On the crown formation and cavity dynamics of free-falling thick disks

The water entry, crown formation, and cavity dynamics of gravity-driven thick solid and annular disks were studied by conducting laboratory experiments. Three different release conditions were examined to study the fully guarded and partially guarded release conditions on the cavity dynamics. The effects of aspect ratio, geometry, and density of disks on crown formation, interface impact, seal development, and air entrainment were investigated. The crown structure was classified based on the dimensionless moment of inertia and the impact Froude number. The crown dimensions were measured and smaller crown geometries were found in annular disks. The total duration of crown formation and destruction was measured which is a critical parameter in water entry problems due to its correlation with momentum transfer and energy dissipation rate. The temporal variations of the cavity dynamics were investigated and it was found that the growth rate of normalized pinch-off depth had a logarithmic correlation with time. The disk's velocities in three different stages were measured and it was found that the settling velocities followed a linear relationship with I* and it was affected by the release conditions. The velocity and vortex fields were extracted from the Particle Image Velocimetry (PIV) data. The velocity fields showed that the solid disks affected a greater surrounding ambient in comparison to the annular disks due to their higher initial momentum. Periodic vortex shedding was formed in the wake of annular disks and the frequency of the vortex field was found to be proportional to the disk density.