Barchan‐Barchan Dune Repulsion Investigated at the Grain Scale.

Barchans are eolian dunes of crescent shape found on Earth, Mars and other celestial bodies. Among the different types of barchan‐barchan interaction, there is one, known as chasing, in which the dunes remain close but without touching each other. In this paper, we investigate the origins of this barchan‐barchan dune repulsion by carrying out grain‐scale numerical computations in which a pair of granular heaps is deformed by the fluid flow into barchan dunes that interact with each other. In our simulations, data such as position, velocity and resultant force are computed for each individual particle at each time step, allowing us to measure details of both the fluid and grains that explain the repulsion. We show the trajectories of grains, time‐average resultant forces, and mass balances for each dune, and that the downstream barchan shrinks faster than the upstream one, keeping, thus, a relatively high velocity although in the wake of the upstream barchan. In its turn, this fast shrinkage is caused by the flow disturbance, which induces higher erosion on the downstream barchan and its circumvention by grains leaving the upstream dune. Our results help explaining the mechanisms behind the distribution of barchans in dune fields found on Earth and Mars. Plain Language Summary: Barchans are crescent‐shaped wind‐formed dunes that grow when the wind is roughly unidirectional and the amount of sand is limited (typically, over a non‐erodible ground). They are found in dune fields on Earth and Mars, where barchan dunes interact with each other, forming different field patterns. One first question that arises is why, in some cases, two nearby barchans do not touch each other. Is there a repulsion mechanism? In this paper, we investigate the barchan‐barchan dune repulsion by carrying out numerical computations of the motion of the fluid and each grain forming a pair of barchans. The outputs are detailed measurements of both fluid and grains, such as trajectories, forces, and mass balances, and they show that the downstream barchan shrinks faster than the upstream one. The results also show that the faster shrinkage is due to the fluid flow disturbance caused by the upstream barchan: the fluid impacts the downstream dune, increasing its erosion at the same time that it entrains further downstream part of grains coming from the upstream dune. As a result, the downstream barchan moves at the same pace or faster than the upstream dune, though receiving grains from the latter (which only loses grains). Key Points: We investigate numerically both the fluid flow and motion of grains for understanding the mechanisms behind barchan‐barchan dune repulsionWe measure the flow rate of grains and show that there is, indeed, greater erosion on the downstream barchanThe disturbed flow impacting the downstream barchan induces higher erosion (of its grains) and low accumulation (of upstream grains) [ABSTRACT FROM AUTHOR]