Application of the Hofmann shape entropy to determine the settling velocity of irregular, semi-ellipsoidal grains

In earlier studies on the settling velocity of irregular particles, it was observed that some of these grains settle faster than regular, symmetrical gains of the same weight and shape (as defined by the Janke shape factor). This was attributed to the smaller projection areas of irregular grains perpendicular to the fall direction. The effect of roundness was considered to be undetectable at Reynolds numbers less than 2. In this study, the original data are re-examined using the more accurate settling equations for smooth particles currently available. It is concluded that the higher settling rates of irregular particles reported by previous investigators should not necessarily be attributed to a reduction in their projection area, but may be partly related to their departure from typical spheroidal shapes. Roundness has a small, but nevertheless positive influence on the settling velocity. A new settling equation for irregular, semi-ellipsoidal gains is proposed, which has a mean error of only 5.15% for the particular data set, compared to 9.89% of the best equation based on the Janke shape factor. This equation can be used for pipette analysis of very fine sediments settling at Reynolds numbers less than 2. (C) 2002 Elsevier Science B.V. All rights reserved.