Effect of Surface Contamination on the Performance of a Section of a Wind Turbine Blade

M. R. Soltani1 and A. H. Birjandi2

Department of aerospace Engineering, Sharif University of Technology Azadi St. Tehran, IRAN

A series of low speed wind tunnel tests on a section of a 660 Kw wind turbine blade which is under constructer were conducted to examine the effects of distributed surface contamination on its performance characteristics. At first model was tested in the clean condition and after that it was tested with tree different types of roughness. The data shows that this particular airfoil is very sensitive to the applied surface roughness. For the contaminated model the roughness, 0.5mm height was distributed over the entire upper surface of the airfoil such that the distribution pattern was denser in the vicinity of the leading edge and thinner in the trailing edge area. Statistical data show that the leading edge contamination is about 4.5 times denser than the trailing edge one. By spreading the roughness on the airfoil upper surface the maximum lift coefficient decreased drastically, up to %42, however, the stall angle of attack increased slightly. In contrast to the clean model, the maximum lift coefficient of the roughed airfoil increased as the Reynolds number was increased. The effect of other types of roughness, zigzag roughness and strip tape were less than that of the contaminated one. It seems that these models just change the laminar flow to turbulent one hence they are not a good model for studding the effect of surface contamination.

I. Introduction bout 20 years ago the first observations were made that wind turbines apparently could have more than one power level in the same wind. The first publication on the phenomenon was made by Madsen [1]. At several

turbine parks in California one noticed different power levels, of which the lowest one was about half the design-level. Several initiatives were taken to understand and solve the problem, for example the study of Dyrmose and Hansen [2], the Joule project on Multiple Stall [3] and the analyses published by Risoe [4]. However since the cause remained uncertain. Blade contamination is one of the theories that researchers answer to this phenomenon by it.

A

Dirt and contaminations accumulate on the wind turbine blade when it operates in the field. The main sources of contamination are insect compacts, ageing, sand impacts and the contaminations which come down with the rain. This contamination has a great role on the rotor performance. When insects, smog and dirt accumulate along the leading edge of the blade, power output can drop up to 40% of its clean value [5]. Modeling the contamination distribution is very difficult and the usual methods to study its effect experimentally are using the leading edge roughness on the model surface. In this ways the transition point moves toward the leading edge and causes early trailing edge turbulent separation. There are different types of leading edge roughness such as, strip tape, zigzag tape, etc. [6].

In this investigation a section of the HAWT rotor blade under construction was selected and various experiments were conducted to examine the effect of different parameters on its performance characteristics. To author’s knowledge, no experimental or theoretical information about the performance of this blade or its airfoil is available. Thus a series of tests were conducted to study the blade behavior under various conditions.

II. Experimental Facilities A low speed wind tunnel was used to conduct tests on this airfoil. Schematic view of the tunnel is shown in Fig.

1. This closed circuit tunnel operates in a velocity range from 0 to 100m/s. Nominal test section dimensions were

1 Professor, msoltani@sharif.edu, non member 2 Graduate student, amir.birjandi@gmail.com, student member

American Institute of Aeronautics and Astronautics

1

45th AIAA Aerospace Sciences Meeting and Exhibit8 - 11 January 2007, Reno, Nevada

AIAA 2007-1081

Copyright © 2007 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

80cm high, 80cm wide and 200cm long. The turbulence intensity in the test section was always less than 0.1% at all speeds [7].

The 25cm chord composiattached to the wind tunnel winner diameter on the wing sEach orifice was connected tosection static pressure. The pthe pressure on the downstrea

A. Clean configuration The clean model was test

the lift coefficient variation about 1.26 and occurred at thto 1.18 in the angle of attasignificant effect on the maReynolds number increased.

starts to decreases at lower athe trailing edge separation.

Figure 1. Schematic view of wind tunnel.

te model was mounted horizontally in the middle of the test section with its two ends alls to eliminate span wise flows. There were 67 static pressure orifices with 0.8mm urface and perpendicular to the surface to measure the static pressure on the model. a differential transducer and the other side of all transducers was connected to the test ressure orifices are located in an oblique line so that the upstream holes do not affect m ones. The airfoil section and the orifices position on the model are shown in Fig. 2.

Figure 2. Holes position on airfoil section and wing.

