# Regular Wave Effects on the Hydrodynamic Performance of Fine-Mesh Nettings in Sampling Nets
## 4 Discussion
Wave transmission coefficients decrease with the increasing wave height at a constant wave period, and nettings with a higher solidity ratio exhibit more pronounced wave attenuation. These findings align with previous studies [23; 27; 33; 38]. This behavior can be attributed to the increase in the water particle velocity with the rising wave height, which leads to higher wave forces at a constant wave period. The increased wave force results in a stronger interaction between the wave and the netting, thereby enhancing the wave attenuation effect. However, the wave transmission coefficients initially increase and then decrease with the increasing wave period for fine-mesh nettings at the same wave height (See Figure 10). [PERSON] et al. found that wave transmission coefficients increase with longer wave periods [38], while [PERSON] et al. reported that, for a single net cage, transmission coefficients increase initially and then decrease with the wave period [27]. In contrast, for multiple net cages, the transmission coefficients continue to increase with the wave period. [PERSON] et al. observed no significant statistical difference in the wave transmission coefficients for three similar wave periods [33]. These discrepancies may stem from the relatively large wave periods considered in this study (all greater than 1 s), where waves do not pass directly through the center of the netting. As a result, the damping effect is more pronounced when the wave trough interacts with the netting during wave propagation, particularly at higher wave periods. Additionally, the wave transmission coefficient is strongly influenced by the attack angle of the netting [38]. Future studies could further investigate the damping effects of varying attack angles under different wave conditions. Developing a numerical model of the netting is crucial for accurately simulating its hydrodynamic behavior under wave conditions. In this study, the small mesh size of the fine-mesh netting would require an extremely high number of computational elements, resulting in impractically long simulation times. To overcome this limitation, a porous media approach was adopted to represent the fine-mesh netting. The porous resistance coefficients were determined by fitting the experimental wave force data obtained from wave tank tests. The simulated wave forces, based on these fitted coefficients, showed good agreement with the experimental measurements (see Figure 6). Previous studies have employed various numerical models to investigate the hydrodynamic performance of nettings in waves, including the porous media model [27; 33; 38; 39] and lumped mass models [25; 32; 37]. Each of these models has its advantages and limitations. For instance, the porous media model offers computational efficiency and is effective in capturing the overall resistance and flow characteristics through net panels [25; 27; 33]. However, it does not resolve the detailed geometry of individual twines and, therefore, cannot capture the localized tension, relative velocity, or oscillation between individual twines [37; 39]. This simplification affects the interpretation of the flow energy distribution by smoothing out small-scale vortical structures, turbulence near twine surfaces, and wake interactions. As a result, localized flow features such as acceleration, separation, and energy dissipation in the near-net region are not explicitly captured. To address this, one-way or two-way fluid-structure interaction methods have been widely applied to analyze the interaction between fluid and netting in waves [37]. However, simulating fine-mesh netting presents challenges for traditional CFD methods due to the complexity of modeling, resulting in substantial computational costs. The mesh grouping method presents an effective approach by representing multiple actuals mesh with a single equivalent fictitious mesh, thereby maintaining consistent hydrodynamic forces on the fine-mesh netting before and after grouping [17].