Numerical Modeling of Fish Cage Structural Responses in Regular and Irregular Waves Using Modified XPBD

Abstract The structural response of gravity-type fish cages in waves and currents is numerically investigated. A Morison model is adopted to compute the hydrodynamic forces acting on the net ropes and a screen model is used to compute the hydrodynamic forces on the net twines. The irregular waves are created based on the JONSWAP wave spectrum. The structure of the aquaculture nets is modeled using a modified extended position-based dynamics (XPBD) algorithm, which was first used in the simulations of cloth and other flexible bodies in games. The proposed method is effectively validated against the published experiment data to ensure its accuracy and reliability. The static response of the fish cage at various current speeds and the dynamic responses of the fish cage in regular and irregular waves are analyzed. Results show that the volume of the investigated fish cage is reduced by 30% in a current with a speed of 0.5 m/s. The tension at the bottom net can reach about 1.8 times of maximum tension of the case without current. In the regular or irregular waves, the drag force of the fish cage is higher than that in a uniform flow with the same current speed and the fish cage volume decreases. The increase in drag force rises from the coupling between the waves and currents. Therefore, the hydrodynamic model including the regular and irregular modeling, Morison model, and screen model is successfully implemented into the framework of the present modified XPBD algorithm to provide the structural response of fish cages in waves and currents.