Measurements for Characteristics of Turbulence over a Streamwise Preferential Porous Substrate

To investigate the turbulence characteristics over a streamwise-preferential porous substrate, we design a layered porous medium that satisfies the turbulent drag-reducing conditions suggested by the direct numerical simulation (DNS) of Gómez-de Segura and García-Mayoral (J Fluid Mech 875:124–172, 2019). Planar particle image velocimetry (PIV) measurements are carried out for fully developed turbulent flows over substrates made of the layered porous medium. Two (square duct and two-dimensional channel) flow cases are considered. Streamwise-wall-normal plane measurements are performed at the bulk Reynolds numbers \(Re_b=\)5000–15000 for the square duct flows. The measurement data indicate that with the drag-reducing conditions, which are suggested by the DNS, turbulence over the porous substrate is suppressed to a similar level to that near a solid smooth wall. For further discussion, we then carry out another PIV campaign for channel flows. With streamwise-wall-normal and streamwise-spanwise plane measurements of channel flows at \(Re_b=\)3000–15000, it is observed that the turbulence level near the porous medium is more significant than that near the solid wall. To investigate why turbulence over the layered porous medium behaves unlike in the DNS, we analyse the data comparing with our previously studied porous medium turbulence. With the spanwise streak distributions and quadrant analyses, their similarity and dissimilarity of turbulence structures are discussed.