Experimental Detection of Transition on Wind Turbine Airfoils

1. Experimental Detection of Transition on Wind Turbine Airfoils Peter Bæk, LM Glasfiber pbk@lmglasfiber.com

2. Outline • Introduction • Experimental Setup • Results • Conclusions EWEC 2009 - Transition Detection - Peter Bæk  Page 2

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 EWEC 2009 - Transition Detection - Peter Bæk  Page 3

4. 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 airfoil’s design conditions! • It is necessary to ensure high quality drag measurements of airfoils. - Surface imperfections on models can be eliminated. 1978 – 2009:From technology pioneering to global industrial excellence EWEC 2009 - Transition Detection - Peter Bæk  Page 4

5. Measurement Setup IR Camera Heat Lamp Quartz Glass View Port Gold Markers EWEC 2009 - Transition Detection - Peter Bæk  Page 5

6. Measurement Concept Transition Point Heat Lamp Heat Lamp Convection Turbulent Convection Laminar Radiation Radiation Internal Conduction Internal Conduction EWEC 2009 - Transition Detection - Peter Bæk  Page 6

7. LM Glasfiber’s Wind Tunnel • Max wind speed: 105 m/s • Reynolds number: 6 millions • Turbulence intensity: 0.1% EWEC 2009 - Transition Detection - Peter Bæk  Page 7

8. Thermographic Image Analysis Zig zag, tape Transition Line NACA0015, AOA=0 deg, Re=6 Mill. Flow EWEC 2009 - Transition Detection - Peter Bæk  Page 8

9. Measurements vs. Xfoil, NACA 0015 airfoil. NACA 0015 =10% higher drag! EWEC 2009 - Transition Detection - Peter Bæk  Page 9

10. Verification using stethoscope Tubes to microphones Goodcorrespondance! EWEC 2009 - Transition Detection - Peter Bæk  Page 10

11. Measurements vs. Xfoil: Wind Turbine Airfoils 18% 25% Good Bad!10% off EWEC 2009 - Transition Detection - Peter Bæk  Page 11

12. Measurements vs. Xfoil: Root Airfoil 40% LSWT Re = 3 x 106 XFOIL, ncr=7 Bad! Good EWEC 2009 - Transition Detection - Peter Bæk  Page 12

13. 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! Wellsuited for an industrialsetup! EWEC 2009 - Transition Detection - Peter Bæk  Page 13

14. 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! EWEC 2009 - Transition Detection - Peter Bæk  Page 14

15. Questions ? Thank you for your attention! Peter Bæk pbk@lmglasfiber.com