Actuator line URANS-to-LES comparison of single and tandem floating offshore wind turbines

Abstract. The motion of floating offshore wind turbine platforms strongly affects wake development, influencing energy production, farm layout, and turbine loads. Unlike fixed-bottom turbines, floating turbine wake interactions are less understood and require high-fidelity modelling. This study investigates the wake dynamics of a single floating wind turbine subjected to surge and pitch motions using an actuator line model. Moreover, the impact on the loads of a downstream turbine operating in the wake of an upstream floating turbine is assessed. A methodological comparison between unsteady Reynolds-averaged Navier–Stokes (URANS) and large-eddy simulation (LES) with laminar and turbulent inflow is performed to isolate the effects of platform motion and inflow turbulence on wake dynamics and to assess URANS' capability in capturing floating turbine wakes. Single floating turbine results are validated against experimental load and wake data. Wake validation shows that LES with turbulent inflow best captures turbulence intensity distribution, while URANS reproduces mean velocity profiles accurately, especially in the near wake. Platform motion enhances wake recovery under laminar inflow, whereas under turbulent inflow, the wake recovers faster and the effect of motion is reduced; URANS shows slower far-wake recovery compared to turbulent LES. Analysis of platform-motion-induced wake oscillations indicates that turbulent LES accurately reproduces amplitudes, while the standard URANS setup adopted in this study underestimates them, and laminar inflow LES is inadequate. No significant differences are observed between surge and pitch cases. Wake meandering is found to be primarily driven by turbulent inflow rather than platform motion: turbulent LES captures wake displacement at a characteristic frequency, whereas URANS fails to reproduce it. The impact on a downstream turbine 5 D away is finally assessed and reveals that URANS underestimates both the mean and the amplitudes of the loads. Blade distributed load analysis shows that URANS captures platform-motion-induced variability but misses turbulence-driven effects downstream. In conclusion, this study provides a detailed characterization of floating turbine wake dynamics; highlights differences in LES and URANS accuracy; and demonstrates that LES yields more reliable predictions of downstream turbine loads, essential for structural assessment within wind farms.