Floating offshore wind turbines (FOWTs) have received growing interest in offshore applications, mainly due to their capability to harvest more offshore wind energy in deep water wind farms. Nonetheless, getting away from the shoreline requires a floating platform associated with more complex phenomena. Compared to a fixed-bottom offshore wind turbine, a floating offshore wind turbine system experiences additional motions up to six-degree-of-freedom (6DOF) caused by both wind aerodynamic and wave hydrodynamic loads. These motions cause a distortion in the wake structure and an oscillation in aerodynamic performance. However, blade element momentum theory (BEMT) is not able to simulate the wake behind the turbine and using computational fluid dynamics (CFD) demands high computational resources.This study focuses on investigating the actuator line model (ALM) applications in the prediction of wake in floating condition. A computational method is developed which implements the ALM with 6DOF coupled with a CFD solver. In ALM, element points with Gaussian distributed body force represent each section of turbine blades. Hence, in this method, grids are cubic and require no boundary layer mesh; this decreases preprocessing time for producing appropriate mesh and decreases computational cost due to the fewer grid density required.Validation of the method was carried out for two different test cases. NREL Phase VI with available adequate data and the NREL 5-MW were chosen for this purpose as they represent two very well-documented test cases regarding two very different turbine sizes. Mesh and time-step independence study were conducted and revealed that approximately 300k grid nodes are necessary to obtain satisfactory accuracy using the ALM method in both test cases, which is truly computationally cost-effective. In fact, the overall results of the ALM code were in excellent agreement with those of the NREL Phase VI experiment. Likewise, compared to the FAST code, the general trend for the angle of attack and lift coefficient was well reproduced.Among rotational motions, the effect of platform pitch motion was more significant. Moreover, the impact of yaw motion was noticeable on the axial thrust. The difference between the velocity of blades in wind direction at the top and bottom of rotor increases the complexity and causes more oscillation in forces on the blades. Among translational movements, surge motion showed the most considerable influence by far.&nbsp; This is because this direction has the most substituted effect on the velocity triangle of the wind turbine. Furthermore, the frequency of power and thrust was almost identical to the frequency of the platform movement. The peak happened when the platform is in its initial position where the platform motion velocity is on its peak.Finally, it is shown that the movement of the wind turbine has a considerable effect on its wake characteristics. The gap distances between wake rings can also change wake interactions and, for the case with platform pitch motion, the condition of the wake is even more complicated as such distance is not the same in all angular sectors. These results are important not only for the investigation of aerodynamic performance of the wind turbine but also for fatigue loading calculation and turbine control. The paper shows that the applied ALM method is highly useful for simulation of the wake behind offshore wind turbines and the complex phenomena in the wake due to platform oscillation.

Development of an Actuator Line Model for Simulation of Floating Offshore Wind Turbines

Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2021/volume/85109

ASME 2021 Power Conference (POWER2021)

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