PT - JOURNAL ARTICLE
AU - DurĂ¡n, Orencio
AU - Claudin, Philippe
AU - Andreotti, Bruno
TI - Direct numerical simulations of aeolian sand ripples
AID - 10.1073/pnas.1413058111
DP - 2014 Oct 15
TA - Proceedings of the National Academy of Sciences
PG - 201413058
4099 - http://www.pnas.org/content/early/2014/10/15/1413058111.short
4100 - http://www.pnas.org/content/early/2014/10/15/1413058111.full
AB - Wind ripples decorate the flanks of dunes in amazingly regular patterns, on both Earth and Mars. Their emergence at a wavelength much larger than the grain size is currently unexplained. We report direct numerical simulations of grains interacting with a wind flow that are, for the first time to our knowledge, able to reproduce the spontaneous growth of ripples with an initial wavelength and a propagation velocity linearly increasing with the wind speed. We propose a new formation mechanism, involving resonant grain trajectories tuned with the ripple wavelength. We also show that the product of the ripple wavelength and velocity is a proxy for the sediment flux, opening a promising perspective from which to perform remote measurements of sand mass transfers, on Mars in particular.Aeolian sand beds exhibit regular patterns of ripples resulting from the interaction between topography and sediment transport. Their characteristics have been so far related to reptation transport caused by the impacts on the ground of grains entrained by the wind into saltation. By means of direct numerical simulations of grains interacting with a wind flow, we show that the instability turns out to be driven by resonant grain trajectories, whose length is close to a ripple wavelength and whose splash leads to a mass displacement toward the ripple crests. The pattern selection results from a compromise between this destabilizing mechanism and a diffusive downslope transport which stabilizes small wavelengths. The initial wavelength is set by the ratio of the sediment flux and the erosion/deposition rate, a ratio which increases linearly with the wind velocity. We show that this scaling law, in agreement with experiments, originates from an interfacial layer separating the saltation zone from the static sand bed, where momentum transfers are dominated by midair collisions. Finally, we provide quantitative support for the use of the propagation of these ripples as a proxy for remote measurements of sediment transport.