A solid object entering a fluid like water represents a classic case of fluid-structure interaction. The practical importance of water entry research extends beyond objects impacting a liquid surface, and it is also relevant for fixed or floating structures that are exposed to the incoming flow or surface waves. When an object impacts the water surface, it initially pushes the upper fluid layer radially outward, then accelerates the surrounding fluid downward as it descends into the water. This process displaces water with air, creating an air-filled cavity in the object's wake. During this dynamic interaction, several complex and nonlinear flow phenomena arise, including air-entraining cavities, splash crowns, cavity pinch-off, vortex shedding, etc. These phenomena influence the kinematics of the object's motion. The study of water entry has garnered extensive attention from academic and technical researchers due to its wide range of applications in various scientific and non-scientific fields. These applications include industrial processes such as inkjet printing, film coatings, and sprayed adhesives; naval architecture and marine engineering tasks like ship slamming and launching; aerospace engineering challenges such as seaplane landings and spacecraft water entry; sports applications like rowing oars and high diving; and natural phenomena observed in predaceous diving beetles, basilisk lizards, and diving seabirds.
Most studies on water entry focus on single water entry scenarios. However, the study of dual or multiple water entry—particularly parallel water entry, which is the focus of this research—is crucial in contexts such as the impact of parallel oars in rowing, lifeboat water entry, synchronized diving, and the interaction of ocean waves with adjacent offshore structures like oil rigs or wind turbine towers. In these situations, differences in size, shape, or surface characteristics add complexity to the problem. Parallel water entry is a highly nonlinear and unsteady process, with significant deviations from single water entry. The interactions affect air-entrainment cavities, cavity pinch-off, splash curtains, and jets, as well as the trajectories of the projectiles, making a detailed investigation both a scientific and an engineering necessity.
The main objectives of this project include studying cavity dynamics and the objects' kinetics/interaction in the context of parallel water entry of two spheres. This research demonstrates the significant impact of neighboring objects on the air cavity and splash sheet shape, spheres' trajectories, and even descent velocities. The results provide valuable information concerning the physics of the water entry process and can serve as reference cases for numerical studies of parallel water entry phenomena.
