A new Blade Element Momentum theory for both compressible and incompressible wind turbine flow computations

The growing trend towards more efficient and cost-effective wind turbines boosting blade length and tip speed. The conventional Blade Element Momentum theory becomes inaccurate due to the assumption of air incompressibility, thus presents an error in predicting aerodynamic loads for extremely large wind turbines. We propose a new Blade Element Momentum theory based on isentropic relations and the Euler equation for accurately calculating the aerodynamic loads of extremely large wind turbines. The new method is validated against computational fluid dynamics on the IEA 15 MW wind turbine at various wind and operational scenarios and an excellent agreement is achieved. Implementing into an aeroelastic code, the study reveals that the air compressibility increases the flap-wise tip displacement, flap-wise root moment, and power up to 4.33 %, 3.49 %, and 1.52 %, respectively, depending on the blade tip speed and pitch orientation. The method provides a new technique to accurately calculate and assess the aerodynamic loads, enabling a more accurate design, safety assessment and power prediction for extremely large wind turbines.