Pore-Scale Flow Effects on Solute Transport Across Free Flow-Porous Media Interface

Abstract:

Free flow-porous media interfaces exist at streambeds, and flow structures near the interfaces exert significant impacts over solute transport and dispersion in rivers. The interaction between turbulence and solute mixing across free flow-porous media interfaces have been previously investigated, while pore-scale flow effects on solute transport across streambed are not yet well understood. We combine laboratory experiments and numerical simulations to elucidate pore-scale flow effects on solute dispersion through free flow-porous media interfaces. We first conduct particle image velocimetry (PIV) experiments in a coupled free flow-porous media flume composed of staggered-arrayed cylindrical pillars. Then, we simulate the hydrodynamics of the experimental case using large eddy simulation (LES) by directly resolving subsurface flows at the pore scale, which is adequately validated with the PIV measurement. For solute transport simulation, we couple the simulated width-averaged flow field with a two-dimensional (2D) Lagrangian particle tracking model. Both PIV and LES results show that the staggered pore structure induces strong upward flows near the free flow-porous media interface. This pore-scale preferential flow structures prevent solute tracers from diffusing deeply into the porous media and bring the tracers back to the free flow region, thus limiting the longitudinal spreading (Fig. 1a). To elucidate these pore-scale flow effects on solute dispersion, we also simulate solute transport using the one-dimensional (1D) profile of streamwise velocity components, neglecting the vertical flow motions. The transport simulation result with the 1D velocity profile shows longitudinal dispersion larger than that of the simulation result that honors the pore-scale flows. This is because the simulation with the 1D velocity profile neglects important pore-scale effects such as preferential flows near the free flow-porous media interface, thereby allowing solute tracers to more readily diffuse into the porous media via turbulent diffusion (Fig. 1b). This study shows that pore-scale flow effects can significantly control solute transport in the coupled free flow-porous media system. Finally, we propose an upscaled transport model that honors pore-scale flow effects on tracer transport.