RIM-PIV Measurements of Turbulent Flow over a Rough Porous Bed

Flow over permeable beds is important in sediment transport and mixing processes, yet detailed velocity and stress measurements remain difficult to obtain, particularly close to the sediment–water interface (SWI). In this work, we use refractive-index-matched PIV to study turbulent open-channel flow over and within a permeable bed composed of monodisperse borosilicate glass beads. Measurements are reported for three low-ReK cases, ReK=0.224, ReK=0.335, and ReK=0.360, to resolve the mean velocity structure and the associated viscous, turbulent, Reynolds, and dispersive stress distributions. The results show that both the mean velocity and the turbulence intensity decrease rapidly below the SWI, indicating strong damping within the porous bed. Above the bed, the flow retains a boundary-layer structure, and increasing ReK enhances the turbulence intensity without changing the overall regime. The results indicate a shift from turbulent transport above the bed to viscous control within the porous layer, while dispersive stresses peak near the interface. Overall, the SWI controls momentum exchange within a thin region and the porous bed suppresses turbulence penetration into the subsurface.