Hydrodynamic Response of Free-Falling Thick Disks

Abstract This 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 those 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. Different correlations were developed to relate the Froude number, dimensionless inertia, and energy dissipation to the observed crown and cavity characteristics, which can be utilized to enhance the current understanding of hydrodynamic impact behavior in finite-thickness bodies and provide predictive relationships useful for applications in marine, aerospace, and environmental engineering.