Sedimentation of a particle cloud across a density interface

The effects of stratification on the sedimentation of suspended particles a re investigated by the experiment in which a two-dimensional particle cloud descends across a density interface. Radically different patterns of the s edimentation 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 th e 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 without experiencing significant modificat ion. When X becomes large (X>>1), however, a particle cloud spreads horizon tally along the density interface forming turbidity currents upon impingeme nt. In this case the settling of particles across the density interface occ urs over much wider area after the turbidity currents propagate to a certai n distance. Measurements of the horizontal length scale I and the downward velocity w(f) of a particle cloud below the density interface show that the stratification increases I 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 (=D(2)Delta b/Q) where Delta b is the buoyancy jump at the density interface. (C) 2000 The Japan Society of Fluid Mechanics and El sevier Science B.V. All rights reserved.