NUMERICAL INVESTIGATION OF THE FLOW THROUGH AND AROUND A NET CAGE

Structure and design of fish cages can be improved by the knowledge of the flow pattern around and inside the net cages. To address this problem, commercially available computational fluid dynamics (CFD) software is used to analyze this problem by calculating the drag and the flow velocity distribution around cylinders with different porosities. The results of these simulations are compared with the data from experiments which have been previously published. Aquaculture cages are very large structures that consist mainly of netting, which can be approximated by small cylinders connected at knots. But due to the large number of these cylinders (millions for a single salmon farming cage), it is computationally expensive to model the exact geometry. Bio fouling is another factor which is of particular interest as fouled nets(lower porosity) can significantly reduce flow of well-oxygenated water reaching the fish during normal rearing conditions. Therefore the numerical approach used to simulate the flow through and around the net cage is to consider it as a circular cylinder with a porous jump boundary. Drag coefficient and flow pattern are compared with available experimental data. Vertical cylinders are used for this study. Different porosities have been used for the simulations as for the experiments (0%, 75%, 82% and 90% open area) in order to simulate the impact of the fouling on the load of the net structures and the flushing of the cage. The results show that a porous jump with a pressure drop proportional to velocity squared has the best agreement with measured data.