CFD Predictions of Transition and Distributed Roughness Over a Wind Turbine Airfoil

Abstract

ows and distributed roughness over a wind turbine airfoil to account for variable life cycle operational regimes are presented. The panel method code Xfoil 6.96 and the CFD commercial code Fluent 12.0.3 Beta are evaluated for the prediction of 2D wind turbine airfoil aerodynamic performance. The aim of the work is to asses the accuracy of the various methods in the determination of integrated loads (i.e. Cl, Cd, L/D and Cm1=4) for airfoils with both clean and rough surfaces that simulate contamination arising from wind turbine operational life. Numerical data are compared to experimental results for the NREL S814 wind turbine airfoil. Xfoil and Fluent are used to predict the three states present in a wind turbine blade life cycle. The aerodynamic characteristics are calculated with transition and no roughness (also called clean conguration), with fully turbulent ow and no roughness and nally applying roughness under fully turbulent ow conditions. Validation cases are presented where computations are compared to experimental data for cases with locally distributed roughness at the leading edge of the airfoil. Roughness of this type simulates airfoil contamination by bugs, dirt or debris. Results for smooth clean surfaces and fully turbulent ows (e.g. tripped boundary layer) are very similar for the two codes for attached or mildly separated ows. The transitional model (k ! Re ) of Fluent gives reasonable results when compared to transition free experimental data and Xfoil computed with free transition. To account for roughness eects, the SST k ! model modied to take into account surface roughness has been used in its Fluent version. CFD calculations, where roughness is modelled, are in good agreement with experimental data. It is shown that roughness originated from contamination has a more damaging eect on aerodynamics than a boundary layer tripping. It is concluded that CFD can simulate all the variety of ows that an operational wind turbine airfoil can encounter throughout its life cycle.