3D Direct Numerical Simulation on the Emergence and Development of Aeolian Sand Ripples

Abstract

A sand surface subjected to a continuous wind field exhibits a regular ripple surface. These aeolian sand ripples emerge and develop under the coupling effect between the wind field, bed surface topology, and sand particle transportation. Lots of theoretical and numerical models have been established to study the aeolian sand ripples since the last century, but none of them has the capability to directly reproduce the 3D long-term development of them. In this work, a novel numerical model with wind-blow sand and dynamic bedform is established. The emergence and long-term development of sand ripples can be obtained directly. The statistical results extracted from this model tally with those deduced from wind tunnel experiments and field observations. A simplified bed surface particle size description procedure is used in this model, which shows that the particle size distribution makes a very important contribution to sand ripples’ final steady state. This 3D bedform provides a more holistic view on the merging of small bumps before regular ripples’ formation. Analyzing the wind field results reveals an ignored development on the particle dynamic threshold during the bedform deformation.

Megaripple mechanics: bimodal transport ingrained in bimodal sands

“…On the other hand, if future missions were to find evidence of bimodal GSDs with a larger coarse-grain width than predicted, it would be strongly suggestive for bedforms with different mechanistic origins. For example, spatial grain sorting and bedform evolution occurs for bidisperse sands even when both grain sizes are saltating 34,39,42 , so that not all bedforms with a perceptible armouring layer on the crest need to be megaripples in the narrow sense of the term used here. If instead future missions were to find evidence of bimodal GSDs with smaller-than-predicted coarse-grain peaks, megaripples could not be completely ruled out because of the possibility of a total lack of immobile coarse grains.…”

“…Unimodal transport: accordingly, for winds beyond the coarse-grain saltation threshold, the coarse grains of the armouring layer are entrained into saltation, and their thus dramatically increased hop lengths render megaripples subcritical and therefore unstable to erosion. (However, small ripples with a perceptible armouring layer may emerge since even unimodal saltation can lead to spatial sorting 34,39,42 .) Selective transport: fine-grain impacts cannot mobilize coarse grains (τ < τr(d (c) )) and megaripple formation stagnates.…”

“…On the one hand, if the wind strength exceeds the saltation threshold of the coarsest available grains, all grains will saltate and the megaripple will quickly be destroyed. In its place, small ripples with a perceptible armouring layer may emerge, since even unimodal saltation can lead to spatial sorting 34,39,42 . On the other hand, if the wind falls below the saltation thresholds for all available grains, stagnation ensues.…”

“…In fact, while megaripples grow quite slowly, they can relatively quickly be destroyed by gusts that exceed the prevailing wind strength, which is yet another anomalous feature that sets them apart from the less fragile ripples and dunes 3,[27][28][29][30] . Since Bagnold's early observations 3 and the pioneering work of Anderson and Bunas 34 , various models have been proposed to explain the formation of megaripples 21,[35][36][37][38][39][40][41][42] . They all share the key hypothesis that megaripples origi-Fig.…”

Effects of Covering Layer of Sand Particles on Particle‐Bed Collision and Aeolian Sand Transport

“…The initial condition of wind field was provided by formula as (Dun et al., 2017): $$$u=\frac{{u}_{\ast }}{k}\mathrm{ln}\left(\frac{z}{{z}_{0}}\right)$$$ where $$k=0.4$$ was the Karman constant, $${u}_{\ast }=0.2,\,0.3,\,0.45,\,0.6,\ \mathrm{a}\mathrm{n}\mathrm{d}\,0.7\ \mathrm{m}/\mathrm{s}$$ was the friction velocity, $${z}_{0}=30{\fracslash}d$$ was the roughness. As previous study (Huo et al., 2021), we used uniform particle size $$d=250\,\mu \mathrm{m}$$. 100 induced particles without initial velocities were randomly released and used to develop naturally into saturated sand flow at the beginning of calculation.…”