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

Contents Background and Motivation Roughness modelling Experimental and CFD Results S814 airfoil  CLEAN  DISTRIBUTED ROUGHNESS Conclusions

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.

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

Roughness modelling Wilcox ω w roughness model.  Changes the turbulent kinetic dissipation rate at the wall, ω w 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.

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

Experimental Cl for the Clean and Rough S814 airfoil

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

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

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

CFD-Experimental Cd Rough configuration

CFD-Experimental Cm Rough configuration

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

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

Conclusions Transition gives reasonable results compare to Xfoil and the Experiment: BL tripping is not equivalent to add distributed Roughness : 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. 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. Distributed LE roughness characteristic of field contamination has been simulated with CFD:

THANK YOU FOR YOUR ATTENTION