Modelling saltation intermittency
Résumé
In order to investigate the intermittency of the aeolian saltation, a saltation model, forced with instantaneous velocity fields, has been introduced in a Large Eddy Simulation airflow model. The coupled model is evaluated on a flat erodible surface under various wind conditions and soil particle-size distribution. It is first shown that the model is able to simulate a well-developed saltation layer in equilibrium with the turbulent flow. The main characteristics of the saltation layer and their sensitivity to wind conditions are in good agreement with previous data set. Then, the saltation intermittency is visualized through the presence of blowing sand structures near the surface, known as aeolian streamers. This is the first time that such structures are reproduced numerically. From a correlation analysis, we confirm previous thoughts that these sand structures are a visual footprint of past turbulent eddies propagating in the surface boundary layer. The streamers appear to be embedded in larger saltation structures with increasing wind conditions. The spatial scales of these streamers change with wind conditions and soil particle-size distribution. This is explained by two mechanisms: (1) the modification of eddy structures with the main characteristics of the saltation layer, and (2) the reduction of saltating particle sensitivity to the near-surface eddies with increasing wind condition and soil median particle diameter, as the eddy lifetime decreases within the saltation layer and the particle response time increases, respectively. The standard deviation of the saltation flux associated to these saltation patterns represents about 10% to 20% of the mean saltation flux.
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