Coupled hyporheicFoam solver for flow and reactive transport in streams and hyporheic zones: strengths, weaknesses, and future development

Abstract:

Hyporheic exchange, driven by pressure difference on sediment bed caused by any natural or anthropogenic flow disturbances, is a vital part of fluvial and biogeochemical processes. In conjunction with lab and field studies, computer models have been developed and utilized widely to reveal the seemly hidden hyporheic dynamics. One recent such model is hyporheicFoam, an open source solver developed using OpenFOAM, a popular computational framework. In this solver, the two computational domains, i.e., surface river channel and subsurface sediment bed are governed by their respective sets of equations. Fluid and species mass exchange and conservation across the bed interface are enforced through an iterative coupling procedure. With this solver, it is now possible to truly couple the surface and subsurface domains at time-step level during a simulation. However, like all models, the current implementation is far from perfect. In this talk, we will appraise the strengths and weaknesses of current formulations and algorithms implemented in hyporheicFoam. For example, the coupling between the two domains may be simulated with some other strategies to improve stability and accuracy. In addition, the current implementation uses a prevailing assumption that the advective flux dominates the hyporheic exchange at the bed interface. This assumption is not correct at low Peclet number. One symptom of this wrong assumption is the flux inconsistency across the interface. Thus, a new algorithm is needed to ensure the continuity of fluxes from both advection and diffusion.