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1.
Aeolian abrasion and modes of fine particle production from natural red dune sands: an experimental study 总被引:3,自引:0,他引:3
A series of experiments was conducted to determine the potential for aeolian abrasion of natural dune sands to produce fine particles (< 125 µm) by (1) the release of resident fines; (2) spalling, chipping and breakage of particles; and (3) the removal of grain surface coatings. Parent samples were obtained from the surfaces of four active continental dunes and abraded using a glass ‘test tube’ chamber for up to 120 h. The fine particles produced by this abrasion process were trapped at varying time intervals and subject to detailed particle‐size analyses using a Coulter Multisizer. The abrasion of untreated parent samples produced fine particles in one of two main size classes, < 10 µm and > 50 µm, but when the parent sample was sieved to exclude particles < 250 µm, relatively more material in the range 10–50 µm was produced. For unsieved parent samples, the size range associated with the dominant mode varied according to the length of the abrasion time. The coarsest mode (> 63 µm) was dominant during the first 16 h of abrasion, then became less significant and is thought to be associated with the release of resident fines into suspension. The finest mode (< 10 µm) was absent or very weak during the first 16 h of abrasion, then became more significant and, in some instances, dominated the distribution as abrasion continued. Removal of grain surface coatings is the main source of fine material < 10 µm, and this may be a significant source of fine material in areas where sands are dominated by subrounded and rounded particles. By comparison with previous studies of aeolian particle abrasion, these natural dune sands produced very low quantities of fine material (by weight), but their spatial extent makes them potentially a significant source of dust‐sized particles at the global scale. 相似文献
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Global dust trajectories indicate that signi?cant quantities of aeolian‐transported iron oxides originate in contemporary dryland areas. One potential source is the iron‐rich clay coatings that characterize many sand‐sized particles in desert dune?elds. This paper uses laboratory experiments to determine the rate at which these coatings can be removed from dune sands by aeolian abrasion. The coatings impart a red colour to the grains to which previous researchers have assigned variable geomorphological signi?cance. The quantities of iron removed during a 120 hour abrasion experiment are small (99 mg kg?1) and dif?cult to detect by eye; however, high resolution spectroscopy clearly indicates that ferric oxides are released during abrasion and the re?ectance of the particles alters. One of the products of aeolian abrasion is ?ne particles (<10 µm diameter) with the potential for long distance transport. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
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Brian?Garvey Ian?P.?CastroEmail author Giles?Wiggs Joanna?Bullard 《Boundary-Layer Meteorology》2005,117(3):417-446
Wind-tunnel measurements of the flow over an isolated valley both normal and at an angle (45°) to a simulated neutrally stable
atmospheric boundary layer are presented. Attention is concentrated on the nature of the flow within the valley itself. The
work formed part of a wider study that included detailed field measurements around an African desert valley and some limited
comparisons with that work are included. A scale of about 1:1000 was used for the laboratory work, in which an appropriate
combination of hot wire and particle image velocimetry was employed. For a valley normal to the upwind flow, it is shown that
the upstream influence of the valley extends to a distance of at least one half of the axial valley width upstream of the
leading edge, whereas differences in mean flow and turbulence could be identified well beyond two valley widths from the downwind
edge. Non-normal wind angles lead to significant along-valley flows within the valley and, even at two valley heights above
the valley ridge level, there remains a significant spanwise flow component. Downwind turbulence levels are somewhat lower
in this case, but are still considerably higher than in the undisturbed boundary layer. At both flow angles, there are significant
recirculation regions within the valleys, starting from mean separation just beyond the leading edge, but the strong spanwise
flow in the 45° case reduces the axial extent of the separated zone. The flow is shown to be in some ways analogous to flow
over an isolated hill. Our results usefully enhance the field data and could be used to improve modelling of saltation processes
in the field. 相似文献
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