Plop, plonk, plunk: The physics of dunking something into water

You may have seen soda commercials where lemons and limes are being dunked into water, dragging down and kicking up air pockets. Beyond looking cool in slow motion, the physics of rigid bodies entering water is relevant to a wide range of real-world applications, from air-dropped torpedoes to the landing of seaplanes.

Yang et al. studied the cavity formations during water entry of deformable spheres with experiments and numerical modeling.

They categorized their experimental observations of the created cavities into three types: dumpy cavity, when the object first hits the water and creates a short-lived and relatively shallow cavity near the water surface; tight cavity, when the now slightly deformed object is below the water surface and the cavity is tightly wrapped around the object; and nested cavity, where the aforementioned tight cavity sinks below the dumpy cavity and becomes more geometrically complex.

The authors then devised a numerical model for predicting how certain things can affect the dynamics and formation of the different kinds of air pockets, including the entry velocity of the projectile and its properties, such as density and surface wettability.

“The flourishing development of hyperelastic material and the trans-media aircraft in recent years inspired us to explore the dynamics of cavity evolution during vertical water entry of deformable spheres,” said author Yingjie Wei. “We hope our research can be useful in future studies of water entry of trans-media aircraft made from elastic materials and the recovery of spacecraft at sea.”

Source: “Dynamics of the cavity evolution during vertical water entry of deformable spheres,” by Liu Yang, Yingjie Wei, Cong Wang, Weixue Xia, Jiachuan Li, Zilu Wang, and Dehua Zhang, Physics of Fluids (2021). The article can be accessed at http://doi.org/10.1063/5.0051401 .