Impact of rotor size and axial spacing on the aerodynamic performance and power generation of offshore floating dual-rotor wind turbines

Large-scale offshore floating wind turbines are being developed to harness deep-sea wind energy, with dual-rotor designs remaining insufficiently understood. This study investigates the impact of rotor size and axial spacing on the aerodynamic performance of dual-rotor wind turbines (DRWT). With the primary rotor fixed to match the NREL Phase VI wind turbine (Experiment: Sequence S, Diameter: D = 10.058 m), six auxiliary rotors employing the FX60-126 airfoil, were designed (diameters ranging from 0.795D to 1.044D), and five axial distances from 0.149D to 0.746D were selected. DRWT numerical models employing the elliptic blending Reynolds stress model were developed using the commercial CFD software STAR CCM+ and validated through experimental data. Results showed that increasing the auxiliary rotor diameter consistently enhanced total power output, with a maximum power coefficient of 0.428 achieved at 1.044D. As axial distance increased from 0.149D to 0.746D, the total power coefficient initially rose, then fell, and finally rose again, aligning with changes in the primary rotor power influenced by the auxiliary rotor's wake flow. Significant interactions between the primary and auxiliary rotors' tip vortexes occurred at 0.597D of the axial distance, leading to a maximum output power difference of 6.3 % among 5 DRWTs at 0.597D and 0.298D.