Offshore wind energy developed rapidly in recent years. Due to the platform motions causing by ocean waves, the aerodynamics of floating offshore wind turbines (FOWT) show strong unsteady characters than onshore wind turbines. The widely used methods to investigate the unsteady aerodynamic performance of wind turbine under dynamic motions are Blade Element Momentum (BEM) and Free Vortex Wake (FVW) methods, which have weak ability to predict the three dimensional and stalled flow characters of wind turbine. CFD simulations using overset or dynamic mesh methods also have been used for FOWT aerodynamic investigations. However, the mesh deforming or reconstruction methods are usually suitable for small movement of wind turbine blade. In this paper, a dual-sliding mesh method is employed to simulate the unsteady aerodynamic characters of an offshore floating wind turbine with supporting platform motions using Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations. Both rotor rotation and platform motions are treated as rigid angular motions. The computational domain was divided into background domain, pitching domain and rotational domain. The relative motion and data exchange were simulated using sliding mesh method. The method was validated using dynamic yaw simulation of the NREL Phase VI wind turbine.

For large scale wind turbine case, the NREL 5MW reference wind turbine is used. The platform pitching and rolling motions are considered. The pitching and rolling motions of floating platform are simplified in the form of a prescribed sinusoidal function. The platform pitching and rolling were considered separately at first. Different conditions with two amplitudes and two frequencies of pitching and rolling motions were investigated. Then, the combined motions with both platform pitching and rolling were investigated. URANS were performed with full structured mesh for wind turbine rotor using commercial software FLUENT. The platform motions were set using User Defined Function (UDF). In total, the mesh has about 11 million grid nodes. Transitional Shear Stress Turbulence (T-SST) model was employed.

The simulation results were compared with BEM method and free wake method results. Both steady status and dynamic pitching processes are investigated. The variations of wind turbine aerodynamic load, as well as the aerodynamic character of airfoils at different spanwise positions, were obtained and analyzed in detail. The simulations results show that the motions of platform introduce remarkable influence on the wind turbine aerodynamic performance. The platform pitching has much larger influence on the wind turbine power and thrust than the platform rolling. The combined motion can reduce the fluctuation caused by platform pitching. The dual-sliding mesh method shows potentials to investigation the dynamic process with multiple rigid motions.


Investigations of Unsteady Aerodynamics of an Offshore Floating Wind Turbine With Platform Motions Using Dual-Sliding Method

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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