The critical role of the boundary layer thickness for the Initiation of aeolian sediment transport

Pähtz, T., Valyrakis, M. , Zhao, X.-H. and Li, Z.-S. (2018) The critical role of the boundary layer thickness for the Initiation of aeolian sediment transport. Geosciences, 8(9), 314. (doi:10.3390/geosciences8090314)

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Abstract

Here, we propose a conceptual framework of Aeolian sediment transport initiation that includes the role of turbulence. Upon increasing the wind shear stress τ above a threshold value τ′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 u¯ induce fluid torques that exceed resisting torques. Upon a slight further increase of τ , rocking turns into a rolling regime (i.e., rolling threshold τt≃τ′t ) provided that the ratio between the integral time scale Ti∝δ/u¯ (where δ is the boundary layer thickness) and the time Te∝√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 Ti/Te is too small, the threshold ratio scales as τt/τ′t∝Te/Ti∝sd2/δ2 , consistent with experiments. Because δ/d controls Ti/Te 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.

Item Type:Articles
Additional Information:This research was funded by the National Natural Science Foundation of China (Grant Nos. 11750410687 and 41171005).
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Valyrakis, Dr Manousos
Authors: Pähtz, T., Valyrakis, M., Zhao, X.-H., and Li, Z.-S.
College/School:College of Science and Engineering > School of Engineering > Infrastructure and Environment
Journal Name:Geosciences
Publisher:MDPI
ISSN:2076-3263
ISSN (Online):2076-3263
Published Online:23 August 2018
Copyright Holders:Copyright © 2018 The Authors
First Published:First published in Geosciences 8(9): 314
Publisher Policy:Reproduced under a Creative Commons License

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