In a liquid blanket system, MHD effect or low heat-transfer property of high Prandtl number fluid makes it difficult to remove high heat load, therefore utilization of ducts with inserted rods or sphere-packed pipes has been proposed to enhance the heat transfer. It is important to reveal influence of arrangement of the rods or spheres upon the heat transfer characteristics. In this study, the influence of a distance between two rods in wall-normal and streamwise directions upon the flow structures is clarified by numerical simulation. When the rod is approaching to another rod in the wall-normal direction, Karman's vortex street is disrupted. On the other hand, the distance between the rod and the wall is shorter than a certain value, a separation occurs on the wall and then the separation position does not move when the distance is much smaller than that value. Moreover, the pressure drop depends on the distance between two rods, and then there exists the minimum pressure drop. When the distance between the two rods becomes shorter in the streamwise direction, the wake behind the upstream rod changes from Karman's vortex street to twin vortexes and the pressure drop decreases. The turbulent kinetic energy near the wall in case of Karman's vortex street, which is generated by the upstream rod, is higher than that in case of twin vortex.