Marine fish farm sites using net cages are gradually moved towards open sea areas. Although new concepts of cages have been proposed to withstand harsh sea conditions, most marine fish farms today still use flexible net cages. For a flexible net cage, the current flow, net deformation, and drag forces continuously interact with each other, resulting in a complex phenomenon. Previous research has mainly focused on the fluid-structure interaction of a single cage. However, at fish farm sites, typical use multiple cages that are connected together. Therefore, the hydrodynamic characteristics of linked flexible cages must be investigated. In the present study, three flexible net cage groups (a single net cage, two net cages arrayed in one column, and three net cages arrayed in one column) were investigated in a flume tank, in order to analyze the hydrodynamic characteristics of the flow and linked flexible net cage, such as the drag force, cage deformation, and flow field inside and around. Based on these results, the fluid-structure interactions of the flexible net cage were discussed.The drag forces and cage deformation of a single flexible net cage were first studied, and their relationships to the current speed were found consistent with existing literature. The averaged current speed (V/V0) inside the single net cage was 0.72 for all incoming current speeds. Furthermore, significant current speed reductions occurred behind the single net cage, at the downstream, for all incoming current speeds. Within the measurement range, the current speed reduction area downstream from the single net cage was almost as wide as the cage diameter, and the length was up to 1.4 times cage diameters along the incoming current direction. The location of this area gradually approached the water surface as the current speed increased. In the case of two flexible net cages arrayed in one column, the differences in drag force occurred when the distance between the two cages was changed. In addition, the current speed incident on the downstream cage tended to decrease, as the distance between the cages increased. The averaged current speeds (V/V0) incident on the downstream cage were 0.54, 0.44, 0.77, and 0.40 when the distances between two cages were 30.0cm, 60.0cm, and 90.0 cm, respectively. In the case of three flexible net cages arrayed in one column, the total drag force of three flexible net cages was 2.2 times that of a single net cage. On the other hand, at the maximum current speed of 50 cm/s, the cross-sectional areas of the first net cage, the second net cage, and the third net cage were 177.10 cm2, 274.19 cm2, and 277.37 cm2, respectively. Overall, the findings of this study can not only help understand and optimize flexible net cage structures but also provide useful information for the configuration of the net cages at the farm sites.

Experimental Investigation of Fluid-Structure Interaction in Linked Flexible Net Cages

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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