Wind turbine rotors in surge motion: New insights into unsteady aerodynamics of FOWT from experiments and simulations

Abstract. An accurate prediction of the unsteady loads acting on floating offshore wind turbines (FOWT) under consideration of wave excitation is crucial for a resource-efficient turbine design. Despite a considerable number of simulation studies in this area, it is still not fully understood, which unsteady aerodynamic phenomena have a notable influence on the loads acting on a wind turbine rotor in motion. In the present study, investigations are carried out to evaluate of the most relevant unsteady aerodynamic phenomena for a wind turbine rotor in surge motion. As a result, inflow conditions are determined for which a significant influence of these phenomena on the rotor loads can be expected.

The experimental and numerical investigations are conducted on a two-bladed wind turbine rotor subjected to a tower top surge motion. A specialised wind tunnel test rig has been developed to measure the aerodynamic torque response of the rotor subjected to surge motions with moderate frequencies. The torque measurements are compared to two free vortex wake methods, namely a panel and a lifting-line method. Unsteady contributions that cannot be captured using quasi-steady modelling have not been detected in either the measurements or the simulations in the covered region of motion frequencies. The surge motion frequencies were limited to a moderate range due to vibrations occurring in the experiments. Therefore, a numerical study with an extended the range of motion frequencies using the panel and the lifting-line method was performed. The results reveal significant unsteady contributions of the surge motions to the torque and thrust response that cannot be modelled in a quasi-steady way. Furthermore, the results show the presence of the returning wake effect, which is known from helicopter aerodynamics. Additional simulations of the UNAFLOW scale model and the IEA 15 MW rotor demonstrate that the occurrence of the returning wake effect is independent from the turbine, but determined by the ratio of rotor speed and surge motion. In case of the IEA 15 MW, a notable impact of the returning wake effect was found at surge motion frequencies in the range of typical wave periods. Finally, a comparison with OpenFAST simulations reveals notable differences in the modelling of the unsteady aerodynamic behaviour in comparison to a FVW method.