The Oscillating Water Column (OWC) is one of the simplest and most studied concepts for wave energy conversion. The commercial scale diffusion of the OWC technology is, however, strongly dependent upon the device optimization. Research at a fundamental level is therefore still required. Analytical, numerical and experimental models are necessary tools for advancing in the knowledge of the system and thus promoting its passage at the commercial level. In this work, a simplified frequency domain rigid piston model has been applied to preliminary select expected ranges of air pressures and air velocities for the instrumental set up of laboratory experiments. The set up of a Computational Fluid Dynamic (CFD) model implemented in the open source OpenFOAM®1 environment is then presented. The multiphase model solves incompressible 3D Navier-Stokes equations, using Large Eddy Simulation (LES) for turbulence modelling, and adopts a Volume of Fluid method (VOF) to track the air-water interface. A preliminary validation of the model with physical tests data is conducted. The numerical approach seems to be promising for an accurate simulation of the OWC device energy conversion process. Hence, the validated model can be a useful research tool for different problems, particularly for systematic parameter studies to extend the range of conditions tested in the laboratory.
A hybrid 2D-3D CFD model is developed for studying water wave loads on a slender pile in a pile group. In the hybrid model approach, a one-way link is established between a model for the far-field and another for studying the fluid-structure interaction in the near field. In the far-field a 2D incompressible Navier-Stokes multiphase solver is considered for the proper reproduction of phase-focused (freak) waves produced in physical experiments. The near-field model is a multiphase 3D CFD model that utilizes compressible Navier-Stokes equations to enhance the simulation of entrapped air compressibility effects during breaking wave impact on structures. Both models use the Volume-Of-Fluid (VOF) method to capture the air-water interface and alternatively a RANS or LES turbulence model. An overlap zone is introduced to both models, in which fluid kinematics and surface elevation are sampled from the far-field model and introduced via a relaxation function to the overlap zone in the near-field model. In the 3D model, the use of a relaxation approach provides absorption for reflected waves from the structure. Further, a procedure is outlined to achieve/enhance the 3D model convergence. This is necessary in case of the development of artificial high velocities at water-air interface at the end of a short overlap (relaxation) zone for wave inlet (or near the boundary if only a wave inlet boundary condition is considered). The model system is developed using the OpenFOAM® framework. The overlap zone is implemented as an extension to the waves2Foam [1] toolbox.
A one-way CFD-CSD coupled model system is presented to reproduce large scale experiments of a caisson breakwater, subject to wave attack. The Computational Structural Dynamics (CSD) model is developed using the finite volume method for the fully dynamic, fully coupled Biot equations. The fully coupled poro-mechanical analysis is handled in a segregated approach in which the skeleton displacement, the pore fluid pressure and the pore fluid velocity (relative to the skeleton) are decoupled at the iteration level. The pore fluid pressure-velocity coupling is resolved using the PISO (Pressure Implicit with Splitting of Operators) algorithm. Two simplifications to the porous media formulations were introduced: (1) neglecting convective acceleration of pore fluid and (2) fully neglecting acceleration of the pore fluid (the u-p approximation). A frictional contact model is implemented to model soil-structure interaction. A multi-surface plasticity model with the Drucker-Prager failure criterion is introduced to model the behavior of sand foundations under cyclic load posed by wave action on the caisson breakwater. An incompressible (constant density) multiphase Computational Fluid Dynamics (CFD) solver is developed for solving flow inside and outside porous media simultaneously using the principle of volume averaged velocity. A seepage model is implemented to model flow resistance of porous media that includes viscous, transitional, inertial and transient terms. An additional term is introduced to the fluid continuity equation to account for fluid mixture (water and air) compressibility (inverse of bulk modulus). The CFD-CSD model system is developed using the OpenFOAM® framework.
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