Experiments on entrainment and mixing in particle-driven gravity currents

Abstract

New experiments with particle-driven, finite-volume gravity currents show that some of the ambient fluid displaced up and over the head of the current becomes mixed into the current. Measurements show that this leads to an increase in volume of the current at a similar rate to that in a single-phase current. Experiments also show that particles gradually settle from the top surface of the current, releasing fluid from the current. Eventually, this begins to dominate the entrainment through the head, and the volume of the current then decreases. Meanwhile, particles continuously sediment from the base of the flow, reducing the particle load. A simplified integral box model is developed to illustrate the evolving balance between these processes as a function of S0, the ratio of the current speed, based on the initial buoyancy, to the particle fall speed, for 30 < S0 < 300. The model combines the entrainment law proposed by Sher and Woods (J. Fluid Mech., 2015, vol. 784, pp. 130–162) for single-phase gravity currents, the sedimentation law proposed by Bonnecaze et al. (J. Fluid Mech., 1993, vol. 250, pp. 339–369) for a constant-volume gravity current, and a new model for the release of fluid from the top surface of the current through particle settling. We show that the maximum volume of the current and the position of the nose when reaching this maximum volume both increase nearly linearly with S0. We discuss the importance of these results for particle-driven gravity currents, especially where the effects of fluid entrainment can lead to a change in fluid chemistry, promoting flocculation and hence sedimentation, or a change in fluid buoyancy through fluid–particle interaction.