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

Abstract

Experimental measurements of terminal fall velocities at sea level are reported for pumices, glass shards, and feldspar crystals with mean diameters between 30 and500 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, C a, as a function of Reynolds number, R a, and shape parameter is: [Display omitted] 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.