Aerodynamic performance of a floating offshore wind turbine (FOWT) is significantly influenced by platform surging motions. Accurate prediction of the unsteady aerodynamic loads is imperative for determining the fatigue life, ultimate loads on key components such as FOWT rotor blades, gearbox and power converter. The current study examines the predictions of numerical codes by comparing with unsteady experimental results of a scaled floating wind turbine rotor. The influence of platform surge amplitude together with the tip speed ratio on the unsteady aerodynamic loading has been simulated through unsteady CFD. It is shown that the unsteady aerodynamic loads of FOWT are highly sensitive to the changes in frequency and amplitude of the platform motion. Also, the surging motion significantly influences the windmill operating state due to strong flow interaction between the rotating blades and generated blade-tip vortices. Almost in all frequencies and amplitudes, CFD, LR-BEM and LR-uBEM predictions of mean thrust shows a good correlation with experimental results.

No abstract

Floating offshore wind turbines operate in a highly unsteady environment; thus, many flow transients occur at the blade cross-sectional level, which affect the rotor aerodynamics. In every rotor aerodynamics modelling technique requiring the blade element theory, the blade cross-sectional aerodynamics need to be predicted accurately on the basis of the flow conditions. At reduced frequencies of 0.01 and greater, the flow unsteadiness can be considered significant and cannot be treated as quasisteady. Floating offshore wind turbines can be expected to consistently operate in some degree of yaw or pitch, which may result in reduced frequencies greater than 0.01 over most of the blade when operating at rated wind speeds and rotor RPM. The Beddoes-Leishman model is a comprehensive but complex model for predicting unsteady airfoil aerodynamics, containing 8 dimensionless time constants. In the present study, the Beddoes-Leishman model was compared with experimental results of 10 different airfoil profiles, each performed under a range of Reynolds numbers, motion frequencies, mean, and amplitudes of angle of attack. An optimization was performed for all time constants in the model, the results of which were used to formulate a simplified model with fewer equations, without any reduction in accuracy. Further, optimizations were performed against the experimental results of each airfoil, and the optimized constants were compared with shape parameters of the airfoils, yielding possible correlations, which were then applied in the simplified Beddoes-Leishman model to yield improved accuracy, measured as a 5% reduction in accumulated error between experimental and predicted coefficients of lift.

Modelling the aerodynamic forces on floating offshore wind turbines (FOWTs) is a challenging task due to the motion of the floating platform, which result in flow transients and associated aerodynamic effects. Each of these needs to be modelled and implemented, before a complete aerodynamic model of the FOWT can be presented. Of special interest is the dynamic wake effect, a result of the time lag between rotor forces and the air flow deceleration within the wake. The dynamic wake effect may present itself significantly in several scenarios including gust loads, changing wind speeds and direction, and the oscillatory motion of the rotor due to platform motion from wave forces. The complexity of the problem is substantiated by the added mass effect, which must be accounted for when considering rotor motions through the air. Computational fluid dynamics (CFD) simulations and blade element momentum (BEM) computations have been performed for the NREL 5MW virtual wind turbine in axial flow and surge motions at different frequencies and amplitudes, to quantify and model the dynamic wake effect, including the added mass effect for rotor motion. This dynamic wake model is then implemented in an unsteady blade element momentum (uBEM) code, for a more complete model of wind turbine aerodynamics.