1DUWIND, Delft University Wind Energy Institute

An overview of NACA 6-digit airfoil series characteristics with

reference to airfoils for large wind turbine

Nando Timmer

DUWINDDelft University Wind Energy Institute

The Netherlands

2DUWIND, Delft University Wind Energy Institute

Outline

• Introduction• Measurements in LTPT• Comparison with RFOIL calculations• (Maximum) lift• Drag

• Roughness effects• Conclusions

3DUWIND, Delft University Wind Energy Institute

Introduction

• Large machines have blades performing at Reynolds numbers up to 9 to 10 million

• Many dedicated wt airfoils have not been tested at these Re-numbers

• Testing at these Re-numbers is relatively expensive

• If blade designers do not want to spend this amount of money they have to rely on the predictive value of codes like XFOIL and/or CFD

4DUWIND, Delft University Wind Energy Institute

Introduction (cntd)

• NACA airfoils were tested in the Langley LTPT up to Re=9x106 and can be used to verify the predictions.

• Main question in this presentation is:

• How good are these data anyway and how well can we predict them with RFOIL.(as a typical example we investigate the 18% thick airfoil from the NACA 63 and 64 series)

5DUWIND, Delft University Wind Energy Institute

LTPT measurements

• Test section 3x7.5 feet (0.914 m x 2.29 m)• Model chord 2 feet (0.61 m)• Maximum velocity at atmospheric pressure is

130 m/s• Maximum Mach number during the tests was

0.17• Models were made of laminated mahogany• Lift from the pressure reaction on the walls

(over a length of 13 feet – 3.96 m), drag from a wake rake.

• Basic wind tunnel wall corrections were applied

6DUWIND, Delft University Wind Energy Institute

RFOIL

• Basically XFOIL• Improvement of the numerical stability by using

the Schlichting velocity profiles for the turbulent boundary layer instead of Swafford’s

• the shear lag coefficient in Green’s lag entrainment equation of the turbulent boundary layer model was adjusted

• Deviation from the equilibrium flow was coupled to the shape factor of the boundary layer

7DUWIND, Delft University Wind Energy Institute

Lift

8DUWIND, Delft University Wind Energy Institute

Re = 3.0x106

-1.0

-0.5

0.0

0.5

1.0

1.5

0.000 0.008 0.016 0.024cd

cl

-1.0

-0.5

0.0

0.5

1.0

1.5

-10 0 10 20a (o)

cl

NACA 63-618

NACA 63-418

NACA 63-218

NACA 63-018

9DUWIND, Delft University Wind Energy Institute

NACA 643-x18

Re = 6.0x106

-1.0

-0.5

0.0

0.5

1.0

1.5

0.000 0.008 0.016 0.024cd

cl

-1.0

-0.5

0.0

0.5

1.0

1.5

-10 0 10 20a (o)

cl

64-018

64-218

64-418

64-618

10DUWIND, Delft University Wind Energy Institute

NACA 643-x18

Re = 9.0x106

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

0.000 0.008 0.016 0.024cd

cl

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

-10 0 10 20a (o)

cl

64-018

64-218

64-418

64-618

11DUWIND, Delft University Wind Energy Institute

NACA 643-x18

Re=6x106

-5

-4

-3

-2

-1

0

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Design lift coefficient, Cl,i

Ze

ro-l

ift a

ng

le (d

eg

ree

s)

LTPT

12DUWIND, Delft University Wind Energy Institute

NACA 643-x18

Re=6x106

-5

-4

-3

-2

-1

0

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Design lift coefficient, Cl,i

Ze

ro-l

ift a

ng

le (d

eg

ree

s)

Inviscid

RFOIL

LTPT

13DUWIND, Delft University Wind Energy Institute

NACA 643-x18

Re=6x106

-5

-4

-3

-2

-1

0

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Design lift coefficient, Cl,i

Ze

ro-l

ift a

ng

le (d

eg

ree

s)

Inviscid

RFOIL

LTPT

Ref. 5

14DUWIND, Delft University Wind Energy Institute

NACA 643-x18

Re=6x106

-5

-4

-3

-2

-1

0

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Design lift coefficient, Cl,i

Ze

ro-l

ift a

ng

le (d

eg

ree

s)

Inviscid

RFOIL

LTPT

Ref. 5

0.4

15DUWIND, Delft University Wind Energy Institute

NACA 64-618

Re = 6.0x106

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

0.000 0.005 0.010 0.015 0.020

cd

cl

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

-20 -15 -10 -5 0 5 10 15 20

a (o)

cl

RFOIL

LTPT-0.4 degr.

