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Open AccessArticle

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

1
Institute of Port, Coastal and Offshore Engineering, Ocean College, Zhejiang University, 866 Yu Hang Tang Road, Hangzhou 310058, China
2
State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, 36 North Baochu Road, Hangzhou 310012, China
3
Infrastructure and Environment Research Division, School of Engineering, University of Glasgow, Glasgow G12 8QQ, UK
4
Division of Environmental Management and Policy, School of Environment, Tsinghua University, Beijing 100084, China
5
College of Environmental Science and Engineering, Peking University, Beijing 100871, China
6
The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Peking University, Beijing 100871, China
*
Author to whom correspondence should be addressed.
Geosciences 2018, 8(9), 314; https://doi.org/10.3390/geosciences8090314
Received: 29 June 2018 / Revised: 15 August 2018 / Accepted: 20 August 2018 / Published: 23 August 2018
(This article belongs to the Special Issue Aeolian Processes and Geomorphology)
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 τ t τ t ) provided that the ratio between the integral time scale T i δ / u ¯ (where δ is the boundary layer thickness) and the time T e 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 τ t / τ t T e / T i s d 2 / δ 2 , consistent with experiments. Because δ / 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. View Full-Text
Keywords: Aeolian sand transport; saltation; aerodynamic entrainment threshold; saltation threshold; initiation threshold; static threshold; dynamic threshold; Venus wind tunnel; Titan wind tunnel Aeolian sand transport; saltation; aerodynamic entrainment threshold; saltation threshold; initiation threshold; static threshold; dynamic threshold; Venus wind tunnel; Titan wind tunnel
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MDPI and ACS Style

Pähtz, T.; Valyrakis, M.; Zhao, X.-H.; Li, Z.-S. The Critical Role of the Boundary Layer Thickness for the Initiation of Aeolian Sediment Transport. Geosciences 2018, 8, 314.

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