Aerodynamic Effects of Painted Surface
Roughness on Wind Turbine Blade
Performance
06/09/2015
Liselle A. Joseph
Aurelien Borgoltz
Matthew Kuester
William Devenport
Julien Fenouil
Special thanks to Wind Turbine Aerodynamics Team of GE Power and Water
College of Engineering
Importance of Roughness Effects
•Roughness is known to
 decrease lift (Abbott and Von Doenhoff, 1959; Jones, 1936)
 Increase drag (Abbott and Von Doenhoff, 1959; Jones, 1936)
 Move transition forward (Timmer, 2004)
•Roughness on wind turbine blades (icing, soiling, coat deterioration
etc.) reduces performance (Sagol, 2013; Ehrmann, 2014; Dalili et al.,
2009)
•These are the main types of roughness currently under study
•No work into the effect of orange-peel type roughness
 Likened to surface of an orange
 More wavy than peaky
 Produced from painting techniques and manufacturing processes
Joseph et al.
NAWEA Symposium 2015
2/14
Roughness Fetches
•Created by painting Contact© paper with latex paint using rollers of various
types
•Number of coats and painting direction were also varied
•3 configurations created and tested
(a)
(b)
(c)
12.5mm
12.5mm
Images of the Roughness Configurations (a) S1 (b) S2 and (c) S3. The scale of the roughness features is
illustrated using the 12.5-mm grid superimposed on the S1 roughness
Joseph et al.
NAWEA Symposium 2015
3/14
Roughness Fetches
•Approximate values of roughness parameters measured using Mahr PS1
Baseline
(Unpainted Contact©
Paper)
S1
S2
S3
𝑅𝑎 , μm
1.6
4.0
6.1
10.7
𝑅𝑡 , μm
13.5
28.6
38.7
62.9
𝑅𝑞 , μm
2.9
10.1
17.7
23.4
•In order of increasing roughness heights: baseline, S1, S2, S3
Joseph et al.
NAWEA Symposium 2015
4/14
Test Matrix
• Two DU96-W-180 models
tested, each at 2 chord
Reynolds Numbers
Chord (m)
Re (x106)
Configuration
Rek1
baseline
0.4
S1
4.4
S2
12.7
S3
21.3
baseline
0.7
S1
7.9
S2
22.5
S3
37.5
baseline
0.7
S3
24.0
baseline
1.1
S2
48.2
2
• Smooth and rough cases tested
for each model
0.8
3
• Roughness Reynolds Number
formulations:
Rek1
𝑅𝑞 𝑢k
=
𝜈
• Below Rek1,crit effects are small,
above Rek1,crit effects become
more noticeable
Joseph et al.
1.5
0.46
2
NAWEA Symposium 2015
5/14
Experimental Set Up
•Experiments done in VT stability
Wind Tunnel
•Lift and drag obtained from
pressure measurements from test
section wall and drag rake
0.8-mm silicone
rubber insulator
Port
mounted IR
camera
Starboard
mounted
IR camera
Aluminum model
with internally
mounted heaters
•Transition obtained from infrared
transition detection system
•Model wrapped in contact paper,
0.8-mm insulator, then roughness
fetch
Joseph et al.
Drag rake
Downstream View of 0.80-m DU96-W-180 Mounted in Wind
Tunnel with Infrared Thermography System
NAWEA Symposium 2015
6/14
Results
0.80-m DU96 Re=2.0M, baseline, Re =0.4
k1
1
•Positive stall: αc~ 9°to10°
0.46-m DU96, Re=2.0M, baseline, Rek1=1.1
0.8
0.6
•Negative stall: αc~-14°
0.4
•Zero-lift αc~ -2°
0.04
0.035
Clwc
0.2
0
0.025
Cdw c
•Baseline cases for two
models of different chord
lengths agree
0.03
-0.2
0.02
0.015
-0.4
0.01
0.005
-0.6
0
-15
-10
0
5
10
c
-0.8
-1
-20
-5
-15
-10
-5
0
c
5
10
15
20
Variation of Lift and Drag for Different Chord Length Models, in Baseline
Configuration, at Fixed Chord Reynolds Number of 2.0x106
Joseph et al.
