Direct simulation of turbulent sediment-water interface

Vertical transport processes across the sediment-water interface play a significant role in biogeochemical processes in aquatic ecosystems. Most numerical studies on residence time distribution and exchange flux and depth are based on the laminar-flow assumption or eddy-viscosity closures. Here, we use direct numerical simulation (DNS) of a turbulent open-channel flow with Reτ= 180 over a grain-resolved sediment bed, with a permeability Reynolds number of 2.56; the Reynolds shear stress dominates the viscous shear stress at the sediment-water interface. The goal is to analyze the link between macroscopic flow-exchange parameters and detailed flow physics. A sediment model similar to the one used in the experiments of Voermans, Ghisalberti and Ivey (2017) is used. The porous medium is modeled as randomly-packed, monodisperse hard spheres represented using an immersed boundary method. The random sphere packing is achieved through molecular dynamics simulations. Overall, single-point turbulence statistics agree with experimental measurements; the form-induced stresses are non-negligible near the interface, and are sensitive to the detailed packing near the sediment top. Discussions will be provided on the link between the overall exchange flux and detailed flow characteristics.