A novel high-fidelity two-way coupling model for fluid-structure interaction in wind energy

To increase the attractiveness of wind energy, wind turbines are continuously scaling up, with diameters now exceeding 200 m. If on the one hand, this trend guarantees an increased power production, on the other hand, it imposes harsher aerodynamical and structural requirements – on the blades in particular – that are difficult to characterise. In particular, the significant size of the state-of-the-art wind turbines suggests a more relevant Fluid-Structure Interaction (FSI) that could alter dramatically the operating life of the full machine. Given the difficulties and the costs of measuring the phenomena occurring at significant scales, researchers advocate the development of high-fidelity numerical models exploiting Computational Fluid and Structural Dynamics (CFD-CSD models). For this reason, in this work we present a novel FSI model for wind turbines combining our Large Eddy Simulation (LES) fluid solver with a modal beam-like structural solver. In the first part of the work, we present the details of our FSI methodology, and we analyse the effects of different coupling conditions. A loose algorithm couples the Actuator Line Model (ALM), which represents the blades in the fluid domain by means of body forces, with the structural model, which represents the flexural and torsional deformations. For a reference utility-scale wind turbine, we compare the results of three sets of simulations. Firstly, we consider one-way coupled simulations where only the fluid solver provides the structural solver with the aerodynamic loads; then, we consider two-way coupled simulations where the structural feedback to the fluid solver is made of the out-of-plane and in-plane bending deformation velocities only; finally, we add to the feedback also the torsional deformation. However, to accurately reproduce the airloads, one should notice that the blades in particular are subjected to many relevant sources of unsteadiness, e.g. tower shadowing, yawed and waked conditions, environmental effects. Therefore, researchers have questioned the use of steady aerodynamics in the numerical fluid and aeroelastic models used in wind energy that do not have the sufficient resolution to solve the flow close to the blade, arguing that the use of tabulated airfoil coefficients could neglect effects that alter the estimation of the turbine behaviour. Different unsteady aerodynamics models have been proposed to account for these effects but have been mainly implemented in low-fidelity engineering models, which lack the complete capability of describing the multiscale and multi-physics phenomena characterising the wind turbine. For this reason, in the second part of the work, a 2D unsteady aerodynamics model is implemented in the sectional estimation of the airloads of the Actuator Line Model. At each section of the blade, a semi-empirical Beddoes-Leishman model includes the effects of additional noncirculatory terms, unsteady trailing edge separation and dynamic stall in the dynamic evaluation of the aerodynamic coefficients of the airfoils, used to determine the ALM body forces. Different inflow conditions and aeroelastic behaviours are examined with the aim of examining the effects of the model, and thus of providing a deeper insight into the unsteady characterisation of large wind turbines by means of a high-fidelity CFD-CSD model.