title: Direct numerical simulations of aeolian sand ripples.
abstract: 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.
authors.affiliation: Laboratoire de Physique et Mécanique des Milieux Hetérogènes, UMR 7636, CNRS, Ecole Supérieure de Physique et de Chimie Industrielles, Université Paris Diderot, Université Pierre et Marie Curie, 75005 Paris, France; and MARUM-Center for Marine Environmental Sciences, University of Bremen, D-28359 Bremen, Germany oduran@marum.de.
authors.forename: Orencio
authors.initials: O
authors.lastname: Durán
authors.affiliation: Laboratoire de Physique et Mécanique des Milieux Hetérogènes, UMR 7636, CNRS, Ecole Supérieure de Physique et de Chimie Industrielles, Université Paris Diderot, Université Pierre et Marie Curie, 75005 Paris, France; and.
authors.forename: Philippe
authors.initials: P
authors.lastname: Claudin
authors.affiliation: Laboratoire de Physique et Mécanique des Milieux Hetérogènes, UMR 7636, CNRS, Ecole Supérieure de Physique et de Chimie Industrielles, Université Paris Diderot, Université Pierre et Marie Curie, 75005 Paris, France; and.
authors.forename: Bruno
authors.initials: B
authors.lastname: Andreotti
keywords: aeolian transport; resonance; self-organization; wind ripples