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Based on the volume of fluid multiphase flow model and the overset mesh technique, a numerical method for an asynchronous parallel oblique water-entry super-cavitating projectile was established. Experimental studies of the oblique water-entry of a high-speed single-launch projectile were carried out to validate the viability of the numerical method. The paper performed the numerical simulations and analyses of cavity evolution and motion characteristics of the front and rear projectiles in different initial intervals and in two sequences of top-side water-entry projectile first and bottom-side water-entry projectile first. The results show that when the initial interval of the first launch projectile is 0.5 time the projectile length, the first launch projectile cannot produce a cavity to completely encapsulate the projectile due to the violent squeezing of the following launch projectile cavity, and its movement is seriously affected and eventually loses its trajectory stability. At the same time, the first launch projectile that enters water from top side is squeezed to a larger degree than the one from bottom side, and the wetting phenomenon occurs earlier and loses stability faster. As the initial interval increases, the influence of the following launch projectile cavity near the first launch projectile is weakened, and the first launch projectile in both water entry sequences move steadily. For the following launch projectile, due to the continuous influence of the first launch projectile cavity, its cavity is always asymmetrical, and its motion stability is affected. The following launch projectile deflects to the inner side and destabilizes when the initial interval is 0.5 times the projectile length. When the initial interval is 1 time the projectile length, it moves steadily. It deflects to the outer side and destabilizes when the initial interval is 2 and 3 times the projectile length. In addition, the motion characteristics of the following launch projectile are basically identical in two water-entry sequences. 基于VOF多相流模型与重叠网格技术建立了超空泡射弹异步并联倾斜入水数值方法, 利用单发射弹倾斜入水实验对数值方法的有效性进行了验证, 在此基础上, 分别对不同初始间距及入水顺序下前后两射弹的空泡演化与运动特性进行了数值模拟与分析。研究结果表明: 对于前发射弹, 在初始间距为0.5倍弹长时, 由于受到后发射弹空泡剧烈挤压, 前发射弹无法产生完整空泡包裹弹体, 运动受到严重影响并最终失稳; 同时, 前发射弹自上侧入水与自下侧入水相比, 其空泡受到的挤压程度更大, 沾湿更早且失稳更快; 随着初始间距的增大, 前发射弹附近空泡受后发射弹的影响减弱, 2种入水顺序下均运动平稳。对于后发射弹, 由于前发射弹空泡的持续作用, 其空泡始终不对称, 运动稳定性受到影响, 在间距为0.5倍弹长时向内侧偏转失稳, 在间距为1倍弹长时运动平稳, 间距为2倍和3倍弹长时向外侧偏转失稳, 且2种入水顺序下运动规律基本一致。

In this study, the tail-slapping behavior of an oblique water-entry projectile is investigated through high-speed photography technology. The experimental images and data are captured, extracted and processed using a digital image processing method. The experimental repeatability is verified. By examining the formation, development and collapse process of the projectile’s cavity, this study investigates the impact of the tail-slapping motion on the cavity’s evolution. Furthermore, it examines the distinctive characteristics of both the tail-slapping cavity and the original cavity at varying initial water-entry speeds. By analyzing the formation, development and collapse process of the cavity of the projectile, the influence of the tail-slapping motion on the cavity evolution is explored. Furthermore, it examines the evolution characteristics of both the tail-slapping cavity and the original cavity under different initial water-entry speeds. The results indicate that a tail-slapping cavity is formed during the reciprocating motion of the projectile. The tail-slapping cavity fits closely with the original cavity and is finally pulled off from the surface of the original cavity to collapse. In addition, as the initial water-entry speed increases, both the maximum cross-section size of the tail-slapping cavity and the length of the original cavity gradually increase. With the increase in the number of tail-slapping motions, the speed attenuation amplitude of the projectile increases during each tail-slapping motion, the time interval between two tail-slapping motions is gradually shortened, the energy loss of the projectile correspondingly enlarges, and the speed storage capacity of the projectile decreases.

