Breakwaters are one of the most common coastal protection structures which their main application is to reduce wave energies and create a calm basin for berthing of the ships in ports. A simple approach to achieve this is to build a solid wall against the waves which are known as caisson breakwaters. Given that caisson breakwaters are used in deep waters, it is important to determine optimum dimensions for this structure in various hydraulic and geotechnical conditions. Hence, several studies have been carried out on hydraulic responses such as flow pressure on the caisson wall and wave overtopping. Reviewing previous studies indicate a rise in the use of numerical models in modeling interaction of waves and marine structures.
In this paper interaction of irregular waves with normal and perforated-wall caisson breakwaters using FLOW-3D is studied. In this paper, the appropriate numerical model has been selected and then the model setup is explained. After the model setup, sensitivity analysis, calibration, and verification of the model results with available laboratory test data are presented. When the model performance is verified, the interaction of irregular wave with the perforated-wall caisson breakwaters is evaluated.
Among many available CFD tools which have the ability to model the interaction of fluid and structure, FLOW-3D is a strong software in this field. This software has been developed for studying one, two, and three dimensional behaviors of fluids in a wide range of applications. Previous research results indicate that it is well capable in simulating fluids around marine and hydraulic structures. The governing equations in this model are the continuity and momentum equations. In addition to the governing equations, this model uses liquid volume method for modeling free surface and also has the ability to use various disturbance models such as RNG, k-ε, and, LES. The most important aspect of FLOW-3D which has increased the use of it in modeling coastal processes, is its ability to use wave border conditions as regular and irregular (spectral) waves.
Calibration of the numerical model is performed in order to achieve best compatibility between measured data and numerical model calculations for simulated variants. Since time history laboratory results of water level alterations with pressure for irregular waves (JONSWAP spectrum) with HS=0.18m and TP=5s are available, calibration of the model has been performed on this wave specifications. Calibration results show suitable correspondence of numerical model results with lab results. Hence, to ensure that the model will work properly in different conditions, it has been verified for wave parameters by controlling all other parameters. In this section, only the overtopping has been checked for two wave conditions with HS=0.18m and TP=5s and HS=0.21m and TP=3s. The generated model has a good capability to simulate irregular wave interaction with caisson breakwaters.
Introducing new geometry for caisson breakwaters for improving their hydrodynamic performance has been a point of interest in recent studies. In this paper, numerical modeling of caisson breakwaters has been studied considering the geometry used in the model presented by Suh et al (1995), where the performance of perforated-wall caisson breakwaters with four different geometries has been compared with traditional caisson breakwaters. The main purpose of this paper is to study the effect of wave chamber and the pattern of holes placed in the wall on overtopping and reflection of waves from caisson breakwaters.
In the first part, the effect of perforated-walls on overtopping values is evaluated. The results show that the wave chamber has a significant effect on wave overtopping, where a 45 percent increase in the width of the wave chamber will lead to approximately 50 percent reduction in overtopping. In the next step, the effect of the number of holes in the wall has been studied by controlling the dimensions of the wave chamber and the overall area of the total holes. The results indicate that the increase in number of holes has no effect on the overtopping performance of these breakwaters. In the end it was observed that reducing the area of the holes into half has no effect on overtopping either. It can be concluded from this last step that most of the wave energy is concentrated near the water surface and the study should be focused on this part.
In the second part, reflection of waves in front of caisson breakwaters is studied. In normal caisson breakwaters (with no holes), wave reflection coefficient is nearly to 1. Also, calculating wave reflection spectrum indicates the formation of short period waves with high energies. By making the caisson breakwaters perforated, the wave reflection coefficient is reduced approximately 15 percent and the secondary waves are not formed in front of the breakwater.
The study results indicate that using numerical models can be effective and useful in a wide range of topics related to waves and marine structures. It also shows that making the front wall of caisson breakwaters perforated will improve hydraulic performance of these structures. In general, perforated-wall caisson breakwaters will have less overtopping and they will affect the wave reflections significantly.