Experimental study on the high-speed water entry of cylinders at shallow angles

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.