Turbulence characteristics in flows over solid and porous square ribs mounted on porous walls

摘要

PIV measurements have been performed for turbulent flows in a rib-mounted channel whose bottom wall is made of a porous layer. The ratio of the rib and channel heights is fixed at 0.5. The effects of the wall and rib permeability are investigated focusing on the separating and reattaching flows at the bulk Reynolds number of 10~3 - 10~4. Three kinds of foamed ceramics are employed as the porous media. They have the same porosity of 0.8 but each permeability is different from the others. Its normalized values by the rib height are 0.89 × 10 ~(- 4), 1.47 × 10 ~(- 4) and 3.87 × 10 ~(- 4). 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 turbulent velocity fields of the solid rib flows indicate that the turbulent intensity behind the rib becomes weak and the recirculation bubble in the clear channel tends to vanish as the the wall permeability increases. In the porous rib flow, the recirculation and the reattachment point shift downstream and turbulence becomes weaker due to the bleeding flow through the rib. In the higher permeability cases, the recirculation bubble hardly exists due to the flows through not only the bottom wall but also the porous rib. From the measurements, it is suggested that in the solid rib flows, a reverse flow region exists inside the porous wall whereas in porous rib flows, such reverse flow does not exist at higher permeability.