As animal welfare has received increasing attention from industry stakeholders, ensuring the welfare of farmed fish throughout the aquaculture production cycle has gradually emerged as an important issue. To further safeguard the welfare of farmed fish and improve the hydrodynamic environment within the cage, this study investigated a net cage featuring flow-diversion and current-attenuation characteristics. Focusing on the in-cage flow-field distribution and the velocity-reduction performance induced by the attenuation device, the internal hydrodynamics were characterized in two dimensions. By measuring and comparing velocity distributions at different locations and regions, the flow velocity magnitude and its attenuation pattern along the inflow direction were quantitatively evaluated. A physical model experiment was conducted, and particle image velocimetry (PIV) was employed to quantify the in-cage flow field. The effects of the flow-diversion and current-attenuation device on internal hydrodynamics were systematically analyzed, together with the flow-field distribution characteristics of the cage under different inflow conditions;

Results At an inflow velocity of 0.1 m/s, the flow-diversion and current-attenuation device exerted a pronounced influence on the in-cage flow field. The velocity attenuation in the leeward region, the windward region, and the lateral regions of the device reached 87.8%, 36.4%, and 10.0%, respectively. Experiments conducted under inflow velocities of 0.08-0.24 m/s further compared the stability of the in-cage velocity attenuation along the inflow direction. The results indicate that the cage maintained a relatively stable attenuation performance across different inflow conditions: in the windward region of the device, the attenuation coefficient ranged from 0.6 to 0.7, whereas in the leeward region, the attenuation coefficient exhibited a maximum range of 0.2-0.4. Under the same inflow velocity, the overall in-cage flow velocity under the action of the flow-diversion and current-attenuation device was significantly lower than that in the cage without the device, indicating that the device can effectively restructure the internal flow field and enhance the flow-reduction performance. In addition, the device markedly decreased the mean in-cage velocity, resulting in a more distinct low-velocity region within the cage. Across different inflow velocities, the flow-attenuation effect exhibited good stability, providing predictable and sustained velocity reduction over a given range of inflow conditions. Collectively, these results suggest that the flow-diversion and current-attenuation device represent an effective approach to improving the hydrodynamic environment of net cages, offering a scientific basis for structural parameter optimization and engineering design, and supporting reduced swimming energy expenditure of fish as well as enhanced habitat suitability and production stability.