A key point in the underground coal gasification process is the cavity evolution in the horizontal segment. The morphological evolution law of the gasification cavity has not been clarified, which is the bottleneck restricting the analysis of its controllability. In this paper, a physical simulation system for cavity generation was developed, and the cavity evolution in a targeted coal seam with overburden pressure was duplicated in the laboratory. A set of temperature field synchronous monitoring devices was developed to realize temperature sampling within a cavity and the surrounding rock. By analyzing the relationship between the overall temperature distribution pattern and the gasification agent injection condition, the morphological propagation law of the cavity is verified to be water drop-shaped, and influencing factors including the injection flow rate and the gasification agent component ratio are investigated. The axial length and volume of the cavity increase with an increasing injection flow rate. Higher oxygen content results in increased size in all dimensions. The research results provide theoretical support and reference for applying controlled cavity formation in the horizontal segment of U-shaped wells.

High-speed underwater vehicles are subjected to complex multiphase turbulent processes, such as the growth, development, shedding, and collapse of cavitation bubbles. To study the cavity evolution and pressure pulsation characteristics, in this paper, cloud cavitation over a conical axisymmetric test body with four pressure sensors is investigated. A multi-field simultaneous measurement experiment method for the natural cavitation of underwater vehicles is proposed to understand the relationship between cavity evolution and instantaneous pressure. The results show that the evolution of cloud cavitation can be mainly divided into three stages: (I) the growth process of the attached cavity, (II) the shedding process of the attached cavity, and (III) the collapse of detached cavities. The evolution of the attached cavity and collapse of the large-scale shedding cavity will cause strong pressure pulsations. It is found that the cavitation number plays an important role in cavitation evolution and pressure pulsation. Interestingly, as the cavitation number decreases, the fluctuation intensity of cavitation increases significantly and gradually presents obvious periodicity. Moreover, the unstable cavitating flow patterns are highly correlated with the time domain and frequency domain characteristics of pressure. Especially, as the cavitation number decreases, the main frequency becomes lower and the pressure band becomes more concentrated.

This study systematically investigates the evolution of the flow field and cavitation behavior during the underwater launch of a rotating sphere. By comparing surface pressure distribution, cavitation evolution, flow separation locations, and re-entrant jet formation under various rotational conditions, this study reveals the significant influence of rotation on both the cavitation processes and sphere’s motion trajectory. It is found that under rotational conditions, cavity detachment tends to occur earlier on the front side, and the re-entrant jet develops more fully, reaching maximum length and intensity at a moderate angular velocity. In additionally, rotation alters the cavity interface and overall flow structure, resulting in noticeable differences in surface wetting, pressure distribution, and separation behavior between the front and rear sides. As the rotational speed increases, flow separation points become less distinct, and pressure fluctuations on the rear side intensify, indicating that rotation plays a critical role in modulating underwater cavitation dynamics. The findings provide theoretical insights into flow control and cavitation risk assessment for underwater launches of rotating bodies.

To explore the water entry flow and impact load characteristics of northern gannets, we conducted water entry experiments using a northern gannet’s head model based on three-dimensional (3D) printing and several cone models under different Froude numbers. A high-speed camera was used to capture flow images, and an inertial measurement unit (IMU) was used to record the water entry impact loads. The results indicate that the geometric topology of the model considerably influenced the water entry flow and impact load. Specifically, the northern gannet’s head model created a smaller water entry splash crown, cavity geometry, and impact load compared with the cone models of similar sizes.

