Numerical investigation on flow field and drag force distribution of a semi-submersible truss net cage with compartmentalization

The internal compartments of large deep-sea semi-submersible truss net cages are of great significance to living environments for farmed fish and facilitating efficient harvesting. The present study focuses on semi-submersible truss net cages with compartments, and combines computational fluid dynamics (CFD) theory and porous media methods to explore the flow speed, drag force distribution and torque of the truss net cage. The results indicate that the internal compartments of the truss net cage have a significant effect on the flow speed distribution. Among three cases, including no netting inside net cage (C0), radial netting configuration (C1) and nine-grid layout (C2), C1 has the most significant damping effect on the flow speed, which is attenuated to 58.1 % inside the net cage, followed by C2, which is attenuated to 49.6 %, and C0 has the least effect, attenuating to 43.3 %. In addition, the drag force exerted on the internal nets and the side nets is the largest, accounting for 22.5 % and 20.1 % respectively, followed by the thick column, accounting for 11.8 %, and the middle cross brace is the smallest, accounting for 3.1 %. As for total force, the drag force exerted on the C1 is higher than that on the C0 and C2, but the difference between the drag forces exerted on the C1 and C2 is small, less than 1.5 %. Furthermore, the torque exerted on the C1 and C2 is higher than the torque on the C0, while the torque amplitude of the C1 and C2 is not much different. We also found a phenomenon that the presence of the internal nets causes the position where the torque direction changes from clockwise to counterclockwise to shift by about 1.0 cm along the positive z direction. The present study comprehensively analyzes the changes in the flow speed, drag force and torque of the semi-submersible truss net cage with compartmentalization, which is of great significance for guiding the design and optimization of practical aquaculture engineering equipment.