The first part of this thesis investigates the mixing of ambient fluid into axisymmetric high Reynolds number gravity currents. A series of laboratory experiments were conducted in which small scale gravity currents travelled along a wedge shaped channel with an increasing width in the downstream direction. The channel was filled with fresh water and the current was generated using saline solution introduced either by a rapid release of a known finite volume from behind a lock gate or by pumping at a constant rate into the apex of the channel. The distribution and evolution of the density of the flow with distance downstream was measured using a light attenuation technique. Additional experiments were performed by injecting parcels of dye in different regions of the flow in order to visualise the motion of fluid in and surrounding the gravity current. Unlike currents introduced by the release of a finite volume of fluid, where most mixing occurs in the head of the flow, currents produced from a steady source develop a steady tail region behind the front which is also found to entrain a significant amount of ambient fluid. In both types of current, we estimate the fraction of displaced ambient fluid that is entrained into the flow. We then derive a new class of self-similar solutions for gravity currents produced from a finite volume release of fluid. The second part of this thesis develops the experimental method of measuring mixing using light attenuation to investigate the mixing of liquid in a vertical conduit which results from a continuous stream of high Reynolds number gas bubbles. The experiments identify that the mixing in the wake of the bubbles leads to a net dispersive transport along the conduit. The process provides an explanation for the heat transfer within a volcanic conduit in the case of a gas-slug flow regime as occurs in the near surface region of volcanic conduits connected to surface lava lakes. We derive a theoretical model to estimate the heat flux associated with such a system using the empirical law for the dispersive mixing. The predicted heat flux associated with the bubbles is found to be comparable to the heat loss at the surface of lava lakes associated with radiative and convective heat loss. Given values for the gas flux, the lake area and the temperature at the surface of the lake, the model enables new predictions for the size of the volcanic conduit.

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