WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2015 57 by a reduced current force (Zhan 2007). For this reason, the deflections of the rear panel were less than on the front panel at all current speeds. Both cage models had a similar distortion with an increase in current velocity but the rear panel deflected less in Model 2. At a current speed of 0.3 m/s, Model 1 deflected 25o and Model 2 deflected 30o. At 0.6 m/s, Model 1 deflected 42o and Model 2 deflected 47o. In both cage models, front deflection angles were greater than rear deflection angles. The greater volume reduction for the cage with bottom ballast weights (Model 2) is due to greater deflection of the front panel along with less deflection of the rear panel (Fig. 8). This indirectly attributed to greater flexibility of the bottom panel that is resulted by dispersed placement of ballast weights. Moreover, the greater outward deflection of the rear panel of the cage model with the sinking collar could be the result of the greater inside water velocity caused by an increased porosity of the front panel. The greater porosity of the mesh resulted from the upper plastic collar and lower rigid rings that drew the net system apart. In contrast, the lower outward deflection of Model 2 might be because of the lower porosity of the front panel that restricts water from entering. When the incoming flow confronted the net structure, the net began to shrink; cage meshes overlapped each other, resulting in reduced outward distortion of the rear panel. A significant difference in degrees of distortion between (CONTINUED ON PAGE 58) TOP, FIGURE 8. Deflection of different sections of net cages at various current conditions. BOTTOM, FIGURE 9. Cage Model 1 and Model 2 in the wave tank. corresponding net panels of the two cage systems arose mainly from the geometry of the ballast weights. As current flows and acts on the cage system, the cage will experience two types of forces. First is a lifting or shearing force, the direction of which is determined by the current velocity and position vector of the mesh (Lee et al. 2008). Second is drag force, which acts horizontally in the direction of current. The drag force is greater than the lifting force and thus the strength of the current in the mooring system is contributed mainly by the drag force. Response of cage models to waves. A regular wave was generated with consistent wave height and period and applied to each cage model. In wave tests, both cage models were held by a two-point mooring system. Two load cells were attached to the front and rear sides of each model to measure tension (Fig. 9). One is aligned in front of incoming regular wave and the other is on the opposite side of the cage. The front line had greater loads in response to waves. Front and rear mooring tensions for Model 1 were greater than corresponding mooring tensions of Model 2 under the same wave conditions (Fig. 10).The reduction of tension on the back of mooring lines might be due to shielding by the netting system. With an incremental increase in wave height, FIGURE 10. Time series graph for wave load on mooring system for the cage with a sinking collar (a) and the model with independent ballast weights (b).
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