  In the realm of granular bedforms, barchan dunes are strong attractors that
can be found in rivers, terrestrial deserts and other planetary environments.
These bedforms are characterized by a crescentic shape, which, although robust,
presents different scales according to the environment they are in, their
length scale varying from the decimeter under water to the kilometer on Mars.
In addition to the scales of bedforms, the transport of grains presents
significant differences according to the nature of the entraining fluid, so
that the growth of barchans rests to be fully understood. Given the smaller
length and time scales of the aquatic case, subaqueous barchans are the ideal
object to study the growth of barchan dunes. In the present paper, we reproduce
numerically the experiments of Alvarez and Franklin [Phys. Rev. E 96, 062906
(2017) and Phys. Rev. Lett. 121, 164503 (2018)] on the shape evolution of
barchans from their initiation until they have reached a stable shape. We
computed the bed evolution by using computational fluid dynamics - discrete
element method (CFD-DEM), where we coupled DEM with large eddy simulation (LES)
for the same initial and boundary conditions of experiments, performed in a
closed-conduit channel where initially conical heaps evolved to single barchans
under the action of a water flow in turbulent regime. Our simulations captured
well the evolution of the initial pile toward a barchan dune in both the
bedform and grain scales, with the same characteristic time and lengths
observed in experiments. In addition, we obtained the local granular flux and
the resultant force acting on each grain, the latter not yet previously
measured nor computed. This shows that the present method is appropriate to
numerical computations of bedforms, opening new possibilities for accessing
data that are not available from current experiments.
