This study investigates the oblique water entry dynamics of tandem cylinders using numerical simulation, validated by experiments data. A classification criterion based on the dimensionless time interval (Mt) is established, distinguishing three distinct entry modes for the trailing cylinder: chasing collision, jet impact entry, and independent navigation. The evolution of flow disturbances and the resulting dynamic changes under each mode are systematically analyzed. Flow field analysis reveals the complex coupling mechanisms through which velocity and vorticity distributions in the preceding flow field influence the evolution and collapse of the trailing cylinder's entry cavity across modes. The closure time of the trailing cylinder's entry cavity is significantly advanced due to the inhibitory effect of the preceding jet. Dynamic analysis indicates that the splash induced by the trailing cylinder's secondary entry causes a sudden surge in the force coefficient of the leading cylinder, and this phenomenon vanishes when Mt exceeds 1.67. The peak impact force coefficient of the trailing cylinder exhibits a trend of initial decrease followed by increases as Mt rises, a pattern primarily governed by the evolution of the preceding jet base. When Mt >= 1.89, the impact resistance coefficient of the trailing cylinder surpasses that of a single cylinder. Following entry, collapsing bubbles induce force fluctuations on the trailing cylinder, with fluctuation amplitude decreasing as Mt increases. Analysis of surface pressure distribution clarifies the cause of moment coefficient fluctuations. Furthermore, the trailing cylinder demonstrates enhanced motion stability within a specific Mt range. These findings deepen understanding of the hydrodynamic characteristics of sequential water entry processes and provide a theoretical foundation for optimizing the design and launch timing strategies of continuous water-entry projectile systems.

Recent studies of supercavitating projectiles primarily focus on the formation and evolution of the cavity, as well as its underwater ballistic characteristics, while neglecting the terminal damage effects. Little attention has been given to exploring the combined damage effects of armor-piercing-explosion supercavitating projectiles (APESP). Therefore, this study comparatively analyzes the response processes and failure modes of an underwater aluminum alloy cylindrical shell target under the action of three different types of loads: armor piercing, explosion, and combined armor-piercing and explosion. This study investigates the underwater combined damage mechanisms of the APESP, clarifies each damage phase under the combined effect, discusses the advantages of damage resulting from the combined armor-piercing and explosion effect based on the target responses and damage modes, and explores the reasons for dissipation of explosion energy. The results show that: the APESP combines localized point damage characteristics of armor piercing with overall surface damage features of underwater explosion. Depending on load stages and target responses, the target response process under the action of the APESP can be divided into the hydrodynamic ram phase, penetration phase, shock wave phase, stable vibration phase, and bubble pulsation phase. The entire physical system can be abstracted as a low-frequency series spring system (equivalent to bubble pulsation frequency) with high-frequency external energy input, based on the energy relationship of the medium and the structure. The concept of the 'blower effect' is proposed based on target behavior during the stable vibration phase. Following the application of different loads, the plastic deformation of the target in a stable state is ranked as: underwater explosion > combined armor-piercing and explosion > underwater armor piercing. Supercavity, shell casing and penetration hole will cause the dissipation of explosion energy.

The structural response of the water-back plate under the combined action of shock wave and bubble loads at water depths of 1-300 m is investigated numerically using an arbitrary Lagrange-Euler method. The accuracy of the numerical model is verified by a comparison with the shallow-water experimental results and theoretical results. Specifically, the influences of the water depth and length-to-diameter ratio of the cylindrical charge on the combined damage effect of the shock wave and bubble loads are analysed. The following conclusions are drawn. As water depth increases from 1 m to 300 m, the plastic deformation energy of the water-back plate decreases, meanwhile the permanent deformation mode of a water-backed plate changes from convex to concave under the combined action of shock wave and bubble loads; when the charge has large length-to-diameter, the plastic deformation energy of radial plate under combined action of the shock wave and bubble loads is higher than that of axial plate at water depths of 1-300 m, and both difference decrease with increasing water depth; the combined damage effect of the shock wave and bubble loads can be enhanced in the radial direction by increasing the length-to-diameter ratio in deep-water environments. When the length-to-diameter ratio increases from 1:1 to 8.2:1, the plastic deformation energy of the radial plate gradually increases by 7.58 % from 10.68 to 11.49 J.

