Forced motion simulations of vortex-induced vibrations of wind turbine blades – A study of sensitivities

Vortex-induced vibrations on wind turbine blades are a complex phenomenon not predictable by standard engineering models. For this reason, higher fidelity computational fluid dynamics (CFD) methods are needed. However, the term CFD covers a broad range of fidelities, and this study investigates which choices have to be made when wanting to capture the VIV phenomenon in a satisfying degree. The method studied is the so-called forced motion (FM) approach, where the structural motion is imposed on the CFD blade surface through modeshape assumptions rather than fully coupled two-way fluid structure interaction. In the study, two independent CFD solvers, EllipSys3D and Ansys CFX, are used and five different turbulence models of varying fidelities are tested. Varying flow scenarios are studied with respectively low to high inclination angles, which determine the component of the flow in the spanwise direction. In all scenarios, the cross-sectional component of the flow is close to perpendicular to the chord of the blade. It is found that the low and high inclination angle scenarios, despite having a difference equivalent to up to only thirty degrees azimuth, have quite different requirements of both grid resolution and turbulence models. For high inclination angles, where the flow has a large spanwise component from tip towards root, satisfying results are found from quite affordable grid sizes, and even with URANS k-ω turbulence the result is quite consistent with models resolving more of the turbulent scales. For low inclination, which has a high degree of natural vortex shedding, the picture is opposite. Here, even for scale resolving turbulence models, a much finer grid resolution is needed. This allows to capture the many incoherent vortices shed from the blade, which have a large impact on the coherent vortices, which inject power into the blade or extract power. It is found that a good consistency is seen using different variations of the higher fidelity hybrid RANS/LES turbulence models, like IDDES, SBES and k-ω SAS models, which agree well for various flow conditions and imposed amplitudes. This study shows that extensive care and consideration are needed when modelling 3D VIVs using CFD, as the flow phenomena, and thereby solver requirements, rapidly change for different scenarios.