Experimental studies of particle-laden plumes

Experiments are described in which buoyant plumes of fresh water and solid particles are generated in a tank of salt water. Dependence of the behavior of the plumes on particle concentration and grain size was studied. At low concentrations, behavior was similar to single-component plumes, and the principal effect of the particles was to reduce the buoyancy flux. Measurements of particle concentration across the plumes demonstrated that there were substantial amounts of particle reentrainment from an outer sedimenting veil. Reentrainment caused the plume margins to become denser than the ambient fluid. Total particle flux in the plumes increased with height and can be several times greater than the flux at the plume source. Plumes with high particle concentration exhibit several kinds of convective instability, which generated gravity currents and rapid sedimentation. Gravity currents were generated at the margins of concentrated plumes. These flows are attributed to reentrainment, causing the flow margins to have excess density. When the plume density was close to but less than the density of the ambient fluid, a fountain forms high in the plumes which collapsed asymmetrically to generate a gravity current on one side of the plume. When the density of the particle-laden flow was greater than the ambient fluid, a symmetrical fountain formed, and gravity currents moved across the tank floor, depositing sediment. At a well-defined distance from the source, buoyant fluid ascended from the flow fronts to form a ring of plumes around the central fountain. These plumes coalesced to form a broad buoyant cloud. The plumes are generated by the sedimentation from the gravity currents, which resulted in the fresh water and some entrained particles being released. All the experimental plumes ascended to the top of the tank and spread out as gravity currents beneath the upper surface. Particles often fall from this layer in discrete parcels of particle-rich fluid with shapes similar to thermals. The fall velocities were much greater than the terminal velocities of individual particles and the sedimenting region is inhomogeneous. These observations indicate that sedimentation is enhanced by convective instabilities when particle-rich fluid is emplaced above particle-free fluid. The experiments indicate that there may be several different mechanisms of generating pyroclastic flows and surges from the collapse of volcanic eruptions columns. The experiments also demonstrate that the sedimentation of particles from a plume has an important influence on dynamical behavior.