Pore-Scale Flow Effects on Solute Transport in Turbulent Channel Flows Over Porous Media

Abstract: Solute transport and mixing at channel-flow–porous media interfaces are strongly influenced by velocity and turbulence structures near porous media, and such coupled channel-flow–porous media systems are commonly observed in nature. However, the effects of pore-scale flows on solute transport in the coupled systems are currently unclear. In this study, we combine particle image velocimetry experiments and large eddy simulations to resolve the pore-scale flow characteristics over and within a porous bed. Then, we perform solute transport simulations by coupling the pore-scale flow fields with a particle-tracking model and show that the pore-scale flows inherent to porous media structure control solute transport. Pore-scale flow properties such as preferential downward–upward flows and vortices occurring near the channel-flow–porous media interface are shown to exert dominant control over interfacial mass exchange and solute transport. To clarify the effects of pore-scale flows on reach-scale transport, we conduct macroscale transport modeling with a spatially averaged stream-wise velocity profile. Because the profile-based model does not incorporate important pore-scale flow features, it significantly overestimates mass transfer into the porous bed, thereby exacerbating late-time tailings in breakthrough curves. Finally, a spatial Markov model, a type of upscaled stochastic transport model, is shown to effectively capture the pore-scale interfacial transport mechanisms via a velocity transition matrix. Our findings confirm that solute transport through channel-flow–porous media interfaces is controlled not only by interfacial turbulent-mixing profiles but also by detailed pore-scale flow structures. Article Highlights: We demonstrate the effects of pore-scale flows on solute transport in coupled turbulent channel-flow–porous media systemsPore structure near the interface exerts dominant control over interfacial mass exchange and solute transportSpatial Markov model effectively upscales the effects of pore-scale flows on solute transport.