Validation and accommodation of vortex wake codes for wind turbine design load calculations

Abstract

Abstract. The computational effort for wind turbine design load calculations is more extreme than it is for other applications (e.g., aerospace), which necessitates the use of efficient but low-fidelity models. Traditionally the blade element momentum (BEM) method is used to resolve the rotor aerodynamic loads for this purpose, as this method is fast and robust. With the current trend of increasing rotor size, and consequently large and flexible blades, a need has risen for a more accurate prediction of rotor aerodynamics. Previous work has demonstrated large improvement potential in terms of fatigue load predictions using vortex wake models together with a manageable penalty in computational effort. The present publication has contributed towards making vortex wake models ready for application to certification load calculations. The observed reduction in flapwise blade root moment fatigue loading using vortex wake models instead of the blade element momentum (BEM) method from previous publications has been verified using computational fluid dynamics (CFD) simulations. A validation effort against a long-term field measurement campaign featuring 2.5 MW turbines has also confirmed the improved prediction of unsteady load characteristics by vortex wake models against BEM-based models in terms of fatigue loading. New light has been shed on the cause for the observed differences and several model improvements have been developed, both to reduce the computational effort of vortex wake simulations and to make BEM models more accurate. Scoping analyses for an entire fatigue load set have revealed the overall fatigue reduction may be up to 5 % for the AVATAR 10 MW rotor using a vortex wake rather than a BEM-based code.

A computationally efficient engineering aerodynamic model for swept wind turbine blades

“…This means the time-marching lifting-line method is computationally heavy even after these modifications. Therefore the method is not practical for the aeroelastic simulation of the whole design load basis (DLB) of a wind turbine, which corresponds to more than 200 hours of real-time simulation (Hansen et al, 2015;Boorsma et al, 2020 In the near-wake model, the trailing functions in Eqs. ( 10) and ( 11) are both approximated with the sum of two exponential functions as shown in Eq.…”

Response to reviewer 1 and 2

“…This means the time-marching lifting-line method is computationally heavy even after these modifications. Therefore the method is not practical for the aeroelastic simulation of the whole design load basis (DLB) of a wind turbine, which corresponds to more than 200 hours of real-time simulation (Hansen et al, 2015;Boorsma et al, 2020). In the near-wake model, the trailing functions in Eqs.…”

Some modelling improvements for prediction of wind turbine rotor loads in turbulent wind

“…BEM has previously been found to provide good agreement with measurements and Computational Fluid Dynamics (CFD) simulations 9 . However, it has also been found to overestimate fluctuations with respect to blade resolved and vortex simulations and experiments 10,11 . Even within the blade resolved simulations, the results arising from different turbulence models are not consistent 12,13 …”