Disentangling wake and projection effects in the aerodynamics of wind turbines with curved blades

Abstract. Advancements in wind turbine technology have led to larger, more flexible blades and an increasing interest in aerodynamic load calculations and design optimization of blades featuring significant sweep, prebend or coning. High-fidelity blade-resolved computational fluid dynamics (CFD) simulations provide precise rotor performance predictions but are computationally expensive. In contrast, the low-fidelity blade element momentum (BEM) method is computationally efficient but unable to model wake-induced effects of non-straight blades and coned rotors. To bridge this gap, mid-fidelity aerodynamic models, which balance accuracy and computational efficiency, are essential for design optimization tasks. Consistent aerodynamic benchmarks are crucial to effectively evaluate these models, particularly for modeling wake-induced effects across different blade geometries. Previous studies typically used the same chord and twist distributions across different curved blade geometries. However, this approach introduces inconsistencies, as it does not guarantee the same local aerodynamic conditions (e.g., angle of attack and local thrust coefficient) along the blade span due to projection effects of velocities and forces between the 2-D airfoil section and the 3-D flow. Consequently, wake-induced effects on loading and induction become entangled with unwanted projection effects, hindering the clear evaluation of how blade curvature alone influences the loads and induction. This study introduces a framework to disentangle wake-induced and projection effects in aerodynamic comparisons of curved blades. Within the BEM framework, we derive the necessary modifications to the chord and twist distributions of curved blades, ensuring the same spanwise circulation distribution as a baseline straight blade. These adjustments remove projection-driven discrepancies, enabling a consistent evaluation of wake-induced effects on loading and induction. Numerical validations using BEM and CFD confirm the effectiveness of these modifications. Additionally, projection effects in existing CFD results can be effectively isolated and removed. Using this framework, we discovered a novel insight from analysis of the CFD results: the wake-induced effects of moderate blade sweep and prebend can be modeled independently and then superimposed. This previously inaccessible insight significantly simplifies the modeling process and provides valuable guidance for developing mid-fidelity engineering aerodynamic models. Overall, this study advances the understanding of blade sweep and prebend effects on normal and tangential aerodynamic loads, supporting future blade design optimization.