Investigating the dynamics of floating wind turbine wakes under laminar flow using large eddy simulations

Abstract. A laboratory-scale model of the DTU 10 MW wind turbine is investigated under prescribed surge, sway, roll, pitch, yaw and coupled surge-pitch motions using YALES2, a high-fidelity large-eddy simulation (LES) tool coupled to an actuator-line model. The prescribed motions are sinusoidal and two cases per degree-of-freedom (DOF) are considered: one with a low Strouhal number St and high normalized amplitude A^*, and vice versa. The turbine is operated in the vicinity of rated conditions, under constant rotor speed and steady uniform free-stream velocity. The different aspects of the methodology are validated including experimental comparison with data from the OC6 Phase III campaign. Cases with low-St/high-A^* exhibit behavior similar to the fixed-bottom case in many aspects of the wake. Conversely, cases with a high-St/low-A^* disturb the wake to a much larger extent. The contrast is caused by differences in how much the wake amplifies the perturbations of the floating motion upstream and is particularly noticeable in the blade tip and root shear layers (a.k.a. turbulent mixing layers), where a higher amplification leads to a faster expansion of the wake in the high-St/low-A^* cases. Prescribed motions with a component perpendicular to the flow are found to have a larger impact than motions exclusively in the flow direction. The prescribed motions also cause wake meandering, with larger amplitudes observed for the cases with low-St/high-A^*. The magnitude of perturbation amplification is quantified by amplification factors relating either the wake velocity or displacement to the rotor center velocity or displacement, respectively. The maximum factors are associated with the high-St/low-A^* cases and are similar among DOFs, with some exceptions.