Peter Bk, LM Glasfiber pbklmglasfiber'com

1
Experimental Detection of Transition on Wind
Turbine Airfoils

• Peter Bæk, LM Glasfiber
  pbk_at_lmglasfiber.com

2
Outline

• Introduction
• Experimental Setup
• Results
• Conclusions

3
Transition Facts
Natural Transtion

• Post-transitional boundary layer is characterized
  by
• Increased boundary layer thickness
• Increased velocity fluctuations
• Increased skin friction

Increased Heat Transfer
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Why do we want to measure transition?

• The transition point is driving the frictional
  drag of the airfoil. - Premature transition can
  reduce annual energy production up to 2!
• The transition point influences separation and
  maximum lift. - Premature separation will reduce
  annual energy production even more!
• Transition cannot be computed accurately enough.
  - It is especially challenging near the
  airfoils design conditions!
• It is necessary to ensure high quality drag
  measurements of airfoils. - Surface
  imperfections on models can be eliminated.

1978 2009From technology pioneering to
global industrial excellence
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Measurement Setup
IR Camera
Heat Lamp
Quartz Glass
View Port
Gold Markers
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Measurement Concept
Transition Point
Heat Lamp
Heat Lamp
Convection Turbulent
Convection Laminar
Radiation
Radiation
Internal Conduction
Internal Conduction
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LM Glasfibers Wind Tunnel

• Max wind speed 105 m/s
• Reynolds number 6 millions
• Turbulence intensity 0.1

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Thermographic Image Analysis
Transition Line
NACA0015, AOA0 deg, Re6 Mill.
Flow
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Measurements vs. Xfoil, NACA 0015 airfoil.
NACA 0015
10 higher drag!
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Verification using stethoscope
Tubes to microphones
Good correspondance!
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Measurements vs. Xfoil Wind Turbine Airfoils
18
25
Good
Bad!10 off
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Measurements vs. Xfoil Root Airfoil
40
LSWT
Re 3 x 106
XFOIL, ncr7
Bad!
Good
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Summary of the new method

• Method was shown to be non-intrusive
• Method was verified using microphones
• High order of repeatability. Variance less than
  2 of chord length
• The technique is fast and reliable!

Well suited for an industrial setup!
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Conclusions

• Overall agreement between measurements and XFOIL
  transition model was good.
• But pitfalls exist!
• Measured and predicted are not always the same!
• Correct prediction of transition does not lead to
  correct maximum lift!
• Possibility to study low level flow details of
  airfoil flow control devices.
• The new method ensures a reliable design basis
  for our products!

15
Questions ?

• Thank you for your attention!
• Peter Bæk
• pbk_at_lmglasfiber.com