An Experimental Study on Separating and Reattaching Turbulent Flows over Permeable Walls (Effects of the Wall Permeability on the Mean Velocity Field)

Abstract

PIV measurements are 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 flow region. Three kinds of porous media are employed. They have the same porosity of 0.8 but each has different permeability from the others. Two types of square cylinder ribs are used, one of which is an impermeable smooth solid rib and another is a permeable porous rib which is made of the same porous medium as that for the bottom wall. The obtained mean velocity profiles of the solid rib cases indicate that the reverse flow in the recirculation behind the rib becomes weak as the increase of the wall permeability. The recirculation area thus shrinks. The main factor of those phenomena is found to be the increase of the bypassing flow rate through the bottom porous wall below the solid rib. Indeed, the local streamwise flux across the clear channel is confirmed to decrease above the rib and then increase behind the rib, according to the increase of the permeability. In the porous rib case, although the magnitude of the reverse flow becomes weak, the reattachment point shifts downstream and the recirculation bubble is slightly expanded. As the increase of the permeability, the reverse flow becomes weaker and the recirculation region tends to vanish. These are due to the bypassing flow not only through the bottom wall but also through the rib. The decrease of the local streamwise flow rate in front of the rib becomes less than that for the solid rib case. This indicates the increase of the bypassing flow through the rib. The bleeding flow from the back of the rib becomes a dominant factor on the shift of the recirculation area.