Comparison of the Lifting Line Free Vortex Wake method and the Blade Element Momentum method for the unsteady aerodynamics of a 15MW FOWT
F.M.A. de Ridder (TU Delft - Electrical Engineering, Mathematics and Computer Science)
Axelle Vire – Mentor (TU Delft - Flow Physics and Technology)
Daniel Van Den Berg – Mentor (TU Delft - Team Jan-Willem van Wingerden)
Pim Van der Male – Graduation committee member (TU Delft - Offshore Engineering)
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Abstract
Accurate wind turbine modelling is essential for reliable aerodynamic performance predictions. The industry primarily uses the Blade Element Momentum (BEM) method with correction models, but BEM’s assumptions become less valid with larger rotors and in Floating Offshore Wind Turbines (FOWTs), where wave interactions and wake dynamics are more complex. The Lifting Line Free Vortex Wake (LLFVW) method offers higher modelling fidelity but is less computationally efficient.
This study compares a BEM and LLFVW model implemented in the software QBlade. The evaluated parameters include power, torque, thrust, root bending moment, tip deflection, and angle of attack using the floating 15 MW UMaine VolturnUS-S reference turbine under various wind and wave conditions taken from several Design Load Cases (DLCs). The aim is to identify any differences between the methods and the met-ocean conditions under which these are most pronounced.
The results show minimal differences in BEM and LLFVW outputs under varying wave conditions. However, wind conditions have a greater impact, particularly around rated speeds where discrepancies were observed, mainly due to different controller dynamics. Above-rated conditions showed similar power, torque, and thrust predictions, but notable differences in angle of attack. The maximum and standard deviation of the root bending moment and tip deflection were found to be consistently lower for LLFVW compared to BEM.