Atmospheric rotating rig testing of a swept blade tip and comparison with multi-fidelity aeroelastic simulations

One promising design solution for increasing the energy production of modern horizontal axis wind turbines is the installation of curved tip extensions. However, since the aeroelastic response of such geometrical add-ons has not been characterized yet, there are currently uncertainties in the application of traditional aerodynamic numerical models. The objective of the present work is twofold. On the one hand, it represents the first effort in the experimental characterization of curved tip extensions in atmospheric flow. On the other hand, it includes a comprehensive validation exercise, accounting for different numerical models for aerodynamic load prediction. The experiments consist of controlled field tests at the outdoor rotating rig at the Risø campus of the Technical University of Denmark (DTU), and consider a swept tip shape. This geometry is the result of an optimized design, focusing on locally maximizing power performance within load constraints compared to an optimal straight tip. The tip model is instrumented with spanwise bands of pressure sensors and is tested in atmospheric inflow conditions. A range of fidelities of aerodynamic models are then used to aeroelastically simulate the test cases and to compare with the measurement data. These aerodynamic codes include a blade element momentum (BEM) method, a vortex-based method coupling a near-wake model with a far-wake model (NW), a lifting-line hybrid wake model (LL), and fully resolved Navier–Stokes computational fluid dynamics (CFD) simulations. Results show that the measured mean normal loading can be captured well with the vortex-based codes and the CFD solver. The observed trends in mean loading are in good agreement with previous wind tunnel tests of a scaled and stiff model of the tip extension. The CFD solution shows a highly three-dimensional flow at the very outboard part of the curved tip that leads to large changes of the angle of the resultant force with respect to the chord. Turbulent simulations using the BEM code and the vortex codes resulted in a good match with the measured standard deviation of the normal force, with some deviations of the BEM results due to the missing root vortex effect.</p

Métholodogie CFD pour la modélisation de l'interaction fluide-structure des éoliennes

Horizontal axis wind turbines are one of the most efficient renewable energy sources. In order to extract the maximum power per machine and reduce the overall energy extraction cost, there is a clear design trend consisting in the up-scaling of the rotor diameter. This implies the consideration of more flexible blades, that significantly deform during operation due to the aerodynamic loading. This thesis proposes an innovative computational approach for the study of this aeroelastic problem that accounts for the interaction of both the fluid and the structure physics.The PhD work tackles two major issues concerning efficient fluid-structure interaction simulations of large horizontal axis wind turbines. One concerning the development of a mesh deformation technology achieving a good trade-off between mesh quality, scalability, robustness and computational cost for 3D flow meshes accounting multi-million points. Another concerning the extension of a frequency domain methodology, namely the nonlinear harmonic method, to handle the 2-way coupling between a fluid in motion and a deformable elastic structure for steady and periodic unsteady aeroelastic interaction.The resulting methodology allows to assess the rotor performance with progressive degrees of model complexity. Both isolated rotor and full machine (i.e. including the tower) calculations can be performed. Fluid modeling relies in a steady or unsteady formulation respectively. For both configurations, a structural model of the blades can be considered in order to assess the influence of aeroelasticity in the rotor performance.The suggested approach was implemented and tested within the FINE™/Turbo software, edited by NUMECA International. It offers a higher fidelity multiphysics flow modeling than current industry standards, and its reduced computational cost enables its direct introduction into the wind energy market.Les éoliennes à axe horizontal sont une des sources d'énergie renouvelables les plus efficaces. Afin de réduire le coût global d'extraction d'énergie, il y a une tendance claire qui consiste en augmenter le diamètre du rotor. Cela implique la prise en compte de pales plus souples, qui se déforment de manière significative pendant le fonctionnement (en raison de la charge aérodynamique). Cette thèse propose une approche innovante de calcul pour l'étude de ce problème aéroélastique, qui tient compte à la fois du fluide et de la physique de la structure.Le travail de thèse aborde deux questions majeures concernant les simulations d'interaction fluide-structure des grandes éoliennes à axe horizontal. L'une concerne le développement d'une technologie de déformation du maillage. Une autre concerne l'extension d'une méthodologie dans le domaine fréquentiel (la méthode harmonique non linéaire), pour gérer le couplage bidirectionnel entre un fluide en mouvement et une structure élastique déformable.La méthodologie résultante. qui permet d'évaluer la performance du rotor avec différents degrés de complexité. a été mis en œuvre et testé dans le logiciel FINE™/Turbo (édité par NUMECA International). Elle offre une modélisation multiphysique avec une fidélité plus élevée que le standard de l'industrie, tout en gardant un coût de calcul réduit