Sedimentation of a particle cloud across a density interface

The effects of stratification on the sedimentation of susp ended particles are investigated by the experiment in which a two-dimensional particle cloud descends across a density interface. Radically different patterns of the sedimentation appear depending on the values of X (=Q/Dw(s)²), where Q is the total buoyancy per unit width of a particle cloud, D is the depth of the density interface, and w(s) is the settling velocity of a particle. When X is small (X << 1), a particle cloud passes through the density interface as individually settling particles with out experiencing significant modification. When X becomes large (X >> 1), however, a particle cloud spreads horizontally along the density interface forming turbidity currents upon impingement. In this case the settling of particles across the density interface occurs over much wider area after the turbidity currents propagate to a certain distance. Measurements of the horizontal length scale l and the downward velocity w(f) of a particle cloud below the density interface show that the stratification increases l and decreases w(f) substantially at large X. On the other hand, they are found to be insensitive to the Richardson number at the interface Ri (=D² Δb/Q) where Δb is the buoyancy jump at the density interface. (C) 2000 The Japan Society of Fluid Mechanics and Elsevier Science B.V. All rights reserved.