Linking the flow across a permeable boundary using a refractive index matching approach: first evidence of amplitude modulation crossing the interface

Abstract:

Turbulent flows bounded by permeable walls are ubiquitous in natural settings as well as industrial applications. River beds are the ultimate example of these dual-region flow systems whereas the surface and subsurface flows, physically separated by a permeable interface, gradually transition from the turbulent to the Darcian regime over a multi-grain thick layer characterized by unique physics. Environmentalists refer to such cushion layer as the hyporheic zone, seat of ecologically important nutrient, contaminant, and energy exchange processes. Recent studies have revealed that mass and momentum fluxes across the interface induce structural modifications on the boundary layer and have suggested the existence of a direct link between instantaneous turbulent flow events in the two regions. Amplitude modulation (AM), a phenomenon whereby the outer large scales modulate the intensity of the near-wall, small-scale turbulence has been widely studied in impermeable-wall turbulence. However, the ability of providing evidence of such phenomenon crossing a permeable interface has been hampered by the challenging nature of the permeable domain, inaccessible to state-of-the-art flow diagnostics. Here we leverage a newly developed large-scale refractive-index matching (RIM) flow environment to investigate the dynamic interplay between surface and subsurface flow in the presence of a permeable boundary using high frame-rate particle-image velocimetry (PIV). We fabricated clear permeable bed models featuring idealized geometries and immersed them in a turbulent flow. The RIM approach effectively afforded optical access into the bed and thus enabled simultaneous free flow and pore space flow measurements. Time-space analysis of the scale-decomposed dual-region flow provides the first direct evidence of AM phenomena propagating beyond the interface. Our results show that large-scale regions of high/low streamwise momentum in the surface flow, downwelling/upwelling across the permeable interface are directly linked to enhancement/suppression of small-scale turbulence, respectively, just above and within the permeable walls.