1 Department of Wind Energy, Technical University of Denmark2 Aeroelastic Design, Department of Wind Energy, Technical University of Denmark
In the design of modern wind turbines with long and slender rotor blades it becomes increasingly important to model and understand the evolving aero-elastic eects in more details. Standard stateof-the-art aero-elastic simulation tools for wind turbines usually employ a blade element momentum (BEM) based aerodynamic model which is computationally cheap but includes several limitations and corrections in order to account for three-dimensional and unsteady eects. The present work discusses the development of an aero-elastic simulation tool where high-fidelity computational fluid dynamics (CFD) is used to model the aerodynamics of the flexible wind turbine rotor. Respective CFD computations are computationally expensive but do not show the limitations of the BEM-based models. It is one of the first times that high-fidelity fluid-structure interaction (FSI) simulations are used to model the aero-elastic response of an entire wind turbine rotor. The work employs a partitioned FSI coupling between the multi-body-based structural model of the aero-elastic solver HAWC2 and the finite volume CFD solver EllipSys3D. In order to establish an FSI coupling of sufficient time accuracy and sufficient numerical stability several coupling strategies are investigated and implemented. The considered coupling strategies incorporate both loose and strong coupling schemes and employ both a conservative and a non-conservative force and deflection transfer. In a specific assessment of the implemented coupling schemes it was found that a relatively simple loosely coupled algorithm with a non-conservative force transfer is well-suited to establish a second order time accurate and sufficiently stable FSI simulation. The use of a strong coupling scheme was found to be redundant. Results of the partitioned FSI coupling between HAWC2 and EllipSys3D (HAWC2CFD) were then compared to the computations of the stand-alone solver of HAWC2 which employs traditional BEM theory to model the aerodynamics. In a first set of comparative simulations the quasi-steady aeroservo-elastic response of the NREL 5MW reference wind turbine was investigated for the wind speed range between 4 m/s and 24 m/s. In a second test case the same turbine was modelled during an emergency shut-down due to a loss of power in which the rotor blades are quickly pitched to feather in order to slow down the turbine. The rapid change in the aerodynamic loading and the severe structural response evoke complex flow regimes which are rather challenging to model with the traditional BEMbased models. The comparisons between the results of HAWC2CFD and HAWC2 revealed a very good agreement in the predicted aero-servo-elastic response of the modelled wind turbine, although some smaller discrepancies could be found in the predicted aerodynamic forces. Additionally, the work includes the description of a generic coupling framework which was developed in order to establish the desired partitioned coupling between HAWC2 and EllipSys3D. The developed framework was then used to also conduct FSI simulations of isolated two-dimensional and threedimensional aerofoil sections by coupling a simple three degrees of freedom structural model with the respective CFD model.
DTU Wind Energy PhD-0033(EN); DTU Wind Energy PhD-0033