Direct numerical simulations of aeolian sand ripples

TL;DR

This study uses direct numerical simulations to reveal that aeolian sand ripple formation is driven by resonant grain trajectories and a balance between destabilizing and stabilizing mechanisms, with implications for remote sediment transport measurement.

Contribution

It introduces a new understanding of ripple pattern formation driven by resonant grain trajectories and provides a scaling law linking wind velocity to ripple wavelength.

Findings

Ripple wavelength scales linearly with wind velocity.

Resonant grain trajectories drive the instability.

Ripple propagation can be used for remote sediment transport estimates.

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 towards 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 mid-air collisions. Finally, we provide quantitative support for the use the propagation of these ripples as a proxy for remote measurements of sediment transport.