Dynamics of Immiscible Gravity Currents

Gravity currents are flows that are driven by a density difference and include pyroclastic flows, landslides and turbidity currents. Gravity currents can be a geohazard and have significant economic impact to connected industries. This thesis focuses on two important questions relating to gravity current dynamics: How do pulses or surges affect the flow dynamics? And, how does a viscosity contrast affect the mixing process between the current and its ambient?

Real-world gravity current flows rarely exist as a single discrete event, but are instead made up of multiple surges. These are studied by the sequential release of two lock gates. The first release creates a gravity current, while the second creates a pulse that eventually propagates to the head of the first current. A shallow-water model is used to analyse the flow structure in terms of two parameters: the densimetric Froude number at the head of the current, Fr, and a dimensionless time between releases, t_re. The pulse speed exhibits negative acceleration for a region of (Fr, t_re)-space. Critically for sediment-laden gravity currents, pulsed flows may change from erosional to depositional further affecting their dynamics. Experimental modelling using glycerol/water mixtures reveals that the pulse can cause a rapid dilution of the current and transition to fully turbulent behaviour.

In a lock-exchange configuration, the effect of a viscosity contrast between the ambient and the current is studied using fully resolved direct numerical simulation of the Navier-Stokes equations. Viscosity acts to both dissipate energy in the bulk of the current and locally inhibit mixing at the interface. Energy lost to viscous heating is dominant when the viscosity contrast is large, i.e. ten times the ambient. However, when the viscosity contrast is small but non-zero, the reduced mixing of the current enables a more efficient transfer between kinetic and potential energy and the total energy lost to mixing and viscous dissipation is reduced when compared to an equal viscosity case.