Effect of Porous Coating on the Water-Exit Dynamics of a Circular Cylinder

Abstract

The effect of a porous coating on the water-exit dynamics of a circular cylinder is investigated experimentally. Cylinders comprising a $30~\mathrm{mm}$ solid core surrounded by a $10~\mathrm{mm}$-thick porous foam layer with pore densities of 10, 20, and 60 pores per inch (PPI) are compared with a smooth cylinder of the same outer diameter. Experiments are performed over $1.50\times10^{4}\leq Re\leq5.00\times10^{4}$ and $0.37\leq Fr\leq4.08$. Time-resolved particle image velocimetry characterizes the wake and reconstructed pressure field, while proper orthogonal decomposition (POD) examines its energetic organization. All porous cylinders reduce free-surface elevation relative to the smooth cylinder, with the largest reduction generally obtained for 60 PPI. The submerged wake, however, is not simply weakened. Porous cases generally develop larger circulation and vortex area, with a non-monotonic dependence on pore density and operating condition. The 10 PPI cylinder frequently produces the largest vortex region, whereas 60 PPI generates stronger negative pressure within the vortex and the largest integrated vortex-associated pressure deficit for $Fr=1.02$, $2.00$, and $4.08$. During the submerged approach to contact, this pressure deficit co-evolves strongly with circulation, with Pearson correlation coefficients typically between $0.90$ and $0.98$. POD shows that 9--18 modes recover 95\% of the modal energy, with 20 PPI requiring only 9--10 modes. Overall, porous coating reorganizes wake strength, extent, pressure signature, and energetic structure while reducing the free-surface response.