The MEXICO rotor aerodynamic loads prediction: ZigZag tape effects and laminar-turbulent transition modeling in CFD

This paper aims to provide an explanation for the overprediction of aerodynamic loads by CFD compared to experiments for the MEXICO wind turbine rotor and improve the CFD prediction by considering laminar-turbulent transition modeling. Large deviations between CFD results and experimental measurements are observed in terms of sectional normal and tangential forces at the blade tip (r/R=0.82 and 0.92) of the MEXICO rotor operating in axial condition at the design tip speed ratio λ=6.7. The first part of this study identifies the effects of ZigZag tape, which is used in the experiment to trigger boundary layer transition, by analyzing the available experimental data of a single, non-rotating MEXICO rotor blade. The analysis indicates that ZigZag tape has a significant impact on sectional aerodynamic tip loads: it alters the boundary layer thickness and additionally reduces the effective airfoil camber besides the expected tripping. These additional effects most likely also occur in the rotating MEXICO experiment, reducing the sectional loads and hence lead to an overprediction by CFD. To eliminate the ZigZag tape interference, experimental data with an untripped blade is preferred to be used as validation case. In the second part of this study, a transitional flow simulation for the MEXICO rotor is performed by using RANS-based transition model k−k_L−ω within OpenFOAM-2.1.1. The numerical results are compared against experimental data obtained from the untripped, new MEXICO experiments. The comparison gives that transitional simulation present a very good tip loads prediction for the untripped blade. The measured data also confirms that the ZigZag tape indeed has a significant influence on the blade tip loads in rotating conditions. The transition onset over 3D MEXICO blade is visualized and transition locations are identified. The results shown in the present study can explain the causes of the large differences between CFD and experiment observed in the MEXICO blind comparisons.