# Monitoring-System Development for a Bottom-Set Gillnet through Time-Domain Dynamic Simulations
## 2 Numerical Model
### Time-Domain Numerical Simulation  
The bottom-set gillnets were numerically modeled using OrcaFlex, a well-known commercial program [24]. In this study, the gillnet models were entirely created using line and 3-DOF-buoy models, as shown in Figure 2. A 3-DOF-buoy model was introduced for modeling other components such as floats, weights, and location buoys, as well as a convenient connection of line models. The Morison-force model for a moving object was employed to estimate the external force of the entire gillnet. In the Morison equation, the added mass coefficient was fixed as 1.0 for the entire line model [25], which corresponded to an inertia coefficient of 2.0. The Morison forces were calculated at each time step at the members' instantaneous locations and inclinations using the instantaneous normal cross-flow relative velocities and accelerations. The drag coefficient for the net should represent the physical net well, and the formulation suggested by [PERSON] et al. [26] was adopted in this study. On the other hand, the drag coefficient for mooring lines and ropes was fixed as 1.2 [27]. The equations and detailed descriptions can be found in [PERSON] et al. [13].  
Creating an equivalent net model is one of the most important parts of developing a numerical model. If the numerical model is to realize the physical net as it is, significant computational time is unavoidable due to the massive number of lines and 3-DOF-buoys that may represent connecting knots. Thus, to reduce the number of line and buoy elements, an equivalent net model is needed, which is much more efficient in terms of modeling and computational time. In the present study, the wet weight as well as the projected area were matched between the physical and numerical nets. When the projected area was matched to produce equivalent viscous drag forces, the numerical net became heavier and stiffer than the physical net. The heavier numerical net was compensated for by changing the buoyancy in the 3-DOF-buoy model used to connect line models. In addition, the issue related to larger axial stiffness can be solved by introducing a modified Young's modulus, as discussed by [PERSON] [28]. The inertial force term has a much smaller magnitude compared with the drag force term considering the large value of the Keulegan-Carpenter (KC) number and given wave conditions [29]. In this case, the inertial force term in the Morison equation can be neglected. [PERSON] and [PERSON] [10] have thoroughly explained the detailed procedure used to develop an equivalent net model. Except for the net, the other components were modeled as they are. For the convergence-test purpose, two numerical models, which are a cruder model (Model A), as presented in Figure 2, and a finer model (Model B), are created. Model A is made up of 568-line and 308 3-DOF-buoy models while Model B has 1831-line and 971 3-DOF-buoy models.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Item**} & \multicolumn{1}{c}{**Value**} & \multicolumn{1}{c}{**Unit**} \\ \hline Head and ground ropes, mooring lines & \begin{tabular}{c} Material \\ Diameter \\ \end{tabular} & \begin{tabular}{c} Polypropylene \\ 10 \\ \end{tabular} & - \\ \hline Floats & \begin{tabular}{c} Material \\ Volume \\ \end{tabular} & \begin{tabular}{c} Plastic \\ 12 \(\times\) 17 \(\times\) 39 \\ \end{tabular} & \begin{tabular}{c} - \\ mm\({}^{3}\) \\ \end{tabular} \\ \hline Weights & \begin{tabular}{c} Material \\ Size (volume) \\ Mass \\ \end{tabular} & \begin{tabular}{c} Lead \\ \(\phi\) 14 \(\times\) 30 \\ \end{tabular} & \begin{tabular}{c} - \\ mm\({}^{3}\) \\ \end{tabular} \\ \hline Buoys & \
\begin{tabular}{c} Material \\ Size \\ \end{tabular} &
\begin{tabular}{c} Styrofoam \\ 50 \\ \end{tabular} & - \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._