Settling of perforated disks in a density-stratified fluid

Settling disks in homogeneous and density-stratified fluids are known to encounter changes in the disks' orientation due to the effects of inertia or buoyancy, respectively. Recent studies have shown that in homogeneous fluids, central holes tend to stabilise the disk's descent, transitioning the settling path from a fluttering motion to a tumbling motion. In linearly stratified fluids, the equilibrium orientation of the disk is found to change from a broad-side on orientation to an edge-wise on orientation. In this work, we start by considering annular disks settling in such environments, followed by introducing different styles of perforations. We employ an immersed-boundary technique for particle-resolved direct numerical simulations within a 3D Cartesian domain of size 4D x 4D x 24D, where D is the outer disk diameter. The settling dynamics are governed by the Galilei, Froude, Schmidt numbers, and the disk's moment of inertia. When introducing these perforations, the simulations are conducted by fixing the mass of the disk with respect to the unperforated base case. The results include an investigation on the disk's orientation when settling, and variations in drift volume due to the introduction of perforations. Lastly, this work explores the effects of introducing a sharp density-transition layer on the disks' settling dynamics.