The influence of shape on the atmospheric settling velocity of volcanic ash particles

Wilson, L. and Huang, T. C. (1979) The influence of shape on the atmospheric settling velocity of volcanic ash particles. Earth and Planetary Science Letters, 44 (2). pp. 311-324. ISSN 0012-821X

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Abstract

Experimental measurements of terminal fall velocities at sea level are reported for pumices, glass shards, and feldspar crystals with mean diameters between 30 and >500 μm. The velocities depend significantly on particle shape and rotation mode as well as density and size. Six tumbling modes were observed, of which two are predominant. The measurements have been converted to drag coefficients and Reynolds numbers so that they can be used to compute terminal velocities at any height in the atmosphere. It was found that if the drag coefficient and Reynolds number are defined empirically in terms of the arithmetic mean particle diameter, the effects of shape and rotation can be fully accounted for by defining a shape parameter, F, for each particle. In terms of the lengths of the longest, intermediate and shortest principal axes of the particle, denoted a, b, and c, respectively, F = (b + c) 2a. A simple formula for the drag coefficient, Ca, as a function of Reynolds number, Ra, and shape parameter is: Ca= 24 RaF-0.828+2 1.07-F. A more fundamental analysis allows the measurements made here to be compared with theoretical curves and experimental data on simple particle shapes from wind tunnel studies.

Item Type:
Journal Article
Journal or Publication Title:
Earth and Planetary Science Letters
Uncontrolled Keywords:
/dk/atira/pure/subjectarea/asjc/1900/1912
Subjects:
ID Code:
149048
Deposited By:
Deposited On:
16 Nov 2020 11:50
Refereed?:
Yes
Published?:
Published
Last Modified:
23 Nov 2020 14:56