Sedimentation and mixing of a turbulent fluid suspension: a laboratory study

In many geological and industrial situations the mixing that occurs across the moving interface between a turbulent fluid layer and a quiescent layer of different density is of fundamental importance. Much of our knowledge of this process has been obtained from laboratory experiments in mixing boxes where the turbulent motions are generated by the vertical oscillation of a horizontal grid located in the interior of the stirred layer. We report here some experiments in which the turbulent layer contains a suspension of small dense particles, with the stirring grid located at the base of the tank in order to simulate the generation of turbulence by stresses acting at the rough bottom over which it traverses. We compare the data with experiments using dense, but particle-free, fluid layers in the same geometry. The results are distinctly different, since the particles responsible for the density difference between the two layers can also migrate through and fall out of the suspension layer as it changes in thickness. We find that the following equilibrium conditions are attained sequentially as the frequency of stirring, and hence the intensity of turbulence, is increased. The particles eventually all precipitate; or only some precipitate while others are held indefinitely in suspension; or all the particles are suspended by the turbulent motions. Both the last two cases produce a stable self-maintained suspension layer separated from the overlying fluid by a sharp density interface. The results cannot be explained simply in terms of particles falling away from an entraining interface: the work done in keeping the particles in suspension must be taken explicitly into account. The relationship between these experiments, and results previously obtained when the interstitial fluid in the lower sediment layer is less dense than the upper layer and can rise across the interface to drive convective motions above, is also discussed briefly. Our results will be applicable to various sediment-laden flows, including crystal-rich magma layers, pyroclastic flows, avalanches, river outflows, turbidites and the deposition of industrial wastes.