III. Results

ed at four different Reynolds numbers. Figure 3 shows effect of Reynolds number on diagram for the clean airfoil. The maximum lift coefficient for the clean airfoil was e angle of attack of 11.2 degrees at the Reynolds number of and decreased ck of 10.7 for the Reynolds number of . Reynolds number did not show ximum lift coefficient and the stall angle of attack but they both decreased as the The linear part of the curves has the same slope, but at higher Reynolds numbers

ngles of attack. The beginning of the nonlinearity in the lift coefficient diagram shows

61043.0 ×61015.1 ×

αlC

American Institute of Aeronautics and Astronautics

2

At low angle of attacks there was a laminar separation bubble in the upper and lower surfaces. As the angle of

attack increased the upper surface laminar separation bubble moved toward the leading edge and the lower surface one moved toward the trailing edge. By increasing angle of attack these bubbles vanished. After vanishing of the laminar separation bubbles the flow started to separate from the trailing edge. At Reynolds number of and zero angle of attack, the laminar separation bubble on the upper surface was located between 150 mm and 165 mm from leading edge while for the lower surface it was located between 120 mm and 150 mm from leading edge, Fig. 4. At 5 degrees angle of attack, the upper surface separation bubble moved toward leading edge and it placed between 125 mm and 135 mm from leading edge, but the lower surface laminar separation bubble moved aft and it was located between 150 mm and 175 mm from leading edge. At higher angles of attack the separation bubble vanished and the flow started to separate from trailing edge. Figure 5 shows the pressure distribution on the airfoil at the different angles of attack,

61085.0 ×

=α 10.5 and 14 degrees. For the 10.5 degrees angle of attack there was no laminar separation bubble on the airfoil surfaces and there was no separation in trailing edge. As the angle of attack was increased to 14 degrees the flow started to separate from about mid chord and moves toward the leading edge.

α

Cl

-5 0 5 10 15 20 25

0

0.2

0.4

0.6

0.8

1

1.2

Re=1150000Re=850000Re=650000Re=430000

Figure 3. Effect of Reynolds number on the lift variation.

X (mm)

Cp

0 20 40 60 80 100 120 140 160 180 200 220 240 260

-1

-0.5

0

0.5

0.05.0

Figure 4. Effect of angle of attack on the pressure distribution.

American Institute of Aeronautics and Astronautics

3

X (mm)

Cp

0 20 40 60 80 100 120 140 160 180 200 220 240 260

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

10.514.0

Figure 5. Pressure distribution on airfoil at 10.5 and 14

degrees angle of attack.

B. 60 and 90 degrees zigzag roughness As mentioned before, various roughness shapes and types were used to investigate the airfoil performance. One

of the shapes that is used commonly on the airfoil is the zigzag type. Two types of zigzag roughness were used. They both had the same geometric characteristics except in their zigzag angle where one of them was 60 and the other one was 90 degrees. The zigzag tapes width were 3mm and their entire width were 12mm as shown in Fig. 6. These tapes were located on the suction side of the airfoil between X/C=5% and X/C=7% from the leading edge. The height of these zigzag tapes was 0.07 mm. After these tapes were installed on the airfoil, their surfaces were covered by 0.45mm average diameter grit to simulate the leading edge roughness.

The lift coefficient variation with the Reynolds number for the 60 degrees and 90 degrees roughness is shown in

Fig. 7 and Fig. 8. The zigzag roughness in the leading edge of the airfoil forced the flow to transit from laminar to turbulent and added thickness to the boundary layer. The resulting thicker boundary layer leaded to an increase in the drag force, a reduction of the effective camber and earlier stall phenomenon. Turbulent boundary layer is thicker than the laminar boundary layer and the leading edge roughness leads to turbulent boundary layer over most part of the upper surface of the airfoil so the effective chamber decreased as can be seen in Fig. 7 and Fig. 8 where shifted the lift coefficient to the right. The zigzag roughness reduced the linear portion of the lift coefficient curve and in comparison to the clean model it can be found that the zigzag model became nonlinear in the angle of attack of higher than 5.

Adding the roughness to the leading edge of the airfoil changes the characteristic of the model. For the clean condition, flow separated sharply and as the Reynolds number increased stall occurred at a smaller angle of attack and the maximum lift coefficient decreased, but for the roughed airfoil the stall phenomenon occurred at higher angle of attack as the Reynolds number increases and stall phenomenon was very smooth, Fig. 9. Leading edge roughness decreases the energy of flow over the airfoil and changes the laminar flow to the turbulent one thus elimination the laminar separation bubble. Pressure distribution over the clean airfoil and the one with the leading edge roughness are the same after the laminar separation bubble, distance more than 140mm from leading edge.

Figure 6. 60 degrees zigzag tape.

American Institute of Aeronautics and Astronautics

4

α

Cl

-5 0 5 10 15 20 25-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Re=1150000Re=850000Re=650000Re=430000Clean Re=1150000

Figure 7. Lift coefficient variation for the 60 degrees zigzag

roughness.

