Barchans are 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 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.