Statistical attributes of turbulent flow across porous walls with a geometrically smooth interface: The role of internal wall structure

This study focuses on geometrically smooth porous walls bounded by a turbulent flow and quantifies experimentally the effects induced by the internal permeability of the wall, K, both on the turbulent boundary layer (TBL) above the interface and on the structure of the transitional layer. Unlike past work, wall roughness is effectively eliminated. Our results suggest that, while in a spatially and temporally averaged sense wall permeability produces effects similar to canonical roughness and supports the universality for wall-bounded flows, it independently controls the TBL. Double-averaged profiles of streamwise velocity closely follow a log-law with a consistent value of the von Kármán coefficient, κ ∼ 0.39. The magnitude of the downward shift, captured by the roughness function Δ U + , increases quasi-linearly with the permeability Reynolds number ( R e K = K u τ / ν ). Our results provide new evidence supporting the hypothesis that the parameter K serves as a characteristic length scale for turbulent flow over permeable walls. The momentum transfer is found to occur along preferential pathways dictated by the internal structure of the porous wall and is modulated by the distribution of up- and down-welling events at the interface. Surface topography, if present, exerts a primary control on the magnitude of the vertical fluctuations. Quadrant decomposition of the transport term within the TKE budget analysis indicates that Q2 events are primarily associated with transporting subsurface kinetic energy from the pore space into the surface flow region, while Q4 events are the main contributors to delivering outer turbulent kinetic energy into the pore space.