Measurements of Turbulence Over a Streamwise Preferential Porous Medium

This study experimentally examines the possibility of the turbulent drag reduction by an orthotropic porous medium whose streamwise permeability is larger than the wall-normal permeability. We made a layered porous medium whose measured streamwise, wall-normal and spanwise permeabilities are K_xx=26.9, K_yy=1.45, and K_zz=4.26 mm², respectively. Hence, ψ_xy = 4.3 and ψ_zy = 1.7, where ψ_xy =√(K_xx/K_yy).These values satisfy the drag-reducing condition suggested by numerical studies in the literature. We carry out particle image velocimetry measurements of turbulent square duct flows over the porous medium at the bulk Reynolds numbers of 3000-15000 and examine the drag reduction probability. Although the present experiments do not show remarkable turbulence reduction over the porous medium, from the detailed analyses of the obtained data, it is confirmed that the present porous medium is suppressed at least to the same level as that of the flat smooth wall side while the near-wall flow structures are different.