  Through high-fidelity numerical simulation, the effect of the arrangement of
micropillars on the flow characteristics and momentum transport has been
extensively investigated. The surface friction due to the complex flow
characteristics and momentum transport mechanism has also been studied in
depth. The micropillars are arranged in a quadrilateral, and different
arrangements are acquired by changing the streamwise and spanwise distances
between pillar rows. The results show that the streamwise and spanwise pillar
distances have their own different influences. When the streamwise pillar
distance is small, the micro eddies in the gaps between the streamwise
neighboring pillars are significantly suppressed. The increase in the spanwise
pillar distance enhances the momentum transport from the flow above pillar
array to the flow in the spaces among micro pillars. When the spanwise pillar
distance is small, the micro eddies in the gaps between the streamwise
neighboring pillars connect with each other and form a tubular eddy between
each pair of spanwise pillar rows. The tubular eddies significantly reduce the
momentum transport from the upper flow to the lower flow. The increase in the
streamwise pillar distance increase the momentum flux slightly. The surface
friction can be decomposed into three components which are associated with two
factors, the dilution effect of the number density of micro pillars and the
multi-faceted effects of micro eddies. These two factors are determined by the
streamwise and spanwise pillar distances. The dependence of the total friction
and its components on the pillar distances has been thoroughly examined.
