# Research on the Water Entry of the Fuselage Cylindrical Structure Based on the Improved SPH Model
## 5 Conclusions  
This paper addresses the fluid-solid coupling problem of the water entry of the fuselage structure and presents an improved smoothed particle hydrodynamics (SPH) model, which includes the low-dissipation GSPH method, the total Lagrangian SFPM method, and the Riemann-based contact algorithm. The rationality of the improved SPH model is demonstrated through the simulation of the water entry of an elastic cylinder via comparison with the theoretical and experimental results. Furthermore, the dynamic characteristics of the skin cylindrical structure, skin-stringer cylindrical structure, and skin-stringer-floor-column structure during water entry are studied. A comparative analysis of the strain distribution and slamming force reveals that in the case of the skin cylindrical structure, deformation primarily occurs at the bottom initially. With an increasing depth of water entry, the bottom's deformation causes it to transition toward a planar shape, and the top of the cylinder becomes pointed, ultimately deforming into an elliptical shape. The deformation pattern of the skin-stringer cylindrical structure is similar, but strains in the stringer are smaller. In the case of the skin-stringer-floor-column structure, the columns exhibit notable deformation due to the upper structure's column support, while the floor undergoes small deformation. Throughout the water entry process, the portion of the structure above the floor remains relatively less deformed, thus better maintaining the upper spatial shape. The trend of the slamming force for the three structures is similar between them. After contacting the water surface, the impact force rapidly rises to a peak and then gradually decreases. In the decreasing phase, the impact force exhibits oscillations, which occur as the bottom of the structure transforms into a plane, resulting in an increased contact area between the structure and water, consequently enhancing the slamming force. The influence of floor thickness and column diameter is also discussed. It is observed that floor thickness has a small effect on the bottom skin's deformation but significantly affects the columns and upper skin deformation. The floor serves as a crucial component supporting the shape of the upper structure, making a reasonable choice of floor thickness vital for structurally safe design. Increasing the column diameter effectively reduces the column, floor, and upper skin deformation, thus playing a favorable role in protecting the overall structure. Based on the damage model, a study is conducted on the water entry of a metallic elastic-plastic skin-stringer-floor-column structure, analyzing the positions of plastic deformation and failure. Compared to elastic structures, greater deformation occurs in the skin's upper and lower portions in plastic structures, but the upper part still maintains its basic shape. Ultimately, failure occurs in the bottom skin section close to the floor, where stress concentration appears. In this paper, the material of the fuselage is aluminum, which is a commonly used metal material in aircraft. The most recent airplanes increasingly use carbon fiber composite material. Hence, we need to further study the water entry characteristics of the composite materials, which is our future work. Methodology, software, validation, formal analysis, and writing--original draft, L.W. ; conceptualization, resources, data curation, and supervision, Y.Y. ; software, investigation, and visualization, Q.Y. All authors have read and agreed to the published version of the manuscript. National Nature Science Foundations of China (grant no. 11972309, 12272309), Natural Science Basic Research Program of Shaanxi Province (2023-JC-QN-0022), and Fundamental Research Funds for the Central Universities (CHD, no. 300102282105). Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: The data are not publicly available due to the participants' privacy.