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.

No abstract

No abstract

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.

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.