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Turbulent flows over porous lattices consisting of rectangular cuboid poresare investigated using scale-resolving direct numerical simulations. Beyond acertain threshold which is primarily determined by the wall-normal Darcypermeability, ${K_y}^+$, near-wall turbulence transitions from its canonicalregime, marked by the presence of streak-like structures, to another marked bythe presence of spanwise coherent structures reminiscent of theKelvin-Helmholtz (K-H) type of instability. This permeability threshold agreeswell with that previously established in studies where permeable-wall boundaryconditions had been used as surrogates for a porous substrate. None of thesubstrates investigated demonstrate any drag reduction relative to smooth-wallturbulent flow. At the permeable surface, a significant component of the flowis that which adheres to the pore geometry and undergoes amplitude modulation(AM). This pore-coherent flow remains notable within the substrates,highlighting the importance of the porous substrate's microstructure when theoverlying flow is turbulent, an aspect which cannot be accounted for when usingcontinuum-based approaches to model porous media flows or effectiverepresentations such as wall boundary conditions. The severity of the AM isenhanced in the K-H-like regime, which has implications when designing poroussubstrates for transport processes. This suggests that the surface of thesubstrate can have a geometry which is different than the rest of it andtailored to influence the overlying flow in a particular way.

Turbulent flows over porous lattices: alteration of near-wall turbulence and pore-flow amplitude modulation