  Turbulent flows over porous lattices consisting of rectangular cuboid pores
are investigated using scale-resolving direct numerical simulations. Beyond a
certain threshold which is primarily determined by the wall-normal Darcy
permeability, ${K_y}^+$, near-wall turbulence transitions from its canonical
regime, marked by the presence of streak-like structures, to another marked by
the presence of spanwise coherent structures reminiscent of the
Kelvin-Helmholtz (K-H) type of instability. This permeability threshold agrees
well with that previously established in studies where permeable-wall boundary
conditions had been used as surrogates for a porous substrate. None of the
substrates investigated demonstrate any drag reduction relative to smooth-wall
turbulent flow. At the permeable surface, a significant component of the flow
is 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 the
overlying flow is turbulent, an aspect which cannot be accounted for when using
continuum-based approaches to model porous media flows or effective
representations such as wall boundary conditions. The severity of the AM is
enhanced in the K-H-like regime, which has implications when designing porous
substrates for transport processes. This suggests that the surface of the
substrate can have a geometry which is different than the rest of it and
tailored to influence the overlying flow in a particular way.
