In aquatic environments such as rivers, the exchange of solutes across the interface between the sediment and the overlying water plays a signifcant role in controlling biogeochemical processes, which are important for an array of topics from nutrient transport and cycling to release of greenhouse gases such as nitrous oxide. Most previous studies on characterizing this exchange are focused on flows with sediment bedforms much larger than individual sediment grains. The physics at the pore or grain scale were typically not resolved. The effects of grain roughness on the sedimentbed surface on the transport across the sediment-water interface (SWI), isolated from those of bed permeability and bedforms, are not well understood. In this work, direct numerical simulations (DNS) of the connected system of turbulent open-channel flow and pore-resolved sediment flow are carried out, with different arrangements of grains at the sediment surface.First, the statistics and structure of the mean flow and turbulence are characterized in flows with a friction Reynolds number of 395 and a permeability Reynolds number of 2.6 over sediments with either regular or random grain packing on a macroscopically flat bed. It is shown that, even in the absence of any bedform, the subtle details of grain roughness alone can signifcantly affect the dynamics of turbulence and the time-mean flow. Such effects translate to large differences in penetration depths, apparent permeabilities, vertical mass fluxes and subsurface flow paths. The less organized distribution of mean recirculation regions near the interface with a random packing leads to a more isotropic form-induced stress tensor, which plays a signifcant role in increasing mixing and wall-normal exchange of mass and momentum.Next, the mass exchange is characterized in detail for macroscopically flat river beds, focusing on the transit time—the time spent by a fluid particle in the sediment—which determines the role of hyporheic zones in transforming the chemical signature of stream water. Results show that bedroughness leads to interfacial pressure variations, which induces deep subsurface flow paths that yield a transit time distribution with a heavy tail. Furthermore, the addition of molecular diffusionis accounted for and is shown to increase transit times regardless of roughness texture. The results demonstrate that particle roughness on a macroscopically flat sediment bed can induce signifcant hyporheic exchange that is fundamentally similar to that induced by bedforms.Lastly, to identify possible interaction between the effect of grain roughness and that of a bedform, DNSs of open channels with a friction Reynolds number of 1580 on a porous dune with two different roughnesses are conducted. Results show that the roughness modifes the wall friction, shear penetration depth and pressure distribution along the interface. Unlike the case on a macroscopically flat bed where the random roughness induces more intense roughness-scale pressure variation than the regular roughness, over a bedform the random roughness reduces the macroscopic pressure distribution at the interface instead due to its higher hydrodynamic drag. The weaker pressure variation in turn weakens the pumping and shortens transit times. The results highlight the nonlinear interaction between the effects of bed morphological features of different scales. Pore-resolved simulations such as the ones herein can be used in the future in direct characterization of pore-scale dynamics to provide insights for pore-unresolved modeling of biogeochemical processes.
Read
