Laminar Boundary Layer Separation Over a Fully Porous Bump

The interaction between boundary layer and porous substrates remains poorly understood, largely due to the difficulty in accessing detailed pore-scale flow information. This study employs pore-resolved direct numerical simulations (DNS) to examine laminar boundary layer separation over a fully porous Gaussian bump formed from packed spheres. The bump has a height of 2.5$\delta$ and a streamwise extent of 11.5$\delta$, where $\delta$ is the inflow boundary layer thickness. Spheres of diameter 0.5$\delta$ are stacked and trimmed to reproduce the Gaussian geometry while allowing the internal flow to be fully resolved via an immersed boundary method. By comparing this porous configuration with a solid bump of identical outline shape, we isolate the influence of pore-scale mass and momentum transfer on separation dynamics. The favorable pressure gradient on the windward side of the bump drives near-wall fluid into the porous media, which then exits near the crest as an upward and outward jet. It uplifts the boundary layer and thus a promoted flow detachment. This flux also weakens the reverse flow in the recirculation region, thereby reducing the mean shear across the separating shear layer. These changes delay the onset of the quasi-spanwise roller vortices -- key to reattachment in laminar separation -- and extend the separated region by approximately 48\%. The total drag increases by 55\% when the bump is porous. These findings demonstrate that pore-scale transport, driven by the surrounding pressure gradients, fundamentally modifies both the separation characteristics and the overall drag associated with a permeable object.