A Numerical Research on the Relationship between Aeolian Sand Ripples and the Sand Flux

Xinghui Huo,Jie Zhang,Ning Huang

doi:10.3390/pr10020354

Abstract

Theoretically, the sand flux will not change after the wind-driven sand particle transport reaches the saturated state. However, it has been found in many wind-tunnel experiments that the sand flux will gradually decrease with time in long-term particle transport duration and will eventually reach a new stable state. In this work, we used numerical simulations to study the source of this kind of decrease and found it is caused by the sand ripple on the bed surface. The ripple index showed a strong correlation to the sand flux, and it decreased during the initial stage of the ripple formation. With a simplified theoretical model, we found the linear relationship between the Shields number and the particle transport load holds. However, the slope of this relationship and the dynamic threshold of particle entrainment decreased with the ripple index. As the sand flux scales linearly with the particle transport load, we finally derived an expression that describes how the sand flux on the ripple bedform varies with the wind strength. From this expression, we found the sand flux increases with ripple index, and it was easier to be influenced by the ripple bed form in small wind strength.

Highlights

While subject to a continuous wind, small particles in the granular bed can always be transported by the means of suspension, saltation, or reptation
Shields number and the particle transport load holds. The slope of this relationship and the dynamic threshold of particle entrainment decreased with the ripple index
We found the sand flux increases with ripple index, and it was easier to be influenced by the ripple bed form in small wind strength

Summary

Introduction

While subject to a continuous wind, small particles in the granular bed can always be transported by the means of suspension, saltation, or reptation. Comparing to the suspension particle who drifts a very long distance without touching the ground (usually in the scaling of kilometers), saltation and reptation particles interact with a local bed surface more frequently, i.e., they bounce forward along the wind direction. The only difference between them is that the former inherits more energy from the wind, making it rebound after every impact and moving forward continuously. Because of these reasons, we realize that the local topology of the bed surface can seriously influence the aeolian particle transport. The bed surface consists of grains that can be modified by the particle transportation as well

Methods

Results

Conclusion