Barchans interacting with dune-size obstacles: details of the fluid flow and motion of grains

TL;DR

This study uses detailed numerical simulations to analyze how subaqueous barchans interact with dune-sized obstacles, revealing flow disturbances, grain trajectories, and vortex formations that influence whether grains bypass, circumvent, or get trapped by obstacles.

Contribution

It provides new insights into the fluid dynamics and grain motion during dune-obstacle interactions, including the role of vortices in grain bypass and trapping mechanisms.

Findings

Particles can pass over, circumvent, or be trapped by obstacles.

A strong vortex forms between the dune and obstacle, affecting grain movement.

Flow deviations caused by vortices explain different interaction outcomes.

Abstract

We investigate details of the interaction of subaqueous barchans with dune-size obstacles by carrying out numerical simulations where the fluid is solved at the grain scale and the motions of individual grains are computed at all time steps. With the outputs, we analyze the disturbances of the fluid flow, the trajectories of grains, and the resultant force on each grain, the latter being unfeasible from experiments and field measurements. We show that in some cases particles pass over the obstacle, while in others they completely circumvent it (without touching it), or are even blocked. For the circumvention and blocking cases, which we call bypass and trapped, respectively, we show the existence of a strong vortex between the lee face of the dune and the obstacle. This vortex results from the interactions of recirculation regions and horseshoe vortices, and has enough strength to deviate the main flow and carry grains around the obstacle in those cases. Our results shed light on the reasons for passing over, circumventing, and blocking, and contribute to our understanding of dunes in the presence of large obstacles such as hills, crater rims, and human constructions.