Universität zu Köln

Kamath, Sandesh Haleangady ORCID: 0000-0001-9260-9777 (2025). A numerical model for aeolian sand transport and the concatenated dust emission. PhD thesis, Universität zu Köln.

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Abstract

Dust is emitted, transported and deposited throughout the year, mainly from the vast sand seas on Earth affecting the weather, climate ecosystems and other cycles of the biosphere. It influences the various feedback mechanisms, and causes the largest uncertainties in the future climate projections. A particle-based 3D numerical model to study the dust aerosol emission is presented in this work. The model was validated using the well characterized sand saltation process which is one of the major mechanisms through which dust is entrained. Using a scalable Discrete Element Method (DEM) model, which is coupled with the fluid dynamics of the turbulent wind, we confirm the existence of a quadratic scaling for the sand mass flux with the wind friction velocity. The impact threshold (minimal wind velocity for sustained transport) and fluid threshold (minimal wind velocity for grain entrainment) values for 200 micrometer sand grains which are key parameters in examining the grain initiation were found. Previous numerical studies never considered the sparsely-covered soils, and thus we developed a scheme to characterize this problem of low-sand availability. Thus we observe a transition from the quadratic scaling in the conventional erodible cases, to a cubic scaling over rigid surfaces. The universality of the model was also tested for varying fluid conditions, due to the comprehensive depiction of the viscous layer close to the bed surface. To simulate mixed sand-dust systems, the model was extended by including the crucial aspect of cohesion, which is modeled using the van der Waals interaction. The rolling resistance, lift force, as well as the stochastic turbulent fluctuations provided the means to verify the grain-size dependency in monodisperse systems on shear velocity which reaffirms the [Shao and Lu 2000] equation. The stochastic nature of cohesion [Shao and Klose 2016] further lowers the threshold values in the presence of turbulent fluctuations, thus stressing on the need to include it in future models. The direct numerical simulations for the first time allow to study the dust emission mechanisms - direct entrainment, saltation bombardment and aggregate disintegration at the micro-scale, as the grain clusters which form and break is captured. In bi-disperse sand-dust beds (10 micrometer dust grains dispersed over 200 micrometer sand grains), under limited supply of dust, we observe the lowering of fluid thresholds as a result of direct aerodynamic entrainment at nominal wind speeds below the saltation threshold. This is due to the fact that, the dust grains, unlike in a monodiperse system are exposed to higher winds because of the roughness elements (sand grains). We observe a quartic scaling for the vertical dust flux with shear velocity, with the scaling exponent having direct implications on the empirical relations in climate models. Finally, we conclude that in size regimes of 5 and 2.5 micrometer grains, they are not easily entrained due to cohesion, but dust is usually present as either mostly coated on sand, or as dust clusters, thus saltation bombardment and cluster disintegration to a certain extent are the dominating mechanisms for emission as predicted before.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Kamath, Sandesh Haleangady skamath@uni-koeln.de orcid.org/0000-0001-9260-9777 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-755745
Date:	2025
Place of Publication:	Köln
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology
Subjects:	Natural sciences and mathematics Physics Earth sciences Technology (Applied sciences)
Uncontrolled Keywords:	Keywords Language Sand transport English Dust emission English Climate change English Sediment process English Discrete Element Method English Computational Fluid Dynamics English Stochastic turbulence English Cohesion English Direct numerical simulation English
Date of oral exam:	28 October 2024
Referee:	Name Academic Title Shao, Yaping Prof. Dr. Wurm, Gerhard Prof. Dr. Parteli, Eric J. R. Dr.
Projects:	German Research Foundation (DFG) - 348617785
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/75574