Pressure Drop Measurements over Anisotropic Porous Substrates in Channel Flow

The use of varying surface microstructures has the potential to be an effective passive flow control method for wall-bounded turbulent flows. Previous numerical simulations have shown drag reduction over streamwise preferential substrates that yield larger effective slip lengths for the streamwise mean flow compared to the turbulent cross-flows. A deterioration in performance is typically observed when the normalized wall-normal permeability √K_yy⁺ is greater than 0.4, linked to the presence of the large-scale motions associated with the K–H instability. However, it remains to be seen if the trends observed in the numerical simulations hold for physically-realizable materials. In this study, a family of anisotropic periodic lattices is manufactured using 3D printing, whereby rod size and spacing in different directions can be varied systematically to achieve different ratios of streamwise, wall-normal, and spanwise bulk permeabilities (K_xx, K_yy, K_zz.) The 3D-printed materials are then flush-mounted in a benchtop water channel. Pressure drop measurements are made in the fully developed region of the flow to systematically characterize drag as a function of bulk permeability for materials with K_xx/K_yy∈ [0.15,6.8]. Although drag reduction as compared to a smooth wall is not observed in the range of bulk Reynolds numbers tested (Re_b ∈[500,4000]), the relative increase in drag is lower for streamwise-preferential materials.