Modelling of two tandem floating offshore wind turbines using an actuator line model

The aerodynamic and wake recovery dynamics of floating offshore wind turbines differ from fixed turbines due to the platform motions. Understanding tandem rotor interactions is essential for both turbines as well as wind farm design. This paper investigates the wake interactions in offshore wind farms by studying the effect of the upstream turbine motion on the downstream wind turbine loads and performance. A previously developed and validated Navier-Stokes actuator line model is used and implemented in the OpenFOAM® solver. The NREL 5 MW turbine is selected as a reference, and the upstream turbine is prescribed both surging and pitching motions (of different amplitude) while the downstream turbine is maintained fixed. Results for the turbine loading, wake and flow development are presented. It was found that the peak-to-peak thrust and power variations depend on modelling the discrete nature of the blades. Although the discrete tip vortices in fixed conditions diffuse within the first two diameters, downstream of the rotor, the platform motion can transform them into a new wake topology form with discrete ring shapes. The frequency spectra of the parameters showed a significant impact from these motion-induced discrete rings. The results indicate the need for higher fidelity modelling approaches when studying floating wind turbine interactions.