# Research on the Water Entry of the Fuselage Cylindrical Structure Based on the Improved SPH Model
## 4 Numerical Simulation of Water Entry of a Cylinder Structure
### Water Entry of the Elastic Skin-Stringer-Floor-Column Structure  
The floor is an essential component of a commercial aircraft's fuselage. The floor bears the load of passengers and seats and needs to be connected to the floor's longitudinal beams, columns, and fuselage frames, making them critical structural elements. Therefore, this section discusses the characteristics of cylindrical structures with floor and columns. Figure 13 presents the equivalent strain at different times for the skin-\"L\" stringer-floor-column structure entering the water, with a 10 mm floor thickness. In the initial stage of water entry, the bottom skin starts to deform upwards. As the depth of immersion increases, the deformation of the bottom skin tends toward a planar shape. Due to the support provided by the columns, the column shows a noticeable deformation tendency toward both sides, while the floor exhibits smaller deformation. Throughout the water entry process, the floor and columns support the upper part of the structure, resulting in relatively small deformation and maintaining the shape of the upper space of the structure. Figure 12: Variation of the slamming force over time for the skin–\"I\" stringer cylindrical structure and skin–\"L\" stringer cylindrical structure. Figure 13: Equivalent strain at different times for the skin–\"L\" stringer–floor–column structure. Figure 14 compares the variation in the slamming force over time for the skin-\" L\" stringer structures with and without floor and columns. After the structure touches the water, the slamming force rapidly increases to its peak value and gradually decreases. It can be observed that the structure with floor and columns has a higher weight, leading to a faster increase in the slamming force but with a smaller peak value. During descent, the impact force exhibits fluctuations and a trend of increase, which is caused by the deformation of the bottom skin and a larger area of contact with the water. Next, the influence of floor thickness on structural deformation is explained. Figures 15 and 16 show the equivalent strain at different times for the structure with a floor thickness of 5 mm and 15 mm, respectively. When the floor thickness decreases, the differences in deformation are minimal at the bottom skin and columns. The influence can be observed on the above floor. A decreased floor thickness reduces the energy absorption from the lower part, resulting in more considerable deformation in the upper skin. Conversely, when floor thickness increases, the differences in deformation also are minimal at the bottom skin, and the deformation of the columns decreases, with almost no deformation observed in the upper skin. Figure 14: Variation in the slamming force over time for skin–“L” stringer structures with and without a floor and columns. Figure 15: Equivalent strain at different times for the skin–“L” stringer–floor–column structure with a floor of thickness 5 mm. It can be concluded that floor thickness has a small influence on the deformation of the bottom skin, but significantly affects the deformation of the columns and upper skin. The floor is an important component for supporting the shape of the upper structure, and selecting an appropriate floor thickness is crucial for a structurally safe design. Figure 17 compares the variation in the slamming force over time for structures with a floor of thickness 5 mm, 10 mm, and 15 mm. The three curves exhibit almost identical trends. The structure with a 15 mm floor thickness shows the smallest peak value. The fluctuations in the slamming force during the descent phase are consistent. Figure 18 shows that there is an equivalent strain at different times for the structure if the diameter of the columns is increased. It can be observed that increasing the column diameter effectively reduces the deformation of the columns, floor, and upper skin, providing beneficial protection for the overall structure.