| Popis: |
As one of the fastest growing renewable energy sources, wind energy is playing an increasingly important part in addressing the climate change and energy crisis issues the world is currently facing. The abundance of wind resource in offshore areas makes them a popular choice for turbine installation. In the past few years, several floating wind projects have emerged where wind turbines are installed far offshore in deepwater sites on moored platforms. Compared to land-based or offshore fixed-bottom wind turbines, an FOWT is a fully coupled system where the wind turbine with flexible blades and the floating platform with its mooring system interact with each other in wind and waves, which makes old design tools inadequate. This work aims to develop a fully coupled high-fidelity aero-hydro-mooring-elastic analysis tool, and to better understand the sophisticated fluid-structure interactions for FOWTs. The numerical tool developed in this work takes advantage of the open source CFD toolbox OpenFOAM to accurately solve wind turbine aerodynamics and floating platform hydrodynamics, and utilises the open source MBD code MBDyn for structural dynamics within a multibody framework while modelling flexible bodies based on a nonlinear beam theory. Coupling of these two solvers is achieved by establishing an interface library to exchange data with the help of the TCP/IP protocol. Additionally, to tackle the complex mesh movement in FOWT simulations, a mesh motion solver is developed in OpenFOAM by combining the sliding mesh technique and the dynamic mesh morphing method. A mooring system analysis module comprising a quasi-static method and a lumped-mass based dynamic approach is also implemented to simulate mooring lines in an FOWT system. A series of test cases is firstly studied to validate the various features of the tool, including basic fluid flow solving, modelling of wind turbine aerodynamics, hydrodynamic analysis of a floating structure with its mooring system, dynamic analysis of a riser or mooring line and coupled analysis of flow induced vibration of a flexible beam. The developed tool is then applied to analyse FSI problems of FOWTs under three different scenarios. Firstly, a coupled aero-hydro-mooring analysis is carried out for the OC4 semisubmersible FOWT under regular waves and uniform wind speed. Blade flexibility is ignored, and mooring lines are solved using the quasi-static method. Interactions between the moored platform and the wind turbine are investigated, focusing on of platform motion on the aerodynamic performance of the wind turbine and the impacts of wind turbine aerodynamics on the responses of the floating platform and its mooring system. Subsequently, an aeroelastic analysis is conducted for the NREL 5-MW offshore wind turbine with flexible blades under uniform wind speed. Effects of blade flexibility on wind turbine aerodynamics and structural responses are studied using the developed CFD-MBD tool. The floating platform supporting the turbine is not directly modelled for simplicity and the influence of platform motion responses on the turbine are analysed via imposing a prescribed surge motion to the turbine base. Fully coupled aero-hydro-mooring-elastic analysis is lastly carried out for the OC4 semi-submersible FOWT under a combined wind/wave condition to demonstrate the capabilities of the developed CFD-MBD tool. Responses of the floating system are investigated in terms of platform hydrodynamics, mooring system dynamics, wind turbine aerodynamics and blade structural dynamics. Interactions between the FOWT and fluid flow are analysed by visualising results obtained via the CFD approach. |
