The Critical Role of the Boundary Layer Thickness for the Initiation of Aeolian Sediment Transport

Abstract

Here, we propose a conceptual framework of Aeolian sediment transport initiation that includes the role of turbulence. Upon increasing the wind shear stress $\tau$ above a threshold value ${\tau }_t^{\prime }$, particles resting at the bed surface begin to rock in their pockets because the largest turbulent fluctuations of the instantaneous wind velocity above its mean value $\overline{u}$ induce fluid torques that exceed resisting torques. Upon a slight further increase of $\tau$, rocking turns into a rolling regime (i.e., rolling threshold ${\tau }_t\simeq {\tau }_t^{\prime }$) provided that the ratio between the integral time scale $T_i\propto \delta /\overline{u}$ (where $\delta$ is the boundary layer thickness) and the time $T_e\propto \sqrt{d/[(1-1/s)g]}$ required for entrainment (where $d$ is the particle diameter and $s$ the particle-air-density ratio) is sufficiently large. Rolling then evolves into mean-wind-sustained saltation transport provided that the mean wind is able to compensate energy losses from particle-bed rebounds. However, when $T_i/T_e$ is too small, the threshold ratio scales as ${\tau }_t/{\tau }_t^{\prime }\propto T_e/T_i\propto s{d}^2/{\delta }^2$, consistent with experiments. Because $\delta /d$ controls $T_i/T_e$ and the relative amplitude of turbulent wind velocity fluctuations, we qualitatively predict that Aeolian sediment transport in natural atmospheres can be initiated under weaker (potentially much weaker) winds than in wind tunnels, consistent with indirect observational evidence on Earth and Mars.

References in corpus (5)

• The physics of wind-blown sand and dust
• A scaling law for aeolian dunes on Mars, Venus, Earth, and for subaqueous ripples
• Difference in the wind speeds required for initiation versus continuation of sand transport on Mars: Implications for dunes and dust storms
• The apparent roughness of a sand surface blown by wind from an analytical model of saltation
• The Cessation Threshold of Nonsuspended Sediment Transport Across Aeolian and Fluvial Environments

Cited by in corpus (5)

• The physics of sediment transport initiation, cessation, and entrainment across aeolian and fluvial environments
• Large Effects of Particle Size Heterogeneity on Dynamic Saltation Threshold
• The role of coherent airflow structures on the incipient aeolian entrainment of coarse particles
• Comment on "Distinct Thresholds for the Initiation and Cessation of Aeolian Saltation From Field Measurements" by Raleigh L. Martin and Jasper F. Kok: Alternative Interpretation of Measured Thresholds as two Distinct Cessation Thresholds
• The role of energetic flow structures on the aeolian transport of sediment and plastic debris