*2.3. FSI Coupling*

The CFD and the structural models outlined in the previous sections were coupled by means of Tango, an in-house code developed at Ghent University, resulting in a partitioned FSI simulation [42]. The Gauss–Seidel coupling algorithm was selected and three iterations were enough within each time step to reach a displacement absolute residual of about 5 mm on the fluid–structure interface. The fluid mesh was deformed according to what is prescribed by the structural solver on the fluid–structure interface. Each blade component mesh was deformed by means of a spring-based smoothing method, therefore adopting an arbitrary Lagrangian–Eulerian (ALE) formulation. Compared to other methods present in the literature [43], the adopted methodology can consistently preserve an appropriate *y*<sup>+</sup> value of the stretched cells in the region adjacent to each wall. This ensured a well-resolved near-wall flow at a reasonable computational cost.

Simulations were performed on 280 cores (10 nodes inter-connected by InfiniBand, each with 2 CPUs of the type 14-core Xeon E5-2680v4, 2.4 GHz) and approximately 10 days were necessary to perform a complete revolution. Starting the FSI simulation from the results of a CFD simulation, approximately 1.2 revolutions were necessary to reach the regime in time. Then, one full rotation was performed and analyzed. During this rotation, the loads, stress, and displacements of each blade could be univocally linked to its azimuth angle. The azimuth angle was set to 0 when the blade was horizontally positioned and in an upward motion. Therefore, a 90◦ azimuth angle corresponded to the blade pointing upward and 270◦ to the blade pointing downward.
