Abstract

Settling rates of irregular glass particles were measured in a settling tube containing glycerine, yielding Reynolds numbers equivalent to silt to very fine sand quartz grains settling in water. The glass particles were collected from beaches and so show varying degrees of rounding due to sand-blasting in the surf. Each was measured for its weight, roundness, and grain shape (sphericity). Roundness was determined using Power's scale, having values from 0 to 6, corresponding to very angular particles to well-rounded particles. Grain shape was defined in terms of the E shape factor of Janke. The ranges of particle weights, roundnesses, and E shape factors are respectively 0.16-5.18 grams, 0.5-5.4, and 0.239-0.809. The settling grains showed some irregular behavior such as oscillations but generally maintained their maximum projection areas oriented perpendicular to the fall direction. Analysis of the data demonstrates almost no correlation between the settling velocity and the grain roundness, the main controlling factors being the grain shape and Reynolds number. Analysis of the data shows that the settling velocity w s can be calculated with the modified Stokes equation W s = [1]/[18] [1]/[mu ](rho s - rho )gD n2 (0.672 + 0.318E) for particles having Reynolds numbers less than 0.5, where rho s and rho are respectively the particle and fluid densities, D n is the grain nominal diameter, mu the fluid viscosity, and E is the grain shape factor. A more complex relationship is presented that is applicable at higher Reynolds numbers.

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