Introduction and Motivation: Offshore fish cages are of strong interest due to availability of open space and the potential in reducing adverse effects on the environment. However, offshore fish cages also introduce significant technological challenges to withstand the harsh environment of the sea, provide sufficient water exchange with the sea while minimising any adverse effect on the sea environment (from altering sea currents and sediment to waste disposal and disease transmission). To assist the design of such a fish cage system, an accurate high-fidelity model of the cage’s hydrodynamics and structural response is highly desirable. In this paper we present a new advanced high-fidelity computational model of the fluid-structure interaction (FSI) of a full fish cage moored and floating on the water surface, and use it for hydrodynamics-structural analysis.
Methodology: The computational fluid dynamics OpenFOAM software has been modified to simulate the moored floating fish cage FSI, where unsteady RANS or Large Eddy Simulation along with a volume of fluid is used as the flow solver. The structural solver includes 1D line (beam, cable, mass, spring) elements modelling the net, collar and mooring lines. Following our previous submerged cage study [1], an immersed boundary method is used to model the load and displacement interaction between the fluid (water) and the fish cage structure. Second order accuracy in space and implicit first order accuracy in time is preserved.
Results and Analysis: The new solver has been rigorously validated against several benchmark problems of multiple planar and circular nets subjected to a range of incoming surface waves and currents. The moored system has also been simulated as illustrated in Fig 1, achieving excellent agreement with known experimental results in terms of surface evaluation and mooring line tension forces. The drag force acting on the fish cage subjected to incoming regular surface waves have been analysed to show sharp peaks in the force’s time variation for the collar and broader peaks for the net, indicating load absorption by the system. The effect of water exchange on waterborne diseases transmission will be briefly discussed in the conference using analysis of particle dynamics simulation (based on Lagrangian approach). Finally, the ability of this computational model to simulate adds-on to the system as a small kinetic turbine (wind/water) to supply green energy to the fish cage assembly will also be highlighted.