NAWEA Symposium 2015
7/14
Effect of Roughness on Lift
•Max lift and lift curve
slope decrease with
increasing Rek1
1.15
1.1
1
1.05
1
0.8
Clwc
0.95
0.6
•effect most apparent at
positive αc, especially
above αc=5°
0.9
0.85
0.8
0.75
0.4
0.7
0.65
Clwc
0.2
6
8
10
12
c
c
c=0.80m, Re =2.0.M, baseline, Re =0.4
0
c
k1
c=0.80m, Rec =3.0M, baseline, Rek1=0.7
•Above Rek1 ~ 23 effect of
roughness becomes much
larger than below this
value
c=0.46m, Rec =1.5M, baseline, Rek1=0.7
-0.2
c=0.46m, Re =2.0M, baseline, Re =1.1
c
-0.4
Re =7.9
c=0.80m, Rec =2.0M, S2,
Re
=12.7
k1
c=0.80m, Rec =2.0M, S3,
Re
=21.3
k1
c=0.80m, Re =3.0M, S2,
Re =22.5
c=0.46m, Re =1.5M, S3,
Re =24.0
c=0.80m, Rec =3.0M, S3,
Re
=37.5
k1
c=0.46m, Rec =2.0M, S2,
Re
=48.2
k1
c
-0.6
c
c
-0.8
-1
•Rek1crit ~ 23
k1
c=0.80m, Re =3.0M, S1,
-15
-10
-5
0
5
k1
k1
k1
10
c
Lift Plots for Varying 𝑅𝑒𝑘1 for the DU96-W-180 (0.46-m and
0.80-m chords) at 𝑅𝑒𝑐 between 1.5x106 and 3.0x106
Joseph et al.
NAWEA Symposium 2015
8/14
Effect of Roughness on Drag
•Drag in bucket increases with
increasing Rek1
0.03
c=0.80m, Re =2.0.M, Re =0.4
c
k1
c=0.80m, Re =3.0M, Re =0.7
k1
c
c
k1
c=0.46m, Re =1.5M, Re =0.7
c
•effect most dominant at
positive αc
c=0.80m, Rec =3.0M, Rek1=7.9
c=0.80m, Re =2.0M, Re =12.7
k1
c
c
k1
c=0.80m, Re =2.0M, Re =21.3
k1
c
c
k1
c=0.80m, Re =3.0M, Re =22.5
k1
c
0.02
c
k1
c=0.46m, Rec =1.5M, Rek1=24.0
c=0.80m, Rec =3.0M, Rek1=37.5
c=0.46m, Re =2.0M, Re =48.2
k1
c
c
Cdwc
•Above Rek1 ~ 24 effect of
roughness becomes much
larger than below this value
k1
c=0.46m, Rec =2.0M, Rek1=1.1
0.025
k1
0.015
0.01
•Rek1crit is between 20-25
(accounting for 10%
uncertainty)
0.005
0
-15
-10
-5

c
0
5
10
c
Drag Plots for Varying 𝑅𝑒𝑘1 for the DU96-W-180
(0.46-m and 0.80-m chords) at 𝑅𝑒𝑐 between
1.5x106 and 3.0x106
Joseph et al.
NAWEA Symposium 2015
9/14
Effect of Roughness on Lift-toDrag Ratio
data
curve fit
120
•Below Rek1~23
L/Dmax slowly declines
100
•Large decrease in
L/Dmax after Rek1~23
•Rek1crit ~ 20-25
L/Dmax
80
60
40
20
0
0
10
20
30
40
50
Rek1
Variation of Maximum Lift-to-Drag Ratio with 𝑅𝑒𝑘1 for the DU96-W-180
(0.46-m and 0.80-m chords) at 𝑅𝑒𝑐 between 1.5x106 and 3.0x106
Joseph et al.
NAWEA Symposium 2015
10/14
Effect of Roughness on Transition
•Infrared transition detection system used to detect transition
•Gradient observed in images is onset of transition
•Image processing techniques used to extract %chord location
AOA=0
AOA=0
(a)
(b)
IR Trans region ~56%
(8.5" from TE)
IR Trans region ~61%
(7.5" from TE)
FLOW
Infrared Images of the Pressure Side of the 0.46-m DU96-W-180 at AoA=0° showing the Forward
Movement of the Transition Front from the (a) Baseline case with Ra=1.58 to (b) S3 Roughness
case with Ra=6.78
Joseph et al.
NAWEA Symposium 2015
11/14
Effect of Roughness on Transition
0.8-m DU96-W-180
Suction Side
Rec=2.0M, Rek1=0.4
Re =2.0M, Re =0.4
c
80
k1
100
Rec=3.0M, Rek1=0.7
Re =2.0M, Re =4.4
Rec=2.0M, Rek1=4.4
c
90
Rec=3.0M, Rek1=7.9
70
c
c
c
80
k1
c
40
k1
Rec=3.0M, Rek1=22.5
70
x/c, %
x/c, %
k1
Rec=3.0M, Rek1=37.5
50
k1
Re =2.0M, Re =21.3
Re =3.0M, Re =22.5
c
k1
Re =2.0M, Re =12.7
k1
Re =2.0M, Re =21.3
60
k1
Re =3.0M, Re =7.9
Re =2.0M, Re =12.7
c
Pressure Side
Rec=3.0M, Rek1=0.7
60
50
30
40
20
30
10
0
20
-10
-5
0
, dg
5
10
10
-8
-6
-4
-2
0
2
, dg
4
6
8
10
12
Variation of transition location with angle of attack on the (a) Suction and (b) Pressure
Side of the 0.8-m for all Rek1
Joseph et al.
