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

TL;DR

This paper presents a conceptual framework highlighting the importance of boundary layer thickness in initiating Aeolian sediment transport, emphasizing turbulence and the ratio of integral to entrainment timescales.

Contribution

It introduces a new model linking boundary layer thickness and turbulence to sediment transport thresholds, explaining differences between natural environments and wind tunnel experiments.

Findings

Transport initiation depends on turbulence-induced particle rocking and rolling.

The ratio of integral to entrainment timescales determines the transport threshold.

Transport can occur at weaker winds in natural atmospheres than in wind tunnels.

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 $\tau^\prime_t$, 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 $\tau_t\simeq\tau^\prime_t$) 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 $\tau_t/\tau^\prime_t\propto{T}_e/T_i\propto{s}d^2/\delta^2$, 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.