•We study the dynamic behaviors of ballistic gelatine behind the soft body armor experimentally.•The cavity expansion–contraction movement is self-similar and can be approximated as a semi-ellipse.•The pressures obtained from the gauges show that three peak waves exist. Understanding the transient cavity formation and transient pressure in the behind armor blunt trauma (BABT) is of fundamental importance in various research fields. In this paper, the transient response of ballistic gelatine behind the soft body armor subjected to the impacting of pistol bullets was studied. The profiles of the transient cavity in real time were captured by a high-speed camera, while the transient pressures were simultaneously recorded by pressure gauges. We find the cavity expansion–contraction movement is self-similar and can be expressed as a semi-ellipse. The gauges-recorded pressures reveal that three peaks on the pressure–time curve.

Understanding the evolution mechanisms of water-exit cavities and flow fields evolve during high-intensity interactions between vehicles and floating ice is critical for advancing the application of submarine-launched marine equipment in low-temperature ice-prone waters. A computational fluid dynamics–finite element method (CFD–FEM) coupled framework was established to simulate bidirectional fluid-structure interactions during the water-exit process of a ventilated vehicle impacting ice in brash environments. Distinct evolution characteristics were revealed by comparatively analyzing the cavity, flow fields, hydrodynamic loading, structural deformation, and trajectory stability across three scenarios: ice-free, single-ice, and multi-ice. Furthermore, the position-dependent impact effects were characterized. The findings reveal that the impact, friction, and compression effects of ice induce bending and wrinkling of the shoulder cavity, aggravating its collapse and increasing the wetting of the vehicle, resulting in a substantial expansion of the high-velocity and vortex-dominated regions within the flow field, accompanied by more obvious water splashes. The impact of ice notably increases the kinetic energy dissipation of the vehicle during the cross-water stage and diminishes its motion stability. In the center-symmetric layout, the vehicle collides with ice only once, with high stress confined to the head. Conversely, the radial-offset layout causes secondary or even multiple collisions, resulting in high-stress areas on the shoulder of the vehicle, making it deflect and ultimately causing the tail cavity to tilt and become destabilized. The design of new vehicles suitable for ice-prone environments should focus on enhancing the impact toughness of the head structure and optimizing the surface shape design to improve the adaptability to low-temperature complex environments. [Display omitted] •The ice impact induces cavity bending, increased collapse, and intensified asymmetric flow turbulence.•The ice impact increases the kinetic energy dissipation of the vehicle and reduces the motion stability.•In the center-symmetric layout, the vehicle collides with ice only once, with high stress confined to the head.•Radial-offset layout causes multi-collisions, resulting in high stress to the shoulder of the vehicle.

The cross-media water entry problem widely exists in fields such as ocean engineering and aerospace. The highly non-stationary characteristics of the cross-media water entry process significantly influence the structural strength and ballistic stability of vehicles. This paper selects air-dropped torpedoes, supercavitating vehicles, and high-speed projectiles as three typical types of cross-media vehicles for study. Based on their unique structural characteristics and typical water entry conditions, this paper focuses on the current status of their respective impact load and load reduction challenges, as well as water entry ballistic stability issues. At the research methodological level, this paper systematically reviews the progress of current research in three directions: theory, experiments, and numerical simulations, and introduces the application of artificial intelligence in solving cross-media problems. Finally, this paper looks forward to future development trends in cross-media water entry research, aiming to provide a reference for structural optimization design, motion stability control, and other related studies of cross-media vehicles.

A two-dimensional model, employing a dynamic mesh technology, is used to simulate numerically the transient multiphase flow field produced by two submerged parallel guns. After a grid refinement study ensuring grid independence, five different conditions are considered to assess the evolution of cavitation occurring in proximity to the gun muzzle. The simulation results show that flow interference is enabled when the distance between the parallel barrels is relatively small; accordingly, the generation and evolution of the vapor cavity becomes more complex. By means of the Q criterion for vorticity detection, it is shown that cavitation causes the generation of vorticity and the evolution of the vapor cavity can result in an asymmetric distribution of vorticity for a certain distance of the barrels. In particular, the evolution of the vapor cavity can hinder the expansion of the gas and force it to flow outward, while an asymmetric distribution of vorticity can lead to a gas jet flowing outward and rotating simultaneously.