16DUWIND, Delft University Wind Energy Institute

NACA 64-618

Re = 6.0x106

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

0.000 0.005 0.010 0.015 0.020

cd

cl

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

-20 -15 -10 -5 0 5 10 15 20

a (o)

cl

LM

RFOIL

LTPT-0.4 degr.

17DUWIND, Delft University Wind Energy Institute

NACA 643-618

Re = 9.0x106

-0.5

0.0

0.5

1.0

1.5

2.0

0.000 0.008 0.016 0.024cd

cl

-0.5

0.0

0.5

1.0

1.5

2.0

-10 0 10 20 30a (o)

cl

NACA LTPT

XFOIL v6.97

RFOIL v2003.1

18DUWIND, Delft University Wind Energy Institute

NACA 643-018

Re = 6.0x106

0.0

0.5

1.0

1.5

0.000 0.008 0.016 0.024cd

cl

0.0

0.5

1.0

1.5

0 8 16 24a (o)

cl

RFOIL

positive angles

negative angles

19DUWIND, Delft University Wind Energy Institute

1.3

1.4

1.5

1.6

1.7

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Design lift coefficient

Max

imu

m li

ft c

oef

fici

ent

NACA Re=6x10E6

RFOIL Re=6x10E6

negative angles

positive angles

NACA 64-x18

3.5%

20DUWIND, Delft University Wind Energy Institute

1.3

1.4

1.5

1.6

1.7

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Design lift coefficient

Max

imu

m li

ft c

oef

fici

ent

NACA Re=9x10E6

RFOIL Re=9x10E6

NACA 64-x18

6.5%

21DUWIND, Delft University Wind Energy Institute

-1.0

-0.5

0.0

0.5

1.0

1.5

-10.0 0.0 10.0 20.0

a (degr.)

cl

Stuttgart

LTPT

RFOIL

NACA 63-418

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

-20.0 -10.0 0.0 10.0 20.0a (degr.)

cl

Stuttgart

LTPT

RFOIL

NACA 63-018

-1.0

-0.5

0.0

0.5

1.0

1.5

-10.0 0.0 10.0 20.0

a (degr.)

cl

LTPT

Stuttgart

RFOIL

NACA 63-421

22DUWIND, Delft University Wind Energy Institute

DU 96-W-180

Re = 3.0x106

-1.0

-0.5

0.0

0.5

1.0

1.5

0.00 0.01 0.02 0.03cd

cL

-1.0

-0.5

0.0

0.5

1.0

1.5

-10 0 10 20a (o)

cL

Delft

Stuttgart

RFOIL

23DUWIND, Delft University Wind Energy Institute

1.0

1.2

1.4

1.6

1.8

0 2 4 6 8 10 12Rex10-6

Cl,max

KKK, M=0.1

KKK, M=0.2Du 97-W-300Mod

Base airfoil, Delft

RFOIL, n=9

RFOIL, n=5

24DUWIND, Delft University Wind Energy Institute

Chord (m)

Span (m)

C/S

LM .90 1.35 1.5

LTPT .61 .914 1.5

Test section top view

wall

Separation line

chord

A B C

span

-0.5

0.0

0.5

1.0

1.5

2.0

-10 -5 0 5 10 15 20 25

Wall pressures

Model pressures C

Model pressures B

Model pressures A

LM wind tunnel test setup

25DUWIND, Delft University Wind Energy Institute

T.E L.E

Stall cells

Pressure orifices

26DUWIND, Delft University Wind Energy Institute

NLF(1)-0416

Re = 6.0x106

-0.5

0.0

0.5

1.0

1.5

2.0

0.00 0.01 0.02 0.03cd

cL

-0.5

0.0

0.5

1.0

1.5

2.0

-10 0 10 20a (o)

cL

RFOIL

LTPT

Lift from model pressures

27DUWIND, Delft University Wind Energy Institute

Drag

28DUWIND, Delft University Wind Energy Institute

NACA 643-618

Re = 9.0x106

-0.5

0.0

0.5

1.0

1.5

0.000 0.008 0.016 0.024cd

cl

-0.5

0.0

0.5

1.0

1.5

0.000 0.008 0.016 0.024

cl

RFOIL

NACA LTPT

Re = 3x106

RFOIL x 1.09

Cd

RFOIL x 1.09

29DUWIND, Delft University Wind Energy Institute

DU 96-W-180

Re = 3.0x106

-1.0

-0.5

0.0

0.5

1.0

1.5

0.00 0.01 0.02 0.03cd

cL

-1.0

-0.5

0.0

0.5

1.0

1.5

-10 0 10 20a (o)

