Experimental study of vortex ring impingement on porous concave hemispherical cavities with different porosities

Discrete vortex rings impinging on porous concave hemispherical cavities with varying porosities (ϕ = 10%, 22%, 40%, 56%) were experimentally investigated at a fixed Reynolds number (Re = 800). Flow visualization was achieved using Planar Laser-Induced Fluorescence (PLIF) and two-dimensional particle image velocimetry (PIV). The geometric ratio between the vortex ring radius (R_v) and cavity radius (R) was maintained at γ = 1/3. Porosity was varied by decreasing the center-to-center spacing (d_c) of circular holes while keeping the hole diameter (d_r) constant. Vorticity fields and Finite-Time Lyapunov Exponents (FTLE) were calculated from PIV data to analyze the evolution of vortex structures. Results indicate that porosity critically governs the downstream transmission behavior and structural development of the vortex ring. At low porosities (ϕ = 10% and 22%), the transmitted flow breaks into incoherent, small-scale vortex rings. In contrast, high porosity cases (ϕ = 40% and 56%) promote vorticity merging and spatial coherence, leading to the formation of large-scale transmitted vortex structures that exceed the size of the incident ring. This scale amplification is attributed to the combined effect of enhanced jet density and geometric divergence imposed by the hemispherical cavity. Despite partial permeability, the upstream dynamics exhibited solid-wall-like features, including the formation of secondary and tertiary vortices. A negative correlation was observed between porosity and the propagation velocity of upstream vortex pairs, governed not merely by flow partitioning but by porosity-dependent redistribution of residual vorticity within the cavity. These findings reveal a distinct mechanism by which porosity and cavity geometry jointly shape vortex ring evolution, bridging behaviors observed in both porous transmission and concave confinement studies.