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.

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Aeolian sand transport drives geophysical phenomena, such as bedform evolution and desertification. Creep plays a crucial, yet poorly understood, role in this process. We present a model for aeolian creep, making quantitative predictions for creep fluxes, which we verify experimentally. We discover that the creep transport rate scales like the Shields number to the power 5/2, clearly different from the laws known for saltation. We derive this 5/2 power scaling law from our theory and confirm it with meticulous wind tunnel experiments. We calculate the creep flux and layer thickness in steady state exactly and for the first time study the relaxation of the flux toward saturation, obtaining an analytic expression for the relaxation time.

Eolian sand transport is a typical gas-solid flow process, and saltating sand particles account for 75% of the total sand transported. Sand particles are considered as perfect spheres, and the forces acted on the particles are calculated using concise formulas in almost all wind-sand movement models. In fact, most of sand particles in natural environment are nonspherical and usually rotating while they saltate in the air, and the complicated turbulent gas-solid flow caused by irregular shape and rotation of particles cannot be predicted. In this paper, the microcosmic midair movements of rotating elliptical sand particles are simulated with dynamic grid method, and the interaction between airflow and sand particles is studied. The results show that the trajectory of a rotating elliptical particle is 18.8% higher and 31.8% longer than that of a spherical one and the drag force acted on a rotating elliptical sand particle fluctuates periodically over time. Separating, colliding, and tumbling behaviors are observed during the saltation process of coupled sand particles. Affected by strong interaction between rotating coupled elliptical particles and airflow, the trajectory is 8.4% lower than one particle system, and the rotation decreases the midair colliding behaviors. A parametric scheme for drag coefficient of sand particle is summarized to modified the macroscopical Eulerian-Lagrangian wind-sand simulation, in which the saltation transport rate varies for different shapes of sand particles. This work indicts that an accurate prediction of shape and rotation effects is required to describe the sand movement and saltation transport.

The Gobi Desert is a vast semidesert area in China, which is mostly covered by gravel with minor vegetation. The Gobi in northern and northwestern part of China is believed to be the main source of sandstorms in East Asia (Sun et al., 2001), and the amount of dust released from the Gobi area (3.7 tons per day) during sandstorm is much higher than that from the sand desert area (0.8 tons per day; S. Chen et al., 2017). Dust emissions due to frequent sandstorms in the Gobi area (Jugder et al., 2012;Natsagdorj et al., 2003) usually accompanied by strong sand surface wind-blown sand movement, which causes great harm to various facilities and railway lines in the vicinity.The wind-blown sand is a typical two-phase (gas-solid) flow. It forms by the interaction between air and solid particulates (Z. Wang et al., 2009;Wu, 2003). Bagnold (1935) first combined theoretical studies on fluid mechanics with his long-time field investigations in the Libyan Desert and further with a series of wind tunnel experiments (Bagnold, 1941) to understand wind-blown sand. Since then, numerous studies have been carried out on the theoretical simulations and quantitative analysis of wind-blown sand (

Wind-break walls along Lanxin High-Speed Railway II were studied and approved as effective measures to reduce strong wind damage to the high-speed trains. The results show that sand sedimentation on the leeward sides of wind-break walls along the railway within Gobi Desert could significantly threaten the operation safety of running trains. Different from the current sand sedimentation prevention measures without adequate consideration of the deposition process of airborne sand particles, this study revealed the mechanism of sand sedimentation on the leeward sides of three wind-break walls within different terrains. A series of wind-tunnel experiments were carried out to measure the horizontal velocity, number density, transport flux, and deposition rate of sand particles, and it was found that the horizontal speed of sand particles was first increased and then decreased on the railway track, and the peak speed over the concave subgrade was much smaller than those over convex and flat subgrades. The number density and horizontal sand flux were largest over the concave subgrade, and were the smallest over the convex subgrade. The sand particle deposition rate and distribution were also the largest within the concave subgrade, and some measures were also proposed to prevent sand sedimentation on the leeward sides of wind-break walls.

As part of a comprehensive environmental assessment of the Dun-Gel railway project located in Dunhuang city, Gansu Province, China, a wind tunnel experiment was proposed to predict surface shear stress changes on a sand dune when a bridge was built upstream it. The results show that the length of the wall shear stress shelter region of a bridge is about 10 times of the bridge height (H). In the cases that the interval of the bridge and sand dune (S) is less than 5 H, normalized wall shear stress on the windward crest is decreased from 1.75 (isolated dune) to 1.0 (S = 5.0 H, measured downwind bridge pier) and 1.5 (S = 5.0 H, measured in the middle line of two adjacent bridge piers). In addition, the mean surface shear stress in the downstream zone of the sand dune model is reduced by the bridge pier and is increased by the bridge desk. As for the fluctuation of surface shear stress ( ζ ) on the windward crest, ζ decreases from 1.3 (in the isolated dune case) to 1.2 (in the case S = 5.0 H, measured just downwind the pier) and increases from 1.3 (in the isolated dune case) to 1.6 (in the cases S = 5.0 H, in the middle of two adjacent piers). Taking the mean and fluctuation of surface shear stress into consideration together, we introduce a parameter ψ ranging from 0 to 1. A low value indicates deposition and a high value indicates erosion. On the windward slope, the value of ψ increases with height (from 0 at toe to 0.98 at crest). However, in the cases of S = 1.5 H, ψ is decreased by the bridge in the lower part of the sand dune at y = 0 and is increased at y = L/2 compared with the isolated dune case. In other cases, the change of ψ on the windward slope is not as prominent as in the case of S = 1.5 H. Downstream the sand dune, erosion starts in a point that exists between x = 10 H and 15 H in all cases.