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Takahiro Miura
Department of Mechanical and Production Engineering, Niigata University Ikarashi 2-no-cho 8050, Nishi-ku, Niigata-shi 950-2181, Japan

Koji Matsubara
Faculty of Engineering, Niigata University: Professor, Doctor of Engineering, Ikarashi 2-no cho 8050, Nishi-ku, Niigata 950-2181, Japan; Pacific Rim Solar Fuel System Research Center, Niigata University, Ikarashi 2-no cho 8050, Nishi-ku, Niigata 950-2181, Japan

Atsushi Sakurai
Department of Mechanical and Production Engineering, Niigata University, 8050 Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan


Direct numerical simulations were performed for turbulent flows in the spatial developing region of a ribbed channel, where square ribs were attached to one wall. The rib pitch-toheight ratio was changed at three steps; Pi/H = 2, 4 and 7. The bulk Reynolds number was given 4,560 through simulations. For the same parameters, simulations were also made for periodic fully developed cases. Although the friction factor decreased as flow goes downstream in the case of Pi/H = 2, it increased after unsteadiness in the case of Pi/H = 7 due to the turbulence enhancement. For Pi/H = 7, the developing flow near the rough wall even in the second cavity had a similar structure to the fully developed flow, whereas developed structures extended further away from the ribbed wall. The vortex structures were revealed by instantaneous fields and the two-point correlation of vorticities. Streamwise and spanwise vortices appeared near the rough wall. The streamwise vortex was smaller than that in a smooth channel, especially in the streamwise direction, and the inclination angle of the vortex was larger. Although the flow near the ribs was more isotropic than the smooth wall, coherent structures existed and produced the high correlation between the streamwise and wall-normal velocity fluctuations.