  Barchan dunes, or simply barchans, are crescent-shaped dunes found in diverse
environments such as the bottom of rivers, Earth's deserts and the surface of
Mars. In a recent paper [Phys. Rev. E 101, 012905 (2020)], we investigated the
evolution of subaqueous barchans by using computational fluid dynamics -
discrete element method (CFD-DEM), and our simulations captured well the
evolution of an initial pile toward a barchan dune in both the bedform and
grain scales. The numerical method having shown to be adequate, we obtain now
the forces acting on each grain, isolate the contact interactions, and
investigate how forces are distributed and transmitted in a barchan dune. We
present force maps and probability density functions (PDFs) for values in the
streamwise and spanwise directions, and show that stronger forces are
experienced by grains at neither the crest nor leading edge of the barchan, but
in positions just upstream the dune centroid on the periphery of the dune. We
show also that a great part of grains undergo longitudinal forces of the order
of 10$^{-7}$ N, with negative values around the crest, resulting in
decelerations and grain deposition in that region. These data show that the
force distribution tends to route a great part of grains toward the crest and
horns of subaqueous barchans, being fundamental to comprehend their
morphodynamics. However, to the best of the authors' knowledge, they are not
accessible from current experiments, making of our results an important step
toward understanding the behavior of barchan dunes.