α

Cl

-5 0 5 10 15 20 25-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Re=1150000Re=850000Re=650000Re=430000Clean Re=1150000

Figure 8. Lift coefficient variation for the 90 degrees zigzag

roughness.

American Institute of Aeronautics and Astronautics

5

The effect of 90 degrees and 60 degrees roughness on the lift coefficient was very similar but they were not

exactly the same. The 90 degrees zigzag roughness had less negative effect on the lift coefficient than the 60 degrees one. On the other hand the stall angle of attack for the 90 degrees roughness was smoother than that of the 60 degrees one, Fig. 10. The negative effect of the leading edge roughness decreases as the Reynolds number increases. At the Reynolds number of and the 90 degrees roughness in the leading edge of the model, the lift coefficient for the angle of attack ranges of 15 to 18 degrees was higher than the clean one, Fig. 11. In this range of angle of attack the lift coefficient of the clean airfoil decreased rapidly but the model with the 90 degrees roughness had a very smooth stall characteristic.

61015.1 ×

x

Cp

0 50 100 150 200 250

-1.5

-1

-0.5

0

0.5

Strip RoughnessClean

Re = 850000α = 2

Figure 9. Pressure distribution on the clean airfoil and the 90

degrees zigzag roughness.

Fig

de

Cl

-5 0 5 10 15 20 25

0

0.2

0.4

0.6

0.8

1

1.2

Clean60 degree zigzag90 degree zigzag

α

Re = 850000

ure 10. Lift coefficient variation for 60 degrees and 90

6

grees roughness in the Reynolds number of . 1085.0 ×

American Institute of Aeronautics and Astronautics

6

Fid

C. Strip tape roughness The strip tape roughness

12mm and 0.1mm height andlift coefficient with angle of compared with the clean airflift coefficient and the correthe airfoil performance decre

In Fig. 13, the lift coefficother. It is seen by inspectiothe zigzag one. Furthermoreone as seen from Fig. 13. Inroughness is much softer thastrip tape is more than the zroughness.

Cl

-5 0 5 10 15 20 25

0

0.2

0.4

0.6

0.8

1

1.2

Clean60 degree zigzag90 degree zigzag

Re = 1150000

α gure 11. Lift coefficient variation for 60 degree and 90 egree roughness in the Reynolds number of 1 . 61015. ×

was located at the same position as the 90 and 60 degrees zigzag ones. It had a width of was covered with the 0.45mm average diameter grit. Figure 12 shows variations of the

attack for four different Reynolds numbers with the strip tape roughness. The results are oil for the Reynolds number of . As the Reynolds number increases maximum

sponding stall angle increases, so at higher Reynolds number the effect of roughness on ases.

61015.1 ×

ient data for the strip tape and the 90 degrees zigzag roughness are compared with each n that the performance degradation caused by the strip tape is much more than that of , the stall characteristic of the zigzag roughness was smoother than the strip roughness addition this figure clearly shows that the stall character of the model with the zigzag n the other two cases; clean and strip tape roughness. Amount of pressure rise after the igzag one so it leads to an earlier and sharper stall in comparison with the 90 degrees

α

Cl

-5 0 5 10 15 20 25-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Re=1150000Re=850000Re=650000Re=430000Clean Re=1150000

Figure 12. Lift coefficient variation for the strip tape

roughness condition.

American Institute of Aeronautics and Astronautics

7

Cl

-5 0 5 10 15 20 25

0

0.2

0.4

0.6

0.8

1

1.2

CleanStrip roughness90 degree roughness

Re = 850000

α Figure 13. Comparison between 90 degree and strip tape

roughness condition.

D. Contamination roughness model Here the contamination phenomenon is attributed to the weather-dependent flying behavior of insects. Figure 14

explains the mechanism in a diagram. It is assumed that the contamination of the wind turbine blades increases only when the insects are flying during the turbine operation. Insects mostly fly when there is no rain, little wind and when it is not too cold, at temperatures above 10°C. If the turbine operates under these conditions, insects will increasingly contaminate the blade near the stagnation line [8].

Contamination distribution on the wind turbine blade surface is not a uniform distribution. At the leading edge the contamination is more than the trailing edge due to the airfoil shape and the direction of rotation. Leading edge contamination is about 4.5 times more than the trailing edge one. On the airfoil model with 250mm chord, at the leading edge there are 18 roughness particles in each centimeter square while at the trailing edge there are only 4 roughness particles in each centimeter square. In this experiment the roughness particles were distributed randomly with no pattern. The roughness distribution pattern which was used on the airfoil surface is shown in Fig. 15.

