The fully-coupled aero-hydrodynamic simulations of a floating offshore wind turbine consisting of a NREL-5MW baseline wind turbine and a semi-submersible floating platform are conducted. The three-dimensional Reynolds Averaged Navier-Stokes (RANS) equations are solved for the coupled aero-hydrodynamic numerical simulation. The in-house code naoe-FOAM-SJTU, which is based on OpenFOAM and overset grid technology and developed for ship and ocean engineering, is employed. Aerodynamic loads on wind turbine are predicted. With the directly viscous simulations, detailed flow information around the turbine blades is available. The coupling effect on the aerodynamics of wind turbine from the platform motion is investigated.
Floating offshore wind turbine (FOWT) has become more attractive as a type of wind energy absorption device in recent years. Due to the additional motion induced by supporting platform, the aerodynamics of the turbine blades turns more complex and unsteady. Accurate prediction of aerodynamic loads and performance of FOWT with coupling effect of floating platform motion becomes one common challenge in the designing of FOWT.
Aerodynamic performance of wind turbine operating in uniform inflow wind condition has been well researched. However, the aerodynamics of a FOWT is much more complicated. Tower shadow effects (Dolan, 2006) and shear wind effect (Thiringer, 2001) both lead to periodical oscillations on aerodynamic forces of wind turbine and asymmetry of the wake flow. And the motion of the floating support platform also induces variation of aerodynamics with motion period (Tran, 2016; Vaal, 2014). To accurately predict the aerodynamics of a floating offshore wind turbine, the fully coupled aero-hydrodynamic simulations should be conducted instead of simplified models.
To meet the technique requirement for FOWT design, fully coupled aero-hydrodynamic simulation solvers for FOWTs have been developed. In earlier studies, most numerical tools for coupling simulation of FOWT are developed based on blade element momentum (BEM) method for aerodynamic simulation and potential flow theory for hydrodynamic computation (Cordle, 2010). As BEM is an empirical method, these solvers achieve quite efficiency computations. However, some researchers (Sebastian, 2013) suggested that the BEM is still questionable in unsteady aerodynamic prediction for FOWTS with various correction models (such as Glauert correction, skewed wake correction, etc.). Additionally, the hydrodynamic simulation based on potential flow theory restricts the usage for more accurate predictions with the viscous effect. With rapid development of compute technology and computing methods, fully coupled studies have been conducted with CFD method are conducted. Tran (2015) has conducted the fully coupled aero-hydrodynamic analysis of a semi-submersible FOWT using a dynamic fluid body interaction approach with Star CCM+ software combined with overset grid technique. Based on OpenFOAM package, Liu (2017) established a fully coupled CFD analysis tool for FOWTs and studied the coupling effect of the OC4 DeepCWind semi-submersible FOWT.
