How does turbulence affect wake development in floating wind turbines? Some insights from comparative large-eddy simulations and wind tunnel experiments

Research is flourishing on how to model, mitigate, or even try to exploit the complex motions floating offshore wind turbines (FOWTs) are subjected to due to the combined loading from wind, waves, currents, and buoyancy effects. While preliminary studies made use of simplified inflows to focus attention on blade-flow interaction, recent evidence suggests that the impact of realistic inflows can be much larger than expected. The present study presents a critical analysis aimed at quantifying to what extent turbulence characteristics affect the wake structures of a floating turbine undergoing large motions. Numerical computational fluid dynamics (CFD) simulations, using a large-eddy simulation (LES) approach coupled with an actuator line method for the rotor, are benchmarked against wind tunnel experimental data from the first campaign of the NETTUNO project on a scaled rotor that was tested both in static conditions and when oscillating in pitch. A comparative analysis of the results at different turbulence levels first confirmed that, whenever idealized flows with no significant turbulence are considered, platform motion in FOWTs indeed leads to the creation of induced flow structures in the wake that dominate its development and the vortex breakdown in comparison to bottom-fixed cases. More interestingly, analyses show, on the other hand, that whenever realistic turbulence comes into play, only small gains in terms of wake recovery are noticed in FOWTs in comparison to bottom-fixed turbines, suggesting the absence of superposition effects between inflow and platform motion, with inflow turbulence contributing significantly to dissipating the structures induced by turbine oscillation. Finally, as an ancillary outcome of the study, evidence provided by LES high-fidelity simulations was used to understand to what extent a less computationally intensive unsteady Reynolds-Averaged Navier-Stokes (URANS) approach can be used to study the impact of realistic turbulence. In particular, an innovative URANS approach featuring improved inflow boundary conditions proved to yield consistent results when mean wake profiles were considered.