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.
Highlights
For wind turbine design certification, load calculations in agreement with IEC guidelines (International Electrotechnical Commission, 2019) are required to define a representative load envelope
The uncertainty margins associated with these models are large, which especially holds true for the unsteady rotor aerodynamics (Boorsma and Schepers, 2018)
Instead of focusing on the equivalent load level determined by the largest ranges with very few occurrences, statistically it makes more sense to study the ranges with a large number of counts when comparing computational fluid dynamics (CFD) to lifting-line simulations
Summary
For wind turbine design certification, load calculations in agreement with IEC guidelines (International Electrotechnical Commission, 2019) are required to define a representative load envelope. Boorsma et al.: Validation and accommodation of vortex wake codes for wind turbine design load calculations flexible blades, has together with the larger inflow variations over the rotor disk resulted in a greater relative importance of unsteady flow features and the modeling thereof This further underlines the need for more accurate prediction of rotor aerodynamics. Over the last decades several publications have researched the added benefit of vortex wake models over BEM-based models (Hauptmann et al, 2014; Gupta, 2006; Boorsma et al, 2016b) and more recently (Perez-Becker et al, 2019) Some of these feature a validation against wind tunnel data, for which the inflow conditions and turbine are not always representative for design load calculations on a multi-megawatt wind turbine.
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