ENTRAINMENT IN COMPOSITIONALLY BUOYANT PLUMES

The forms and growth histories of compositionally buoyant plumes were investigated experimentally by emitting a lower-density fluid into a higher-density fluid from a point-source and also by allowing multiple plumes to rise from an interface between layers of fluid with different densities. Of special interest is the observation that some lower density plumes entrain higher density host material as they rise. This process, which occurs even at very low Reynolds numbers, may be analogous to the mixing of geological materials in igneous plumes and salt diapirs. Entrainment of host fluid up through the base of the carapace occurs in plumes that experience simultaneous cap expansion and vertical ascent at a rate that is dependent on the internal vortical volocity. Such plumes have a viscosity contrast between the host and source fluids near unity and a cap/stem diameter ratio of about 2-3. Plumes that entrain host material change shape from spherical to a form with a flat base and bluntly-pointed tip and decelerate due to an increase in effective density relative to the density of the surrounding host material. The ascent of all plumes with a host/source viscosity ratio within the range 0.1-21 and a density contrast of about 1.5 can be modeled as Stokesian provided that entrainment is accounted for by use of an effective density. Effective density is simply the weighted average density of the plume and entrained host material.