In this paper, high-speed photography was employed to experimentally study the high-speed, shallow-angle water entry of cylinders. By varying the density, length–diameter ratio, and launch speed of the cylinders, three typical trajectories were observed: arc, S, and ricochet trajectories. This study examined the cavity evolution, motion trajectory, force state, and stability of the cylinders under these three typical trajectories. Additionally, the influence of each cylinder's length–diameter ratio and density on the stability of its motion during shallow-angle water entry was explored. The experimental results indicated that during the impact stage, the cylinder generates a head-down torque, resulting in an upward deflection after entry. The combination of head force and angle of attack generates lift, which increases with a positive angle of attack. Consequently, the cylinder's deflection speed accelerates, while it slows with a negative angle of attack. During the tail-slap process, the combined forces from the head and tail both generate lift, but in the opposite directions. The motion stability decreases sequentially in the arc, S, and ricochet trajectories, which is closely related to the first tail-slap. Increasing the cylinder length–diameter ratio or density delays the occurrence of the first tail-slap, thereby enhancing motion stability during shallow-angle water entry.

Tail slapping is considered to be a prominent mode for facilitating the movement of supercavitating projectiles underwater. Stability mechanism of the tail slapping and stability criterion of projectiles was numerically investigated in this paper. A numerical method was conducted to calculate the free flight of projectiles by combining the finite volume method, the mixture multiphase model, and a dynamic meshing scheme. The accuracy and applicability of the numerical calculation method are verified by experiments. Three typical stability modes of tail slapping motion, namely, stable, conditionally stable, and unstable, were identified through the free flight numerical simulations of projectiles with varying length-to-diameter ratios under different initial angular velocities. The stability criterion of projectiles was proposed, indicating that the tail slapping stability can be determined by analyzing the curve of the moment variation with the angle of attack at any velocity above 200 m/s. Furthermore, it was found that the stable projectiles possess inherent motion states that are exclusively determined by their structural features and are unaffected by the initial motion conditions.

The high velocity of supercavitating projectiles in a wave environment alters the flow characteristics and water entry stability, which significantly impacts the development and application of supercavitating weapons. This paper, investigates the effects of waves on the oblique water entry of high-speed supercavitating projectiles using computational fluid dynamics, with Stokes' second-order wave theory as the foundation for wave simulations. The numerical simulation method is validated through high-speed water entry experiments. The analysis explores the impact of wave inclination on cavity formation and the forces acting on the projectile. The results reveal that variations in wave inclination change the actual water-entry angles, affecting the cavitation structure near the free surface, modifying the impact intensity on the tail fins during water contact, and ultimately influencing the hydrodynamic forces acting on the projectile. When the actual water-entry angles are similar, the forces on the projectile during entry remain consistent under different conditions, with the trajectory being determined by the entry angle. Additionally, a reduction in the actual water-entry angle improves the projectile's entry stability but increases the amplitude and frequency of tail slap, ultimately affecting the stability of the projectile's trajectory after water entry.

To define the guidelines for the design of a Mewis duct for a small bulk cargo ship, a numerical study was carried out to investigate the effects of a fan-shaped Mewis duct on propeller performance of a 38,000 t bulk cargo ship with a three-blade propeller. Calculations were performed using the STAR-CCM+ software based on the solution of the Reynolds averaged Navier–Stokes (RANS) equation. Computations were carried out for a wide range of locations from 0.11D to 0.21D upstream from the propeller disc, a radius of trailing edge from 0.65R to 1.05R, and angle of attack of the inner fins, where from 0° to 10° of the fan-shaped Mewis duct, where D is the diameter of the propeller and R is the radius of the propeller. With the installation of the duct, the axial mean wake fraction on the propeller disc decreased, while the tangential mean wake fraction increased significantly. As the installation position was far away from the propeller, the axial mean wake fraction varied in a wave pattern, while the tangential mean wake fraction increased. With the increase of the radius and the angle of attack of the inner fins of the duct, the axial mean wake fraction first decreased and then increased, while the tangential mean wake fraction increased gradually. Correspondingly, the propeller force coefficients and efficiencies with the duct were all more than those without the duct and changed with the variations of the duct parameters. When the propeller and duct were taken as a propulsion system, the efficiency of the system was less than that of only the propeller. At the same time, with variations of the location, radius, and angle of attack of the inner fins of the duct, the efficiency of the system first increased and then decreased gradually, which indicates that there was an optimum design value for all of the duct parameters. Moreover, only when the design of these duct parameters are reasonable can energy savings be achieved; otherwise, the opposite effect is produced. Eventually, with the installation of the fan-shaped Mewis duct, the wake harmonic degree on the propeller disc can be significantly improved.