## 3 Numerical Results: Performance Evaluation of the Stow Net
### Effects of Current Velocities  
Figure 4 shows height- and area-reduction ratios at different current velocities without wave excitations to validate the numerical stow-net model. The reduction ratio is the ratio of decreased value under current to the value in fully stretched state. The trends of net-entrance height and area are compared with those of previous researches [19; 20] as a validation purpose. In the previous research, they considered different types of stow-net designs/materials/solidity ratios and conducted experiments to analyze the behaviors at different current velocities. As shown in Figure 4, the ratios decrease as current velocity rises. Higher current velocity causes higher drag force acting on the net, and the net pulls the head and ground ropes in the current direction decreasing the net-entrance height and area. Since different models/materials are used in the experiments, direct quantitative comparisons are not possible. However, as a qualitative comparison, the general trends (e.g., decreasing ratios with increasing current velocity) in the present numerical simulations coincide with the experimental results. Figure 3: Theoretical JONSWAP wave spectrum and reproduced spectrum from wave time histories using FFT (**a**) and wave time histories generated from JONSWAP wave spectrum (**b**) for \(H_{s}=1.0\) m, \(T_{p}=5.25\) s.  
From this point on, let us consider the dynamic behaviors of the stow net system in steady sheared currents and irregular waves. Higher current velocity contributes to larger submergence depth of the stow net. Once the net floats close to the surface at low current velocity, twisting motions of the assembly might occur by wave excitations that will deteriorate the overall net functionality. The stretching performance of the stow net is also highly dependent on current velocity. Therefore, it is interesting to find the relationships between current velocity and both submergence depth and stretching performance. Figure 5 shows the snapshot of the stow net after 30 min of simulations at different current velocities, under the same wave condition (\(Hs=1.0\) m \(Tp=5.25\) s). Figure 6 shows time histories of net horizontal length, net vertical deflection, and net-entrance area of these cases. Table 3 summarizes the statistical results of these parameters for all simulated cases. Figure 5 clearly demonstrates that for current velocity lower than 0.75 m/s, the stow net floats close to the free surface, and the submergence depth of stow net gradually increases with increasing current velocity. It can be expected that as current velocity is lower than 0.75 m/s, the net assembly is significantly affected by wave excitations, which can be further demonstrated in Figure 6. The time-history results demonstrate that the fluctuations in net horizontal length, net vertical deflection, and net-entrance areas decrease with increasing current velocity. If current velocity is higher than 0.75 m/s, we can observe only small variations in those parameters. The net horizontal length is the longest at current velocity of 1.0 m/s, and at lower current velocities, the current force is not enough to enable the net to fully stretch in horizontal direction, as can be observed in Figure 6. While the net vertical deflection decreases with current velocity, the absolute value is the smallest at current velocity of 1.0  
Figure 4: Height (**a**)- and area (**b**)-reduction ratios at different current velocities without wave excitations for validation. m/s as demonstrated in Figure 6b. Because current velocity decreases with depth, higher current force acts on upper parts of the net and head rope than lower parts of the net and ground rope, which causes vertical deflection. Moreover, as shown in Figure 6c, the net-entrance area decreases as current velocity rises, and significant decreases occur when current velocity is greater than 1.0 m/s. Maintaining the net-entrance area well at high current velocity is directly related to the performance of stow nets.