro

ugh

Ro

ugh

Thick airfoils at the tip: Improvement in

structural properties

Mod. Design with 21% af @ tip (from 80m to the tip)

Mod. Design with 25% af @ tip (from 63m to the tip)

Mod. Design with 30% af @ tip (from 50m to the tip)

With thick airfoils at the tip, it is possible to significantly improve the structural properties of the rotor blade with only minor reduction in the AEP. As a result, the total blade mass is also expected to be reduced.

• 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.

• RFOIL is used in order to predict the effects of Reynolds numbers together with some validations.

• As a result of the effect of high Reynolds numbers, rotor blades get more slender and the optimum (Cp) operating conditions are improved.

• Due to the improvement in performance of the thick airfoils both in clean and rough conditions it may be possible to use thick airfoils at the tip section of the blade which brings significant improvements in structural properties and the overall weight of the rotor.

Conclusions

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

• Those effects can already be included in the existing or the next generation (7-10 MW) wind turbines.

• More detailed design work is necessary to be performed in order to choose the right airfoils with right thickness. (stall, dynamic effects, stability etc.)

• These results are based mainly on the numerical predictions that show a lot of improvement possibilities. However, wind tunnel tests of thick airfoils for high Reynolds numbers is required in order to understand the effects and afterwards apply these in real life problems!

Final Remarks

• Herman Snel

• Arne van Garrel

Special Thanks to

Questions...

ceyhan@ecn.nl