Coastal zones have been a dynamic area and most favoured locations utilized for living, leisure, recreational activities, tourism, commerce and many other human activities. Submerged and floating breakwaters have been used as effective systems to protect these zones from wave attack. However, they are only effectively functional if the incident wave height is relatively low. Under such condition, these systems can reduce the wave transmission with significant wave dissipation and hence achieving a desirable tranquillity in designated areas. Therefore, the focus of this research is mainly to investigate the possibility of using a combination of a submerged porous breakwater (SBW) with a floating breakwater (FBW) as an innovative coastal protection system that can provide adequate calm conditions in the coastal zones with minimum visual impact. This study utilizes the open-source CFD model, REEF3D to simulate such wave-structure interaction. This CFD model is based on the RANS equations coupled with the level set method and the k − ω turbulence model. In the present study, the first section deals with the simulation of irregular wave breaking over an irregular bed profile with the use of wave reconstruction method to generate irregular waves. An excellent agreement between the computed results and the experimental data is obtained showing that REEF3D model is capable of capturing the dominant features of the evolution of the wave breaking process, both in the shoaling region and the surf zone. The second section deals with the simulation of regular wave interaction with the SBW. The simulation is conducted using the VRANS method to resolve the porous flow. The wave interaction with the SBW is validated by comparison with experimental data. An impressive agreement between the numerical results and the experimental data is achieved with very small RMSE values. Finally, the validated model is then used to simulate the combination of the SBW and the FBW. Three different cases are investigated with three different spacing between these structures. For each case, five different configurations related to the geometry of the FBW are simulated. It is found out that an effective reduction of more than 90%, on average, of the incident wave height, can be achieved for this combined breakwaters system. This means that a transmission coefficient (Kt) of less than 10% is calculated across this combination. Besides, it is found out that 1.75 FBW length to wavelength (L/λ) ratio produces a very low transmission coefficient (Kt). Further, an effective distance of 1-2 wavelength between the SBW and the FBW subsystems can also result in lower transmission coefficients (Kt).

This study focuses on water-structure interaction of submerged floating tube in homogeneous and stratified flows, as well as gravity current development. The latter was done in order to set up and validate numerical model that treats water stratification properly and in a computationally efficient manner. First part of the study is dedicated to gravity current development. Grid convergence study was performed for simulation of two-dimensional gravity current, which showed that optimal cell sizes for the problem in question are 5 mm and 2.5 mm. In addition, Large-Eddy Simulation (LES) model of turbulence was tested, and its performance in simulation was compared to performance of simulation without any turbulence modelling. It was established that the latter represents gravity current development in two dimensions more accurately. After that, gravity current development in three dimensions was performed with LES modelling of turbulence and without any turbulence model. Both simulations were in good agreement with results of physical experiments, therefore LES model is a preferable choice as it is less computationally demanding. In second part of the project, rigidly fixed horizontal cylinder in flow was considered in order to investigate in-line forces, acting upon the cylinder. At first, model with homogeneous flow was set up and validated. It was established that two-dimensional model with cell size of 2.5 mm and CFL criterion of 0.3 is a good choice in terms of balance between accuracy and computational efficiency. After that, system of two cylinders with varying distance between them was tested. It was established that in-line forces for both cylinders are affected, with influence on the second cylinder being more stark. Third part of the study is dedicated to horizontal cylinder in stratified flow. Parameters of produced internal waves were in agreement with results of physical experiments.

More and more aquaculture structures are installed in open water and further offshore due to the fact that a variety of conflicts have arisen with coastal water, eco-system, and inhabitants who live nearby. They can additionally benefit from a high energy environment, for example, strong currents and waves could disperse fish farm waste. However, an open water environment accompanies longer or steeper waves and harsher weather conditions where the structures could be exerted by a higher environmental loading and damaged by larger displacements or motions. Therefore, non-linear interaction between wave and structure needs to be investigated to get a better understanding of an accidental phenomenon which might impact on structures. The collar structure above nets, shaped typically in a torus, consists of either single or double pipes, usually made of high-density polyethylene. Due to its slenderness and the material properties, the structure should be regarded as a flexible body and treated so in the structural analysis. In order to study its non-linearity, in addition, a coupling between structure and fluid surrounding the structure should be taken into account. In this thesis, a 3-dimensional beam model is used for the collar structure and Newmark method is adopted for the time integration of structural dynamic analysis. To verify and validate a structure solver, which is developed in MATLAB, the results from the solver are examined with reliable sources including literatures and a commercial code, ANSYS Mechanical APDL 19.1. A free floating fluid model is established using an open-source CFD (Computational Fluid Dynamics) solver REEF3D. This fluid solver is capable of solving the Navier-Stokes equations for both water and air. With both solvers this thesis presents the numerical models and these set-ups as a good foundation to tackle nonlinear fluid-structure problem under the assumption the structures are flexible bodies. As a further step from the starting point this structural solver will be integrated into REEF3D to investigate fluid-structure interaction (FSI).