Experimental study of modulating effect in permeable-wall turbulence*

In natural and engineering flow systems, turbulent flow overlying permeable walls are encountered across a broad range of length scales. Understanding the coupling between the free and the pore flows is key to accurately predicting many biochemical processes occurring in such systems. In an intermediate region between these two distinct flows, non-linear interactions take place across the interface and render permeable-wall turbulence unique compared to more canonical turbulent boundary layers over impermeable walls. The aim of this study is to explore the unique nature of these flow interactions, particularly modulation of the near-wall and pore flow by larger-scale motions in the outer layer in flow over cubically-packed uniform spheres. Low- and high-frame PIV measurements were in a refractive-index matching (RIM) environment that allows almost full optical access in the vicinity of the permeable interface for the current wall models. Conditional averaging with a proper orthogonal decomposition (POD) filter and two-point amplitude modulation (AM) correlation coefficients revealed the prevalent AM effect in permeable-wall turbulence. Furthermore, these results also showed that the AM phenomenon propagates into the pore space along the penetrating flow path.