Experimental investigation on turbulent flow overlying permeable walls

A turbulent flow overlying a permeable wall can be subdivided into two distinct flow regions separated by a permeable interface. The first is the surface (or free) flow region, which overlies the interface. The second is the subsurface (or pore) flow region, which occurs within the permeable wall. While the near-wall surface flow can be turbulent, deep within the bed the subsurface flow is often laminar and can be described by Darcy's law (a balance of viscous and pressure forces). Thus, a region must exist between these two extremes where the flow undergoes a transition from inertia-dominated turbulence to viscous-dominated, laminar flow across the permeable interface. This region, typically termed the 'transitional layer,' develops across the permeable interface where non-linear flow interactions between the free flow and the pore flow take place. Accessing this region either experimentally or computationally is extremely challenging, with the latter limited by direct physical of optical access near and within the bed. The goal of this study was to explore the role of wall permeability and surface topography in flow interactions across a permeable interface and the corresponding turbulent boundary layer modifications in the surface flow region. The turbulent flow overlying impermeable and permeable walls with identical surface topography was explored experimentally using the particle-image velocimetry technique coupled with a unique refractive-index matching flow environment, whereby the latter provided full optical access to the flow in the vicinity of and within the permeable interface and the former allowed the acquisition of instantaneous velocity fields in this region with this optical access. Utilizing velocity statistics and conditional averaging, quantitative assessments were made for turbulent boundary layer modifications imposed by permeability and topography as well as the role of these effects in the mutual interplay between the surface and subsurface flows. Surface topography is found to intensify these interactions across the transitional layer, meaning that it enhances mass, momentum and energy transport between these two flow regimes. In addition, it was found that the larger scales of the surface-layer flow modulate the smaller scales near the permeable interface and within the bed itself. This effect was previously identified in canonical turbulent boundary layers (both smooth and rough), but the results presented herein highlight the enhancement of modulation effects owing to permeability. This physical linkage between the surface and subsurface flows across the transitional layer could provide a new framework for modeling such effects based on this unique dynamic connection between the two flow regimes