Experimental investigation on the fluid-structure interaction of a flexible net cage used to farm Pacific bluefin tuna (Thunnus orientalis)

Resource protection of Pacific bluefin tuna (Thunnus orientalis) fish catching has become stringent over the past years. Meanwhile, tuna farming in Japan, the largest consumer of this fish, using a circular flexible net cage is continuously growing as a promising business. A stable aquaculture production of tuna can be achieved by understanding the characteristics, such as a clean environment and space in the net cage and flow inside and near the circular flexible net cage. We measured the drag force, cage deformation, and flow field inside and around a scaled net cage model composed of different bottom weights under various incoming current speeds in a flume tank. Results indicated that the drag force and cage volume increased and decreased, respectively, with the bottom weight. Owing to the significant deformation of the flexible net cage, a complex fluid-structure interaction occurred and a strong negative correlation between the drag force and cage volume was obtained. Furthermore, an area where the current speed was often reduced was identified. The intensity of this reduction depended on the incoming current speed. However, in particular cases, this reduction did not occur, or a local acceleration of the fluid was observed. In this work, the depth data of several positions on the net cage were also analyzed to verify if it could be used to estimate the cage volume. The findings of this study can be used to understand and design optimal flexible net cage structures, which can be used for modern aquaculture.

Automated summary

The study conducted drag, deformation, and flow field tests on a circular flexible net cage used for tuna farming, finding that an increase in bottom weight led to changes in drag force and cage volume, and a complex fluid-structure interaction was observed due to significant cage deformation. Additionally, an area of reduced current speed was identified, with the intensity of the reduction depending on the incoming current speed.