CFD predictions of transition and distributed roughness over a wind turbine airfoil

1. CFD predictions of transition and distributed roughness over a wind turbine airfoil ESTEBAN FERRER XABIER MUNDUATE 47th AIAA Aerospace Sciences Meeting and Exhibit Orlando, Florida, 5 – 8 Jan 2009

2. Contents • Background and Motivation • Roughness modelling • Experimental and CFD Results S814 airfoil • CLEAN • DISTRIBUTED ROUGHNESS • Conclusions

3. Background and Motivation • Roughness affects Transition. • Traditionally via numerical panel methods or wind tunnel test by tripping the BL. • But Roughness affects Fully Turbulent flow. • Experimentally changing the roughness on the surface on the aerofoil. • As result: Modification log-law velocity distribution with increase shear. Not only earlier transition. • Traditionally “insensitive to Roughness” implies only effect on transition. Here, it is shown roughness effects beyond transition.

4. Background and Motivation • Wind turbine blade load calculation, design and certification still relay on 2D profile steady data: TRANSITIONAL FLOW S814 CLEAN AEROFOIL Menter-Langtry k – ω – γ - Reθ correlation. 2D CLEAN TURBULENT FLOW S814 CLEAN AEROFOIL SST k – ω TURBULENT FLOW S814 ROUGHAEROFOIL SST k – ω with Wilcox ωw roughness model. ROUGHNESS 2D

5. Roughness modelling • Wilcox ωw roughness model. • Changes the turbulent kinetic dissipation rate at the wall, ωw as a function of friction velocity and the equivalent sand grain roughness height. • The roughness model includes. • From hydrodynamic smooth surfaces to full rough flow condition (our case). • The turbulence model is the SST k – ω. • Computations are steady solution, no transient, therefore only calculate until small amount of separation, AOA < 8 deg.

6. Roughness modelling: S814 2D Grid Max t/c=24%. Distributed Roughness around LE, x/c=0.1. C type hexa 100.000 nodes.

7. Experimental Cl for the Clean and Rough S814 airfoil

8. Experimental Cd for the Clean and Rough S814 airfoil Re=1.5x10^6 Cdw measurements -5<AOA<8 deg

9. CFD-Experimental Cl Clean configuration More turbulence than the nominal 0.1% at the wind tunnel ?

10. CFD-Experimental Cl Rough configuration Roughness is different from tripping to fully turbulent

11. CFD-Experimental Cd Rough configuration

12. CFD-Experimental Cm Rough configuration

13. CFD-Experimental L/D Rough configuration There is not error compensation on L/D Roughness is more severe than tripping to fully turbulent

14. CFD-Experimental % Change in L/D Rough configuration Deterioration on Cl 15% - Cd 75% - Cm 15% - L/D 55%

15. Roughness has a more damaging effect on Cl-Cd-Cm than a localized tripping. Indeed roughness can reduced up to 55% the L/D on wind turbine airfoils. • Distributed LE roughness characteristic of field contamination has been simulated with CFD: Xfoil has shown not to be valid for predicting contamination roughness effects, only fully turbulent flows. CFD provides valuable qualitative and quantitative results on this respect: less than 10% error compare to experimental. Conclusions • BL tripping is not equivalent to add distributed Roughness : • Transition gives reasonable results compare to Xfoil and the Experiment:

16. THANK YOU FOR YOUR ATTENTION