Critical study of the effects and numerical simulations of boundary layer transition in lift-based wind turbines at moderate Reynolds numbers

Early laminar-to-turbulent transition due to fouling typically degrades the lift and amplifies the drag of airfoils. Few studies of medium-size horizontal and vertical axis wind turbines (HAWTs and VAWTs, respectively) provide information on this turbulence. This reflects uncertainties in the reported power coefficients (Cp) and contradictory recommendations for computational fluid dynamics (CFD) models. This paper investigates the Cp degradation of wind turbines (Rechord = 0.7–1 × 106) and its CFD-modeling aspects. Published experimental data for the modern DU12W262 airfoil, for both the clean and dirty surface conditions, are taken as the baseline. CFD is employed to reproduce airfoil characteristics for both cases. Using the blade element momentum and computational fluid dynamics (CFD) models, the experimental and numerical HAWT and VAWT performances are assessed. Non-transition turbulence closure models [Spalart–Allmaras and Shear Stress Transport (SST)], which have been recommended against in previous works without due consideration for surface aspects, are shown to successfully mimic dirty surface conditions but to underpredict the dramatic experimental degradation of 82% in maximum CL/CD to 67% when uncalibrated. The resulting experimental Cp reductions for HAWTs and VAWTs are 34% and 65%, respectively, values that are also underpredicted by CFD as 17% and 34%, respectively, without calibration. The detailed findings may serve as pioneering data for transition effects and aspects of CFD for both HAWTs and VAWTs at moderate Reynolds numbers.