Numerical investigation of the effect of nappe non-aeration on caisson sliding force during Tsunami breakwater over-topping using OpenFOAM

Abstract

During the 2011 Great East Japan Tsunami, the world’s deepest breakwaters in Kamaishi and Ofunato, which had been designed to reduce tsunami damage, partially failed after over-topping. The failure was due to a combination of sliding, over-topping induced foundation scour and foundation bearing capacity failure. Hindcasting this failure required detailed two-phase (air-water) Computational Fluid Dynamics (CFD) (OpenFOAM in this case) simulation, though uncertainty in the turbulence model used had a large effect on the simulated forces on the breakwater. This study focuses on understanding the physics necessary to correctly model the problem of breakwater over-topping by the tsunami (Bricker et al. 2013). To correctly model the physical behavior of the system, physical model studies were carried out (Mudiyanselage 2017). However, validation of the numerical model will allow easy up-scaling of the flow physics. In order to have complete flexibility in the CFD code while at the same time have sufficient reliability, OpenFOAM will be used.
As the physical model studies have already been carried out, this study will only focus on replicating the experimental results in the numerical model. A validation procedure will study the effect of grid size (using grid convergence
study) and type of simulation (2-Dimensional/3-Dimensional) used to model the experiment numerically. The final goal of the study is to understand the relationship between the overflowing discharge and the additional force generated due to the non-aeration of the over-flow jet. The entrainment of air from the cavity under the over-flow jet is also of prime interest in this study.
Overall, the numerical results indicate that non-aeration could provide about 17.85% additional force leading to failure by sliding/overturning. It was observed that the forcing on the caisson is not constant but cyclic. This is
a key result from this study affecting the design of the caisson breakwater. Additionally, the aeration mechanism and overflow jet breakup during the flow were also investigated. A mechanism for aeration of the cavity has been proposed as observed in the numerical model results. However, due to the limited scope of the study and time constraints, further investigations with respect to the overflow discharges was not possible.