Flow Field Inside and Around a Square Fish Cage Considering Fish School Swimming Pattern

Abstract The interaction between fluid and fish cage with stocked fish is extremely complex, including fluid and structure, as well as fluid and fish swimming behavior. The on-current swimming pattern of fish schools was found toward the incoming flow in the previous laboratory studies, which is different from the circular swimming pattern commonly observed in the farming site. In this study, a pseudo fish school structure model (PFS) was proposed to reproduce the five circular swimming patterns of farmed yellowtail, and to investigate the influence of fish school behaviors on the flow field inside and around a model square fish cage in laboratory experiments. The results showed that the drag force acting on the square fish cage increased with the increase of the current speed for all fish school swimming patterns, but no clear difference was observed between the fish school swimming behavior patterns. Overall, the drag force of the square fish cage considering the farmed fish behavior decreased by 11.8%, compared to the drag force of the fish cage without PFS. The current speeds inside and downstream of the fish cage increased almost linearly with increasing current velocities. Compared with the case of the fish cage without PFS, the current speed inside the cage under motionless closely PFS (C0), revolving closely PFS (CR), motionless loosely PFS (L0) and revolving loosely PFS (LR) conditions changed by 10.8%, 9.4%, 65.8% and 39.7%, respectively. In addition, compared to the case of the fish cage without PFS, the current speeds under C0, CR, L0 and LR conditions decreased by 89.8%, 16.3%, 58.2%, and 31.9%, respectively, at 16.0cm downstream from the fish cage, and decreased by 69.2%, 19.4%, 62.7% and 26.3%, respectively, at 63.6cm downstream from the fish cage. Furthermore, the current speed distribution and relative horizontal turbulence intensity distribution inside and around the fish cage under different fish school swimming pattern was discussed. In the future, we will use live fish to conduct experiments to evaluate fish school models.