NAWEA Symposium 2015
12/14
Effect of Roughness on Transition
0.46-m DU96-W-180
Suction Side
80
Rec=1.5M, Rek1=0.7
80
Re =2.0M, Re = 1.1
c
Re =1.5M, Re =24.0
70
c
k1
Rec=2.0M, Rek1=48.2
Pressure Side
Rec=1.5M, Rek1=0.7
k1
Re =1.5M, Re = 24.0
c
70
k1
Re =2.0M, Re = 48.2
c
60
k1
60
50
x/c, %
x/c, %
50
40
40
30
30
20
20
10
10
0
0
-10
-5
0
, dg
5
10
-10
-5
0
, dg
5
10
Variation of transition location with angle of attack on the (a) Suction and (b) Pressure
Side of the 0.46-m for all Rek1
Joseph et al.
NAWEA Symposium 2015
13/14
Conclusions
Orange-peel type painted surface roughness on wind turbine blades have
an effect on the performance
It was found that:
•Roughness effects show dependence on Rec and Rek
•The effect of the roughness is more pronounced at positive angles of attack
•Lift decreases gradually with increasing Rek, up to the critical Rek
•Drag increases gradually with increasing Rek, up to the critical Rek
•Transition moves forward slightly with increasing Rek, up to the critical Rek
•Critical Rek for orange-peel roughness is between 20 and 25.
Joseph et al.
NAWEA Symposium 2015
14/14
Q&A
Joseph et al.
NAWEA Symposium 2015
Supporting Slides
Joseph et al.
NAWEA Symposium 2015
Effect of Roughness on T-S Waves
•Roughness induced disturbances grow and overtake natural T-S waves
•Roughness-induced T-S waves cause linear transition front upstream of
natural transition
(a)
0.46-m chord, Re = 1.5x106






1.4
x 10
Normalized Integrated Growth
0.9
-3
S1
S2
S3
1.2
1
0.8
0.7
0.6
1
=
=
=
=
=
=
-5 deg., Pressure Side: x/c=10%
0 deg., Pressure Side: x/c=50%
5 deg., Pressure Side: x/c=70%
-5 deg., Suction Side: x/c=55%
0 deg., Suction Side: x/c=50%
5 deg., Suction Side: x/c=40%
Normalized Integrated Growth
1
0.5
0.4
0.3
0.2
0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
10
15
20
25
Wavelength, mm
30
35
0
40
0
5
0
5
0.6
(b)
1
0.4
1
0
2
4
6
8
10
Wavelength, mm
12
14
16
Averaged wavelength spectra of the painted
roughness surfaces
18
0.9
Normalized Integrated Growth
0.2
0
0.80-m chord, Re = 2.0x106
0.9
Normalized Integrated Growth
Normalized PSD
0.1
0.8
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
10
15
20
25
Wavelength, mm
30
35
40
0
Wavelengths of unstable Tollmien-Schlichting disturbances for (a) 0.46-m
DU96-W-180 at Re=1.5x106 and (b) 0.80-m DU96-W-180 at Re=1.5x106
Joseph et al.
NAWEA Symposium 2015
1.5
0.02
Analysis of Effect on Performance
0.018
1
0.016
0.5
0.014
Cl
Cd
•XFOIL used to investigate whether changes in lift and drag are from
0.012
changes0 in transition
Clean - Experiment
0.01
S3 - Experiment
Clean - XFOIL
S3 - XFOIL
•XFOIL-0.5‘tripped’ at where transition is observed
on IRT images for rough
0.008
cases -1
0.006
-10
-5
0
, deg.
5
10
-10
-5
0
, deg.
5
10
5
10
•Differences compared to that observed between clean and rough
results
(b)
0.01
3.5
0
3
-0.01
2.5
-0.02
2
C d
C l
(a)
-0.03
-0.04
-0.05
-0.06
x 10
-3
C - Experiment
d
Cd - XFOIL
1.5
1
0.5
C - Experiment
l
0
C - XFOIL
l
-0.07
-10
-5
0
, deg.
5
10
-0.5
-10
-5
0
, deg.
XFOIL analysis of the effect of transition location on lift and drag. XFOIL transition locations were set
from IR transition measurements for the 0.46-m DU96-W-180 Model at Re = 1.5x106. Differences in (a)
lift and (b) drag are between the clean model (covered in insulator) and the S3 roughness condition.
Joseph et al.
NAWEA Symposium 2015