Modelling Sand Foundation Behaviour Underneath Caisson Breakwaters Subject to Breaking Wave Impact

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.