Edgewise instabilities of a wind turbine blade section in attached flow conditions

Abstract. With the persistent trend towards larger, lighter turbine blades and the resulting increase in blade flexibility, the risk of edgewise instabilities is elevated. This emphasizes the need for a better understanding of the accuracy of simulation models and the mechanisms affecting the damping of edgewise modes. This study investigates edgewise instabilities in wind turbines under attached flow conditions using a two-dimensional, three-degree-of-freedom typical section model. The primary goal of this work is to validate the stability analysis predictions from common low-fidelity aerodynamic models by comparing them with a high-fidelity computational fluid dynamics (CFD) reference result. In addition, the study aims to deepen the understanding of the fundamental mechanisms behind edgewise instabilities. The comparison of the stability analysis results obtained with aerodynamic models of different fidelity demonstrates excellent agreement in both the trends and quantitative damping predictions for the edgewise mode. An additional sensitivity study highlights the importance of the steady load vector on the damping prediction. The flutter mechanism analysis shows that the edgewise instabilities are caused by a coupling of flapwise motion with the structurally coupled edgewise-torsional motion. Edgewise instabilities can occur when there is sufficient structural edge-twist coupling, both in edge-twist coupling to stall or edge-twist coupling to feather. The findings in the paper increase the confidence in common low-fidelity aerodynamic models and contribute to a better understanding of edgewise instabilities.