HYDRODYNAMIC DISPERSION BROADENING OF A SEDIMENTATION FRONT

Hydrodynamic dispersion is responsible for the spreading of the sedime ntation front even in a noncolloidal monodisperse suspension. Measurem ents of the broadening of the top front observed during sedimentation have been used in determining the hydrodynamic dispersion coefficient. Hindered settling has an opposed effect and leads to the self-sharpen ing of the front. Both effects have to be taken into account simultane ously. This Letter provides a simple, but complete determination of th e space and time concentration profile and shows that the final front should consist of a steady-shape profile propagating at constant veloc ity. With such a solution, the data of Davis et al. [AIChE J. 34, 123 (1988); J. Fluid Mech. 196, 107 (1988)] give hydrodynamic dispersion c oefficient five times larger than their former analysis, in agreement with Lee et al. [Phys. Fluids A 4, 2601 (1992)].