In this thesis we present experimental studies and theoretical models on the rise height and entrainment of axisymmetric multiphase fountains. The aim of this thesis is to improve our understanding of these fundamental flows frequently observed in industry and nature.

In each chapter we present a series of laboratory experiments and we employ various dye visualisations and imaging techniques to extract experimental data. This data is compared with theoretical models based on the conservation of volume, momentum and buoyancy fluxes. We focus our attention on the bulk dynamics of the flow rather than on the motion of individual bubbles or particles.

In chapters 2 and 3 we consider unconfined bubble fountains. We begin by investigating the maximum depth reached by bubbles in turbulent water jets issuing downwards into a reservoir. We compare the experimental data with a theoretical model based on the conservation of volume, momentum and buoyancy fluxes. We proceed in chapter 3 by investigating the depth reached by bubbles in low Reynolds number bubble fountains and we present a scaling for the maximum depth reached by bubbles in such flows. In the experiments we observed bubbles accumulating at the surface, forming a filling-box flow. This observation encouraged a subsequent investigation of filling-box flows of bubbles and particles in confined reservoirs.

In chapters 4 and 5 we consider turbulent fountains in confined environments. In chapter 4 we investigate the fate of particles carried into a confined space by a turbulent particle-laden fountain. We identify four distinct regimes by comparing the particle settling velocity with three characteristic velocities in the system. In the fifth chapter we focus our attention on extremely confined turbulent fountains. We measure the rise height and entrainment of fountains in very narrow tanks and identify the regime in which the fountain is affected by the confinement. We present a scaling argument for the entrainment of such flows.

In chapter 6 we consider the run-out length and entrainment of particle-driven gravity currents, fed by turbulent particle-laden fountains. We present a theoretical model accounting for the presence of a sedimentation front.

The thesis concludes with a summary and discussion of the findings in chapter 7.