Axisymmetric Underflows from Impinging Buoyant Jets of Dense Cohesive Particle-Laden Fluids

Abstract

Underflows generated from the impingement of descending dense cohesive particle-laden buoyant jets on a horizontal bottom of a water body were investigated theoretically and experimentally. Laboratory experiments were conducted by discharging fluid mud (suspension of cohesive clay particles in water) at a constant volume flow rate from a submerged vertical pipe into a body of water. The experiments involved a wide range of initial parameters (e.g., height of the discharge pipe from the bottom, flow rate, and concentration of the fluid mud at the time of discharge). The discharged fluid mud descended as vertical buoyant jets and flowed away as axisymmetric underflows after impinging on the bottom. The underflows exhibited two well-documented propagation phases: (1) a radial momentum-driven wall jet phase near the impingement region, and (2) an inertial gravity current phase after the completion of the wall jet phase. Experiments revealed that, depending on the initial discharge parameters, underflows underwent a viscous propagation phase in which the propagations were influenced by the non-Newtonian rheology of the discharged fluid mud. The dynamics of the propagation phases of underflows were analyzed and compared with previous studies on saline underflows. The radial wall jet lengths (radial extent of underflows at which they transitioned from the wall jet phase to the inertial gravity current phase) were found to scale on the characteristic jet length scale and discharge source-bottom separation distance when the buoyant jets impinge on the bottom as momentum-dominated jets. When the buoyant jets impinge on the bottom as buoyancy-dominated plumes, however, they scaled with only the source-bottom separation distance. Radial wall jet length parameterizations for both types of the impingements were developed. The predictive capabilities of two different shallow water model solutions were tested for the inertial propagation of fluid mud gravity currents. The transitions of fluid mud underflows into a viscous propagation phase were analyzed in terms of the discharge parameters and their non-Newtonian rheology. A box model solution for the viscous propagation of axisymmetric non-Newtonian gravity currents was developed. Predictions of the developed box model were then evaluated using the authors’ experimental observations. Of those underflows in which viscous propagation occurred, few showed an abrupt settling phenomenon after propagating some distance in the viscous propagation phase in which clay particles abruptly settled en masse from the underflows and only a turbidity cloud propagated. Discussions on the abrupt settling phenomena are also provided.