Hydrodynamic Response of Free-Falling Thick Disks

Abstract

The present study investigates the hydrodynamic response of free-falling thick disks impacting a quiescent water surface, emphasizing how impact energy governs crown evolution, cavity dynamics, and energy dissipation. A systematic series of laboratory experiments was conducted with six disks of varying density ratios (ρd/ρw = 1.08 – 2.33), two aspect ratios (χ = 1.5 and 3), and four release heights (hr/do = 2.6 – 6.6), yielding a wide range of impact Froude numbers and dimensionless moments of inertia. High-speed imaging revealed that a distinct transition in flow regime occurs near hr/do = 4, where the morphology of the crown and the topology of the underwater cavity shift abruptly. Disks released below this threshold generated smooth, partially sealed crowns, while higher-energy impacts produced rough, fully sealed crowns accompanied by secondary jet formation and cavity oscillation. A critical normalized impact energy of Ei/Eo = 0.36 was identified as the minimum required for full crown sealing. Crown diameter and height at pinch-off scaled linearly with the impact Froude number, whereas crown height showed negligible dependence on release height. Gravity-driven disks exhibited shallower pinch-off depths and shorter collapse times than that of force-driven disks and spheres under equivalent conditions. Energy loss analysis indicated an inverse correlation between normalized dissipation and Froude number, with lower-energy impacts dissipating proportionally more energy.