A Direct numerical simulation (DNS) is conducted for a turbulent channel flow having a rough wall and a smooth wall using spectral method. On the rough wall, rib elements are placed for spanwise direction with some extent streamwisely, which are realized by virtual forcing to satisfy adhesive condition on the rib's surface based on the numerical treatment proposed by D.Goldstein et al.(1994). The rib's height (h{sup}+)is 20{sup}+, that means transitional regime of rough-wall flow. Global Reynolds number nondimensionalized wall-friction velocity and half channel width is 150. Two types of roughness element are examined, i.e., d- and k-type roughness. As visualizing the coherent structures such as coherent vortex and streaky structure, they are observed above the rib element although the rib element modulate strongly turbulence phenomena over the wall.
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