Numerical simulation of wave overtopping above perforated caisson breakwaters

A two-dimensional numerical model was used to study the wave overtopping performance above perforated caisson breakwaters under regular waves. The turbulent flow was simulated by solving the Reynolds Averaged Navier-Stokes (RANS) equations and the k-epsilon turbulence model equations. In the numerical wave flume, the free surface was tracked by the Volume of Fluid (VOF) method, and a relaxation zone was added to avoid the unwanted re-reflection from the inflow boundary. Based on the ideal gas equation at a constant temperature, the pressure change caused by the compressed air inside the wave chamber was incorporated into the numerical model for accurately simulating the wave motion inside the wave chamber. The numerical results of the free surface elevations inside and outside the wave chamber, the wave overtopping discharges and the reflection coefficients of perforated caisson breakwaters were in good agreement with the experimental data. This means that the numerical model is valid in estimating wave action and overtopping on complicated coastal structures with perforated thin walls. The distributions of flow velocity and turbulence kinetic energy (TKE) around the perforated caissons were clarified, which gave a better understanding of the wave overtopping mechanism above the perforated caisson breakwaters. Numerical results showed that the wave energy transferred by the overtopping flows is much smaller than that dissipated by the perforated caisson breakwater. When the relative freeboard (the ratio of the crest freeboard to the incident wave height) of the perforated caisson breakwater was larger than 0.58, the existence of the overtopping flow has no significant influence on the reflection coefficient. Besides, avoiding the air compression inside the wave chamber can effectively reduce the overtopping discharge. Increasing the caisson porosity or the wave chamber width can also reduce the overtopping discharge above the perforated caisson breakwaters.