## 1 Introduction
Thus, in order to successfully capture the gas-water interface and simulate UNDEXs satisfactorily, researchers have made a lot of efforts to combine the front capturing method and the front tracking method. The Ghost Fluid Method (GFM), which was first proposed by [PERSON] et al. [26] in the year 1999, is a typical representative of them. In the GFM method, the interface is traced implicitly by using the level-set method [27; 28; 29]. Moreover, \"ghost points\" are defined in which the parameter values such as the normal velocity are taken from the real fluid. Then, the governing equations of the flow field are solved at both sides of the interface after the implementation of the ghost points. Based on this idea, a variety of GFM methods [30; 31; 32; 33; 34] were then developed. Among them, the MGFM (Modified GFM) method proposed by [PERSON] et al. [32] performs better in simulating the evolution of the gas-water interface with a high pressure ratio. This method predicts the interface status, and then uses it to re-define the density in the real fluid and the parameters of the ghost points on the other side of the interface. The results shown in [33] proved that the MGFM method is less problem related and more robust in simulating the interaction between the shock wave and the gas-water interface. Later on, on the basis of MGFM, [PERSON] et al. [34] proposed the RGFM (Real GFM) method, which allows for modifying the values of more variables on the real fluid side, and obtained a more accurate solution to the wave impedance problem. The MGFM and RGFM methods are proved to have a third-order accuracy [35] in simulating the strong shock-interface impacting problem, while the original GFM methods proposed by [PERSON] et al. [26] and [PERSON] et al. [31] possess only at most first-order accuracy. Thus, the MGFM and the RGFM methods are capable of providing more accurate and consistent computational results in simulating the gas-water interface problems with a high pressure ratio. At present, in various areas, attention is paid to investigating the impacts of the shape change of warheads on the dynamics of the UNDEX. For example, in oil engineering, warheads with special shape designs are detonated in a water environment to form cracks in the rocks so that more oil can be drilled. Moreover, in the military anti-warship industry, efforts were also made on designing the shape of the warhead so that it can cause greater damage to the ship by the same amount of the explosive. However, the existing numerical methods mentioned above were so far mostly applied to one- or two-dimensional problems. In addition, most of the simulations treated the explosives of a spherical shape instead of the complicated shapes practically used in engineering. Therefore, until now, three-dimensional numerical studies of UNDEXs with complex shapes of the explosives are still lacking. Thus, in the present study, we adopted a three-dimensional numerical model with the implementation of the RGFM method, to simulate UNDEXs with various initial shapes of a high-pressure gas bubble. Through this numerical study, we were able to obtain the flow field of UNDEXs at different time steps after the initiation of the explosion. The pressure history at the boundaries of the computational domain was also recorded. In addition, the computational results for different shapes of the bubble were compared and discussed, which helps to gain a deeper understanding of the dynamics of an UNDEX and its impact on solid structures by the shape change of the warhead. The organization of the manuscript is as follows. In Section 2, the governing equations of the model used in the present study are given. The interface treatment and the boundary conditions used in this model are also presented. Later on, in Section 3, the major computational results obtained in a series of simulation scenarios by varying the shape of the hot-gas bubble immersed in a water environment are shown and discussed.