## 4 Analysis Results and Discussion  
This section presents the results of the numerical simulations and a comparison between the numerical and experimental results. Figure 8 shows images of the water-steel box interaction as time progresses for a drop height (\(h\)) of 2 m and a plate thickness (\(t\)) of 3 mm. Figure 9 shows the pressure value at each stage shown in Figure 8a-d, i.e., the pressure exerting on the center of the bottom plate. Two peak pressures were observed in the wet drop test of the flat plate owing to the air pocket effect immediately after immersion at the water surface. In general, the second peak pressure was higher than the first peak pressure owing to the air pocket effect. However, only one peak pressure response was observed in this study because no air pocket was considered. Figure 8: Images of water–steel box interaction in CEL analysis (\(h\): 2 m; \(t\): 3 mm). Figure 9: Pressure time history for center of bottom plate (\(h\): 2 m; \(t\): 3 mm). The primary purpose of this study was to examine the plastic deformation of a plate during a slamming impact event. The plastic deformation of the plate was investigated after the dynamic impact of water on the plate had disappeared. Figure 10 shows the plastic deformation contour in the vertical (plate thickness) direction. For comparison with the experimental results, the relative displacement was calculated by fixing both ends of the plate centerline to zero, as shown in Figure 11. The formula used to calculate the relative deformation is shown in Equation (22). \[ \begin{split} X^{\prime}&=\cos\alpha(X-X_{0})+\sin \alpha(Y-Y_{0})\\ Y^{\prime}&=-\sin\alpha(X-X_{0})+\cos\alpha(Y-Y_{0}) \end{split}  \tag{22}\]  
Figure 10: Plastic deformation contour in vertical direction. Figure 12 shows a comparison of the simulated relative deformation results derived using Equation (22) with the experimental results. It is observed that the plate deformation becomes increased as drop height is higher and plate thickness is thinner. In addition, various deformation patterns were observed for experimental works; whereas simple deformation patterns were found in FE simulations. Figure 12c,e,f show excellent agreement in terms of deformation between the simulation and experimental results, i.e., one deformation peak, whereas Figure 12a,b,d show discrepancies between the simulation and experiment, i.e., different numbers of deformation peaks. The different deformed shapes shown by the experimental and numerical results are thought to be attributable to the initial deformation, which was not reflected in this study. As the current test specimen is an unstiffened flat plate, the initial imperfection mode and shape significantly affect the dynamic structural behaviors. If the initial imperfection mode of the test specimen shows a three-half-wave cycle pattern, then better simulation results can be expected by applying a similar initial imperfection pattern. However, the simulation results agreed well with the experimental results for cases involving a low amount of initial imperfection or a one-half wave cycle pattern of the initial imperfection. Further study is required to accurately reflect the initial shape of the bottom plate before performing the simulation. In addition, the air pocket effect, which is primarily observed in the wet drop of a flat plate, is attributable to the difference between the experiment and simulation [31]. Generally, when a flat plate is dropped, an air pocket is generated in the middle, and the edges of the plate establish contact with water, which may change the deformation of the plate. In particular, the presence of air pockets can significantly affect the deformation of the plate, which exhibits an initial imperfection. In this study, an air pocket was not implemented because air was not considered in the simulation. Additional numerical studies are required to realize an air pocket by considering air, which is a compressible fluid. Figure 11: Calculation of relative deformation.