In order to analyze the influence of a water medium on the forming process and penetration performance of a shaped charge jet, a comparative study was carried out on an underwater shaped charge jet (USCJ) and a shaped charge jet (SCJ) in air. The virtual mass hypothesis is proposed to analyze the forming mechanism of USCJs. The arbitrary Lagrangian–Eulerian algorithm is adopted to carry out a series of simulation calculations considering the impact of standoff height and liner cone angle. The penetration test of the shaped charge jet in two media is carried out, and the experimental results verify the effectiveness of the theory and simulation. The differences of SCJ formation and penetration in air and water are analyzed. The results demonstrate that the USCJ exhibits a higher jet head velocity, higher cumulative kinetic energy, and a greater penetration ability than those of the jet in air. The depth of penetration (DOP) initially increases and subsequently decreases with an increase in the standoff height. The optimal standoff height of the USCJ is approximately 4–4.5 times greater than the charge diameter, whereas the SCJ in air is approximately 3.5–4 times greater than the charge diameter. Additionally, the DOP of the jet decreases at the optimal standoff height with an increase in the cone angle.

In order to study the directional enhancement effect of underwater explosion for cylindrical explosive with large length-to-diameter ratio, based on the experiment and ALE method, the influences of length-to-diameter ratio on the shock wave and bubble load as well as the structural response of nearby water-back copper plate are analysed. Meanwhile, the influence of the explosive weight on the directional enhancement effect is considered. Some instructive conclusions can be drawn. i) As the length-to-diameter ratio increases, the propagation velocity increases in the radial direction and decreases in the axial direction, and the high-pressure area gradually concentrates in the radial direction. ii) In the range of 1:1–8.2:1, increasing the length-to-diameter ratio enhances the shock wave and bubble load in the top direction and facilitates the formation of a stable bubble jet in this direction. iii) Increasing the length-to-diameter ratio enhances the damage effect of combined load in the top and bottom directions. When the length-to-diameter ratio increases from 1:1 to 8.2:1, the maximum plastic deformation of the water-back copper plate is increased by 26.2%. iv) The directional enhancement effect will be more obviously if the maximum bubble radius is larger than the distance between the structure and the explosive.

This study aims to investigate the formation characteristics and penetration performance of an underwater conical shaped charge jet. First, a new theoretical model for underwater jet formation is proposed by the virtual mass method. Based on the theoretical model and the ALE algorithm, the influences of standoff height, cone angle, liner thickness and initiation manner on the jet formation characteristics and penetration ability are analyzed. Then the experiments of underwater shaped charge jet are carried out to verify the theoretical and simulation results. Finally some instructive conclusions are drawn: (i) the theoretical model based on the virtual mass method can accurately predict the initial jet parameters and calculate the penetration depth, and the error between the theoretical and experimental results is less than 8%. (ii) The penetration depth initially increases and then decreases with increasing standoff height. The optimal standoff height is approximately 4–4.5 times the charge diameter. (iii) at the optimal standoff height, the penetration depth decreases with the increase of the cone angle, and the optimal cone angle is 40°–50°. (iv) The penetration depth decreases with increasing the liner thickness, and the jet shape and penetration depth formed by the ring initiation manner are better.

An underwater multipoint synchronous explosion can enhance the damage effect of an underwater explosion shock wave on a structure. This study investigates the damage effect of an underwater multipoint synchronous explosion on an air-backed clamped circular plate. First, we derive the theoretical formula for the superposition of underwater explosion shock wave pressure. A numerical simulation was conducted using the ALE (Arbitrary Lagrange-Euler) algorithm to investigate the damage effect of underwater explosion shock wave pressure on a clamped circular plate, and an experiment was conducted in a tank. By changing the conditions (explosive distance, explosive spacing, and number of explosives), the damage effect of an underwater multipoint synchronous explosion on an air-backed clamped circular plate was thoroughly investigated. The results show that the shock wave pressure of an underwater two-point synchronous explosion has a nonlinear superposition effect that is approximately 1.48–2.52 times greater than that of a single-point explosion. As the distances between the explosives and between the explosives and the plate decrease, the damage effect increases. When the total mass of the explosives is constant, the synchronous explosion of eight explosives has the greatest damage effect on the clamped circular plate.