The transition in the sedimentation pattern of a particle cloud

Experiments were carried out to study the sedimentation of a two-dimensional particle cloud. When a large number of particles (glass beads) of uniform size are released from a two-dimensional opening into a column of fresh water, the mixture initially descends as a thermal; however, after some time, the particles start settling individually, thus leaving the parent fluid behind. For a given type of particle, the critical depth z_c at which this transition occurs, measured from a virtual origin, was found to change as z_cw_s/v∼(Q/vw_s)^α, with α≅0.3, where w_s is the terminal velocity of a single particle, v is the kinematic viscosity, and Q is the total buoyancy of the released particles per unit length. The descending velocity and the spatial growth of the particle cloud were found to depend on its sedimentation characteristics.