Abstract
The study of wave energy converters (WECs) is largely conducted with only experiments and low- or medium-fidelity simulations. Rigorous computational fluid models are ignored either for lack of computer resources or computation speed. This research status of lacking high-fidelity simulations can be changed with the development of advanced numerical algorithms and access to modern supercomputers. This study conducts high-fidelity simulations of the wave-WEC interactions to predict the energy output of a point absorber WEC in response to incoming waves. The Navier-Stokes equations are simulated using an in-house code to provide a detailed description of the flow field at every time step.
Our code addresses various WEC features, such as two-phase fluid solvers and fluid-structure interaction solvers, allowing for fully realized WEC simulations. The air-water interface is modeled by a coupled level set volume-of-fluid (CLSVOF) method. The level-set accurately tracks the air-water interface's geometry, while the volume-of-fluid method ensures mass conservation. Depending on the grid size used, the two-phase solver can track not only the wave interaction with a floating WEC but also the bubbles and droplets created in wave breaking. The fluid-structure interaction solver uses a discrete immersed boundary method, which allows for a standard Cartesian grid without the need for re-meshing for the moving WEC. Using a static grid significantly saves computational time and allows for an even higher resolution in the simulations. Waves are generated at the inlet of the computational domain and damped in a sponge layer near the outlet of the domain to prevent artificial wave reflections.
Based on our simulation results, nonlinear effects such as vortex shedding are captured and quantified using the force partition method and vortex identification method. These effects would be ignored in lower-fidelity simulations. To investigate the energy extraction by the WEC, a comparison is made between complex conjugate control and sliding-mode control, two promising methods for optimizing WEC performance.