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FLOW CHARACTERISTICS IN TURBULENT SEPARATING AND REATTACHING FLOWS OVER PERMEABLE POROUS WALLS

S. Tominaga
Department of Mechanical Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai 599-8531, Japan

M. Mori
Department of Mechanical Engineering, Osaka Prefecture University 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan

Masayuki Kaneda
Department of Mechanical Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai 599-8531, Japan

Kazuhiko Suga
Department of Mechanical Engineering, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai, Osaka 599-8531, Japan

Аннотация

PIV measurements have been performed for turbulent flows in a rib-mounted channel whose bottom wall is made of a porous layer. The effects of the wall and rib permeability are investigated focusing on the separating and reattaching flows. Three kinds of porous media are employed. They have the same porosity of 0.8 but each permeability is different from the others. Two kinds of square cylinder ribs: an impermeable smooth solid rib or a permeable porous rib which is made of the same porous medium as that for the bottom wall are used. The obtained mean velocity profiles of the solid rib flows suggest that the reverse flow in the recirculation behind the rib becomes weak as the increase of the wall permeability. The main factor is found to be the increase of the by-pass flow rate through the bottom porous wall underneath the solid rib. Due to the reduction of the magnitude and the size of the reverse flow region in the clear channel, turbulence becomes weaker than that of the solid-rib mounted solid-wall channel flow. However, because of the entrainment flow to the shear layer through the porous wall, the near wall turbulent intensities are enhanced. In the porous rib flow, the recirculation and the reattachment point shift downstream further. As the increase of the permeability, the recirculation is finally vanished. These are due to the flows not only through the bottom wall but also through the rib. Because of the flow through the porous rib, which weakens the shear layer behind the rib, turbulence become weaker than that of the solid-rib flows. In both the solid and porous rib flows, it is confirmed that a reverse flow region exists inside the porous wall whereas the reverse flow is not obvious in the clear channel region at a higher Reynolds number and/or higher permeability.