Abstract
The presence of a row of circular cylinders along the centerline in a flume gives a significant drag to the flow (J. JSCE B1 71(4) I_1057 2015). The row can be seen macroscopically as a porous flat plate having both the permeability and roughness effects, and the flow over semiannular stripe roughness elements is the system obtained by removing only the permeability from the flow past the cylinders. In the present study, these flows have been studied comparatively by a two-dimensional numerical experiment to elucidate the impacts of the permeability and the roughness on the flow resistance. The drag on the row of cylinders is found to be roughly one order of magnitude higher than that over the roughness elements. In other words, permeability causes much more resistance to the flow than roughness. The importance of the former tends to be decreased with decreasing the number of cylinders (i.e., increasing the plate porosity). Visualization of the instantaneous flow fields reveals the presence of large scale vortices in both types of flows, and the strength of the vortices is significantly enhanced by the boundary permeability. This then promotes fluid mixing and momentum exchange across the whole flume width, and increases the resistance to the flow.
