A Two-dimensional Numerical and Experimental Study of a Floater with Net and Sinker Tube in Waves and Current
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- Institutt for marin teknikk 
In this thesis, nonlinear wave and current forces on a coupled system of a moored floater with net panel and sinker tube is studied. Due to the complexity of the hydrodynamic analysis of a complete three-dimensional fish cage, a two-dimensional approximation is selected to gain insights into the problem. Experiments and numerical simulations including Computational Fluid Dynamics (CFD) are performed in order to solve the problem. First part of the model tests has been carried out to investigate the effects of nonlinear waves on this multi-body system. Ten wave periods and one wave steepness were considered. The net panel was flexible and three solidity ratios, Sn, were selected. Sn is the ratio of the projected area of the net twines on a parallel plane to the net panel to the total area of the net panel. The horizontal and vertical accelerations of the floater were recorded. These were used to find the sway and heave motions of the floater. Forces in the mooring lines at both sides of the model and forces in the connection net twines to the floater were also measured. The measurements were used to validate the developed numerical wave tank. Snap loads on the connecting net twines to the floater and wave overtopping for some wave periods were observed. They were also simulated using the numerical wave tank. Second part of model tests were carried out to consider the current effect on the net panel and sinker tube and also combined wave and current effects on the moored floater, net panel and sinker tube system similar to the first set of experiments. Here, a carriage was used to tow the model with constant velocities towards the incoming waves. Three current velocities were investigated. In this case the mooring line forces on the floater were used for comparisons. The developed numerical wave tank is based on the Constrained Interpolation Profile (CIP) method. Navier Stokes equations are solved for a Cartesian grid. A first order immersed boundary method is used to define the floater and sinker boundary in the domain. Although the net structure is not defined in the Numerical Wave Tank (NWT) due to the complexity of the implementation considering the net twines diameter compared to the floater and sinker tube diameters, its effects is taken into account by using the force screen model. The force model is verified with experimental results of the net panel in current only cases. When the net becomes slack and snap loads happen, a simplified model is developed to predict such loads.