Figure 14. The Insect Hypothesis.

American Institute of Aeronautics and Astronautics

8

To apply the roughness pattern on the model, a flat plate was used and the roughness pattern was spread over it.

Small holes were drilled for each roughness. After drilling the holes on the plate surface, a two side banderole was put on one side of the plate and the grit was fully distributed on the other side such that at least one grit particle was in each hole. The banderole was then took off from the plate surface and put on the airfoil surface. The average diameter of the roughness was about 0.45mm.

By adding the surface contamination model of the roughness on the airfoil the maximum lift coefficient decreased about 37% for the Reynolds number of and 43% for the Reynolds number of while the stall is very smooth, Fig. 16 and Fig 17. Like the other roughness patterns the maximum lift coefficient increased as the Reynolds number increased, hence it seems that the contamination effect decreases by increasing the Reynolds number, Fig. 17.

61015.1 × 61043.0 ×

For the contaminated airfoil the flow separates at very low angle of attacks because the surface roughness consumes the free stream energy and increases its pressure over the suction part of model (Fig. 16). As see from Fig. 17, dose not change significantly with the Reynolds number and lC stallα is almost independent of the Reynolds too. Contamination model of the roughness on the airfoil has the greatest effect on the aerodynamic coefficients. It increases the stall angle up to 13 degrees. In the contaminated airfoil stall procedure is very smooth and after a slight reduction in the lift coefficient, the lift increases again. This is because of pressure reduction in the wake area. Figure 18 shows the pressure distribution for the contaminated airfoil in the post stall region. It can be seen that as the angle of attack increases the minimum pressure coefficient increases and the separated region expands but in the separated region the pressure decreases when compared to the lower angle of attack one.

Figure 15. Contamination distribution model on airfoil.

x

Cp

0 50 100 150 200 250

-1.5

-1

-0.5

0

0.5

CountaminatedClean

Re = 850000α = 2

Figure 16. Pressure distribution over the contaminated airfoil.

American Institute of Aeronautics and Astronautics

9

α

Cl

-5 0 5 10 15 20 25-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Re=1150000Re=850000Re=650000Re=430000Clean Re=1150000

Figure 17. Lift coefficient variation for the contaminated

airfoil.

x

Cp

0 50 100 150 200 250

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

A = 17A = 25αα

Figure 18. Pressure distribution over the contaminated airfoil

in the post stall region.

IV. Conclusion A section of a wind turbine blade was tested in a low speed wind tunnel at various conditions. Contamination

distribution on the blade surface varies from leading edge to the trailing edge and it is denser in the leading edge. Our results show that surface roughness and contamination distribution strongly affect the performance of this blade section. Maximum lift coefficient is decreased up to 43% due to the surface contamination. At high angle of attacks differences between the clean and roughed airfoil were not significant.

All the roughness models changed the airfoil characteristics. The roughness caused a drastic drop in and

increased in maxlC

stallα . Maximum lift coefficient for the clean model increased by the Reynolds number reduction but this trend changed as the roughness installed on the model. This change leads to the result that Reynolds number decreases the roughness sensitivity.

American Institute of Aeronautics and Astronautics

10

References 1 Madsen, H. A., “Aerodynamics of a Horizontal Axis Wind Turbine in Natural Conditions”, Risoe M 2903, 1991. 2 Dyrmose, S. Z., Hansen, P., “The Double Stall Phenomenon and how to avoid it”, IEA, Lyngby, Dec. 3-4, 1998. 3 Snel, H., Corten, G. P.; Dekker, J. W. M., e.a., “Progr. in the Joule Project: MUST”, EUWEC, Mar. 1999, Nice, France. 4 Bak, C.; Madsen, H.A.; Fuglsang, P.; Rasmussen, F., 'Double Stall', Riso-R-1043(EN), June 1998. 5 A.C. Hansen, C.P. Butterfield, “Aerodynamics of Horizontal- Axis Wind Turbine”, Annual Review of Fluid Mechanics,

1993. 25, pp.115-149 6 R. P. J. O. M. van Rooij, W.A. Timmer, “Roughness Sensitivity Considerations for Thick Rotor Blade Airfoils” Journal of

Solar Energy Engineering , Vol. 125, November 2003 7 F. Askari, “Experimental Study of a Wind Turbine Blade Section”, M.S. Dissertation, Aerospace Dept. Sharif University of

Technology, Tehran, IRAN, 2004 8 Corten, G. P., “Insects Cause Double Stall”, ECNCX-- 00-018, Feb. 2001

American Institute of Aeronautics and Astronautics

11