Numerical Simulation Of Complex Flows Around Floating Offshore Wind Turbine In Winds And Waves

The present thesis deals with the dynamic response prediction of the floating offshore wind turbines(FOWT)due to the coupled aero-hydro-mooring effects under combined wind and wave conditions.Several essential problems show great challenges in the dynamic response analysis:(i)accurate aerodynamic loads prediction of unsteady wind turbine.In aerodynamic simulations,the interaction effects from the dynamic motions of floating platforms should also be taken into account in addition to tower shadow and shear wind effects.(ii)precise simulations of hydrodynamic responses of its supporting platform.Several important issues should be included such as the wave loads,mooring forces,as well as the extra forces from the rotating wind turbine(iii)multi-level motions of floating platform and turbine blades.The coupled effects makes the dynamic response prediction of the FOWT more complex to be dealt with due to thedynamic responses of the supporting platform and the rotational motion of thewind turbine around its axis(iv)coupled aero-hydro-mooring simulations of an FOWT in complicated wind-wave conditions.A coupling aero-hydro-mooring simulation of a whole FOWT system in is a great challenge when considering the sophisticated environmental loads and the complex interaction between the turbine aerodynamics and the platform hydrodynamics.It can be seen that the coupled effects due to the complex emvironmental loads and the interaction between the wind turbine and supporting platforms motions makes the dynamic response analysis of the FOWT more diffuclt,which is also essential problem to be solved in the present work.This thesis focuses on studying the coupled aero-hydro-mooring effects on an FOWT under combined wind and wave conditions.This was carried out by using the extended in-house code naoe-FOAM-SJTU which was developed based on the open source CFD platform OpenFOAM and overset grid technique.The wave generation module was integrated to the full-domain simulation of the whole wave-wind field.These improvemnets are aimed to simulate the complex flows around floating offshore wind turbine in winds and waves for further dynamic analysis of FOWTs.Accurate prediction of the aerodynamic performance of an FOWT is a key point.A series of aerodynamic studies on a NREL-5MW baseline wind turbine were carried out based on OpenFOAM with the help of the overset grid technique.The aerodynamic simulations of a NREL-5MW wind turbine consisting of three turbine blades,a hub and a nacelle,were conducted under uniform wind conditions.The corresponding results of erodynamic loads were compared with that obtained from other numerical tools to validate the present code of modelling wind turbine aerodynamics.The aerodynamic response analyses of the wind turbine were carried out under high wind speed conditions to study its performance under extreme wind conditions.The tower shadow effects were further studied by performing unsteady aerodynamic simulations of a coupled wind turbine and tower structure with various distances.Then,the shear wind effects are investigated by conducting unsteady aerodynamic simulations of the NREL-5MW wind turbine under various shear wind conditions with different distribution types.Lastly,further validation was carried out with respect to the dynamic response of the supporting platform effects on the aerodynamic performance of a wind turbine by setting an additional periodical sinusoidal surge or pitch motions on the wind turbine.Numerical analyses were conducted in the extended solver by simulating complex flows around the FOWT with a Spar type floating platform as well as a semi-submersible floating platform under combined winds and waves conditions.First of all,the wave generation module of naoe-FOAM-SJTU solver was validated by generating regular waves in a numerical wave tank and comparing numerical results with analytical solutions.Furthermore,this was validated by the free decay tests of the DeepCWind platform in calm water as well as hydrodynamic motion responses in regular waves.Based on the aerodynamic studies of the NREL-5MW wind turbine,the impacts of the wind turbine were simplified as equivalent loads,which were then applied to the floating platform in order to preliminarily analyze the influence of turbine aerodynamics on the hydrodynamic performance of its supporting platform.The dynamic performance of the coupled FOWT system was studied by analyzing the numerical results under different wind-wave conditions.The coupling effect between the aerodynamics of the wind turbine and the hydrodynamics of the floating support platform was investigated.This was achieved bycomparing the aerodynamic loads on the turbine blades and the 6DOF motions of the floating support platform in both coupling and non-coupling simulations.This is aimed to provide guidance for future studies and designs on FOWTs.Decoupling analyses between the hydrodynamic responses of floating platform and the aerodynamic loads on turbine blades were further carried out for investigating the coupling effects between them.Firstly,the rotating turbine blades effects were simplified as equivalent loads on the supporting platform.The simplification was made based on previous aerodynamic numerical results of turbine blades.This is aimed to fully understand the aerodynamic effects of turbine blades on the platform motions.Secondly,the floating platform effects were simplified as the prescribed displacements which were further applied in the aerodynamic simulations according to the hydrodynamic responses.This is aimed to study the platform motion efffects on the aerodynamic performances of turbine blades.Therefore,the differences were invesitigated with respect to the aerodynamic loads between the Spar and semi-submersible platforms.