  The focus of this paper is to systematically study the influence of solid
obstacle surface roughness in porous media on the microscale flow physics and
report its effect on macroscale drag and Nusselt number. The Reynolds averaged
flow field is numerically simulated for a flow through a periodic porous medium
consisting of an in-line arrangement of square cylinders with square roughness
particles on the cylinder surface. Two flow regimes are identified with respect
to the surface roughness particle height: fine and coarse roughness regimes.
The effect of the roughness particles in the fine roughness regime is limited
to the near-wall boundary layer around the solid obstacle surface. In the
coarse roughness regime, the roughness particles modify the microscale flow
field in the entire pore space of the porous medium. In the fine roughness
regime, the heat transfer from the rough solid obstacles to the fluid inside
the porous medium is less than that from a smooth solid obstacle. In the coarse
roughness regime, there is an enhancement in the heat transfer from the rough
solid obstacle to the fluid inside the porous medium. Total drag reduction is
also observed in the fine roughness regime for the smallest roughness particle
height. The surface roughness particle spacing determines the fractional area
of the solid obstacle surface covered by recirculating, reattached, and
stagnating flow. As the roughness particle spacing increases, there are two
competing factors for the heat transfer rate: increase due to more surface area
covered by reattached flow, and decrease due to the decrease in the number of
roughness particles on the solid obstacle surface. Decreasing the porosity and
increasing the Reynolds number amplify the effect of the surface roughness on
the microscale flow.
