SOME EFFECTS OF PARTICLE-BED GEOMETRY IN SELECTIVE SEDIMENT SORTING.
CHICAGO UNIV ILL FLUID DYNAMICS AND SEDIMENT TRANSPORT LAB
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In an effort to better understand the initial phases of sediment sorting, the resistance to movement due to particle-bed geometry was studied in a wind tunnel. In order to reduce the complexity encountered in the natural situation a simplified experimental design using spherical particle size, particle density and the size of bed roughness. Initial motion of the test particles was directly observed and correlated with the corresponding average surface shearing stress acting on the bed. A re-evaluation of published data of wind flow over sand surfaces indicates that the surface roughness effects are not fully developed at the inception of particle transport of fine and medium sands. For the range of variables studied, the threshold shear velocity was found to be a function of the particles fall velocity, the relative density and the ratio of particle exposure to the viscous sub-layer thickness. The experimental data indicate that spheres of different density but equal in diameter and resting on spheres of uniform size, have a threshold equivalent equal to the ratio of their fall velocities. Author