WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2015 55 Detailed material descriptions and physical properties of each elemental structure of the cage models are provided in Table 1. Studies were carried out at the National Fisheries Research and Development Institute in Busan, Korea. Tests involved two main activities: mooring tension measurement and deformation analysis. Positions of markers were detected on the periphery of the network at a specific water flow rate and were then used for deformational analysis. The operation later was used to approximate the deformed mesh in a two-dimensional view. Current tests were performed in a water channel with dimensions of 8 m long × 1.4 m deep × 2.8 m wide (Fig. 2a). Water flowed uniformly in a canal system in a closed circulation. The wave tank had geometrical sections 85 m long × 3.5 m deep × 10 m wide (Fig. 2b). The wave tank is capable of producing a regular wave through a controlled system with an optimal wave height of 0.5 m. Cages were connected to a two-point mooring system consisting of two dead-weight stainless-steel anchors laid on the floor of the wave tank and 5 m from each other. For the current test, both models were configured with a single-point mooring (Fig. 3). Two-dimensional displacement of selected points on the network in horizontal and vertical directions under the influence of water current was determined by digitizer. Photographs were taken to compare and quantitatively characterize cage deformation. Tension in mooring lines resulting from wave and current loads were measured. Cage volume was assumed to be proportional to the cross-sectional area of the side panel (Lader et al. 2008). Cross-sectional area of cages was determined based on the position of selected points on the net. Volume reduction was calculated as the ratio between the cross-sectional area of the cage at a specific flow rate and the initial cross-sectional area. The second descriptive parameter was the deflection angle of different sections of the cylindrical net. The front panel is the half side of the net facing towards the water flow and the aft panel is the half section of the net-cage facing the direction of water flow. The bottom section is aligned along the current direction; a large portion of panel is shielded by front twines around its circumference, so its contribution to the reduction in volume was considered to be negligible. Results and Discussion Response of cage models to water current. When current flows horizontally at a specific velocity, cages experience movements of translation and rotation. Deflection and deformation movements were also observed. The degree of deformation of each cage model was different. Increasing flow rate resulted in a greater deformation and high inclination angle of the floating collar. For Different methods of analysis were used to determine responses to uniform currents and regular waves. Study Methods Two models of floating circular cages were evaluated. One (Model 1) was a circular net-cage with a weighted collar at the bottom and the other (Model 2) was a circular cage with evenly distributed ballast weights at its bottom, with each model having the same mass (Fig. 1). Other material structures, including the net, floating collar system and buoys are identical in both models. (CONTINUED ON PAGE 56) TABLE 1. Physical specifications for model cages. Item Specification Buoy Floating Collar Netting Bottom weight Sinking collar Mooring Line Material Density (g/cm3) Diameter (mm) Material Density (g/cm3) Diameter (mm) Pipe diameter (mm) Buoyancy force (kgf) Material Density (g/cm3) Net depth (mm) Mesh type Mesh size (mm) Twine diameter (mm) Total mass (g) Solidity Material Unit mass (g/piece) Number of pieces Total mass (kg) Material Diameter (mm) Total mass (kg) Material Diameter (mm) ABS (acrylonitrile butadiene styrene) 1.08 200 HDPE (high density polyethylene) 0.953 1375 89 19.32 Nylon 1.14 800 Square 18 1.35 920 0.15 Stainless steel 500 14 7 Stainless steel 15 7 Nylon 3.5 FIGURE 3. Side views of cage arrangement in the flume tank for Model 1 (a) and Model 2 (b) and for the arrangement of the cage in the towing tank (c). A. B. C.
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