cL

Delft

Stuttgart

RFOIL

RFOIL*1.09

30DUWIND, Delft University Wind Energy Institute

Roughness

31DUWIND, Delft University Wind Energy Institute

NACA 632-615

Re = 6.0x106

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

0.000 0.008 0.016 0.024cd

cl

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

-10 0 10 20a (o)

cl

Roughness off

Roughness on

32DUWIND, Delft University Wind Energy Institute

NACA 63-x15

Re=6x106

-5

-4

-3

-2

-1

0

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Design lift coefficient, Cl,i

Zer

o-lif

t an

gle

(deg

rees

)

Roughness

Clean

RFOIL

33DUWIND, Delft University Wind Energy Institute

Roughness?

34DUWIND, Delft University Wind Energy Institute

NACA 643-x18

Re = 6.0x106

-1.0

-0.5

0.0

0.5

1.0

1.5

0.00 0.01 0.02 0.03cd

cL

-1.0

-0.5

0.0

0.5

1.0

1.5

-20 -10 0 10 20 30a (o)

cL

64-01864-21864-41864-618

35DUWIND, Delft University Wind Energy Institute

0.6

0.8

1

1.2

1.4

1.6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Design lift coefficient

Cl-

max

Clean

Wrap-around roughness

Naca 64-x18

Re=6x106

Reduction 18% - 20%

36DUWIND, Delft University Wind Energy Institute

Roughness configurations

• NACA wrap-around roughness (no. 60 grid distributed sparsely from 8% at the lower surface to 8% on the upper surface (worst case?)

• NASA roughness (no. 80 grid strips , 2.5 cm wide at both the upper and lower surface 8% chord stations

• Zigzag tape, various thicknesses and positions• Fixed transition on the leading edge in

calculations

37DUWIND, Delft University Wind Energy Institute

LS(1)-0417

Re = 6.0x106

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0.000 0.015 0.030 0.045

cd

cl

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

-10 0 10 20 30

a (o)

cl

NACA roughness

NASA roughness

Free transition

38DUWIND, Delft University Wind Energy Institute

NACA 64-418

Re = 6.0x106

Grid roughness

-1.0

-0.5

0.0

0.5

1.0

1.5

0.00 0.01 0.02 0.03cd

cL

-1.0

-0.5

0.0

0.5

1.0

1.5

-20 -10 0 10 20 30a (o)

cL

NACA LTPT

RFOIL

RFOIL: b.l. tripped at x/c=5% u.s. and 5% l.s.

39DUWIND, Delft University Wind Energy Institute

0.4

0.6

0.8

1

1.2

1.4

1.6

0 2 4 6 8 10 12Rx10-6

Cl,max

Clean

zigzag tape 0.4 mm

Carborundum 60

Du 97-W-300Mod

40DUWIND, Delft University Wind Energy Institute

Conclusions

• The measured zero-lift angle of several NACA airfoils needs to be adjusted with absolute values ranging from 0.4 to 1 degree

• The maximum lift coefficients predicted with RFOIL match the LTPT data well at Re=3x106, but under predict the Cl,max at 6x106 by 3.5% up to 6.5% at Re=9x106

• Though it may be possible that the higher Cl,max in the LTPT data partly originates from the wall pressure method, RFOIL also under predicts the maximum lift measured with surface pressures.

41DUWIND, Delft University Wind Energy Institute

Conclusions (cntd)

• RFOIL consistently under predicts the drag coefficient with about 9% for a wide range of airfoils and Reynolds numbers

• NACA standard roughness causes a reduction in the lift coefficient of 18% to 20% for 18% thick airfoils from the NACA 64-series

• The effect on airfoil performance of various types of roughness has been measured in the past, but it is unclear what type of roughness may be expected, though wrap-round roughness may serve as a worst-case scenario

42DUWIND, Delft University Wind Energy Institute

How to proceed?

• Roughness investigations in the wind tunnel at the appropriate Reynolds numbers and field tests with zigzag tape on the blades are necessary to be able to better quantify the effect of blade soiling on the rotor performance

• Side-by side tests are necessary to better understand the amount of soiling during operation.