Porous interfaces are encountered across a broad range of length scales, both in natural and engineering flow systems. Understanding the coupling between the free flow and the pore flow is key to accurately predicting many important biogeochemical processes occurring in such systems. This is particularly true when the free flow is turbulent and the coupling involves an intermediate region within which the flow undergoes a transition from turbulent to laminar regime. Such region, typically called “transitional region”, develops across the permeable interface where non-linear flow interactions between the free flow and the pore flow take place. The very existence of the transitional region and the unique nature of these interactions may explain previously observed modifications of the turbulent structure of the free flow, as compared to the classic turbulent boundary layers (TBL) over impermeable walls. The aim of this study is to explore the instantaneous flow interactions across the interface in order to elucidate the mechanisms that lead to the TBL modifications. In this paper we experimentally investigate the transitional region in a permeable smooth wall constructed packing five layers of cubically arranged spheres (d = 25.4mm). Surface topography was removed by cutting half of a diameter on the top layer in order to render the interface smooth and thus isolate the permeability impact on the flow. The investigation was conducted performing particle image velocimetry (PIV) measurements in a refractive index matching (RIM) environment. Free flow and pore flow were simultaneously imaged and the link between them was examined. The results of our analysis reflect the dynamic interplay between these two flows. Instantaneous events of upwelling and down-welling flow across the permeable interface were explored. These are found to be consistently associated with near-interface low- and high- momentum free flow, respectively. Preliminary visualizations of instantaneous velocity fields suggested that near-interface surface and subsurface flow are negatively correlated, indicating that low-momentum surface flow may be associated with high-momentum pore flow, and vice versa. Conditional averaging confirmed this conjecture, providing statistical evidence of our observations. Our results suggest that wall sweep events are associated with high momentum fluid from the free flow being pushed downward thus penetrating into the bed near the location where the event occurs. Likewise, low-momentum fluid is ejected out from the bed at locations upstream, presumably driven by a large low-pressure region. This is indicative of a “pumping” process sustained by free flow turbulent events which alternatively push fluid in and out the bed. While it is evident that this process clearly controls mass exchange, it may be also responsible for exchange of momentum. Temporal correlations between the two flows allowed us to observe the dynamics of structures transporting momentum. This quantitative information provides the means for a better understanding of the underlying physics. In addition, it allows us to investigate the potential existence of an amplitude modulation mechanism between the large-scale structures in the free flow and the finer structures in the pore spaces.
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