Free and Confined Buoyant Flows
Creators: name: Ezzamel, Adam
Publisher: name: Imperial College London
Year: 2011

description: Flows driven by density differences, whether natural or induced by man, surround us on a wide range of scales. As a density difference is generated by commonly occuring temperature or salinity differences, these flows are ubiquitous. On the largest scales, they transport and mix the water in our oceans and air in our atmosphere. At an intermediate scale, frequently referred to as the mesoscale, the cold outflows which flow downwards from thunderstorms and impact the ground are a common aviation hazard. On a smaller scale, the toxins emitted by fire plumes or dense gas releases are a threat to health. This work focusses on a continuous, localised release of buoyant fluid from a horizontal source whose dimension is significantly smaller than the horizontal and vertical scales of the quiescent, uniform environment into which the flow propagates. We consider both passive releases, which are only driven by the density difference between the source fluid and the denser ambient, and forced releases, where the fluid has source momentum in addition to its buoyancy. In general, these releases give rise to turbulent plumes, a familiar example being the cloud of smoke and water vapour seen rising from a chimney stack on a cold, still morning. The first part of the research presented in this thesis focusses on the freely-propagating plume. Velocity and temperature measurements are presented which contribute considerably to the existing experimental data available in the literature. This data-set is used to validate classical plume theory and make a check of the experimental set-up so that the subsequent results can be presented with confidence. It is also possible that this dataset will be used by other researchers to validate numerical simulations of buoyant flows. The effect of varying the source balance of buoyancy and momentum upon the plume dynamics is investigated. Measurements also reveal the development region or ‘zone of flow establishment’. Frequently, plumes are restricted by some form of confinement, either vertically, horizontally or both, for example the plumes rising from the occupants of a room. Whether this restriction takes the form of a solid wall, free surface or density discontinuity, the disturbance to the flow is typically significant. The simplest confining boundary is arguably a horizontal surface located some distance H from the source of buoyant fluid. The horizontal boundary forces the vertical flow to change direction and propagate radially outwards. This type of semi-confined flow can be frequently observed in the natural world with examples including the impingement of a fire plume against a ceiling and a plume of volcanic ash with the tropopause. An investigation into this type of flow, which we refer to as the ‘impinging buoyant plume’, constitutes the second part of the research. Plume impingement has not been studied as extensively as jet impingement and several key questions remain unanswered. For example, how much energy is lost as the vertical flow is forced to turn and propagate horizontally? What effect does buoyancy have on the horizontal flow? How does the flow evolve with increasing radial distance and what is the effect of changing the source-boundary separation? These are just some of the questions addressed in this thesis guided by the novel application of highly-resolved Particle Image Velocimetry measurements to this relatively low-speed, buoyant, turbulent flow. The free and impinging plume studies both employed similar experimental techniques and analysis methods. Statistics of the steady flow were determined from a highly-resolved data-set. The third part of the research concerns a time-dependent flow and is of a more qualitative nature. The complexity of the impinging plume increases considerably when a radial confinement is added to the geometry. This restricts the radial propagation of the flow produced by the impinging plume. The plume is now effectively enclosed and buoyant fluid begins to accumulate within and thereby fill the enclosure, a configuration known as the ‘filling-box’. While previous work, which we shall go on to review in detail, has contributed analytical solutions for the density profiles in the enclosure after a certain time-scale has elapsed, in many applications, such as the spread of smoke carried by a fire plume in a room, what happens in the early moments of a confined release following impingement with the horizontal and then vertical boundaries, may be critical. This has been overlooked in earlier studies, yet is crucial as it is during these early transients that the fire is best tackled by fire-fighters. Visualisations and velocity measurements of these early filling-box transients are reported. This work provides the first detailed measurements of the velocity field induced in the filling-box by the turbulent plume during the early transients and resolves the turbulent structures that comprise the plume outflow. The experiments which investigated the impinging jet were conducted on thermal air plumes in facilities at the Laboratoire de Mécanique des fluides et de l’Acoustique (LMFA) of École Centrale de Lyon (ECL). Filling-box experiments were performed on brine plumes in fresh water in visualisation tanks in the Department of Civil &amp; Environmental Engineering at Imperial College London (ICL). The set of experiments at ECL used a combination of Particle Image Velocimetry (PIV) and thermocouples to measure flow velocities and temperatures. At ICL, Light-Induced Fluorescence (LIF) enabled visualisation of a plane through the centre of the axisymmetric flow to complement the PIV work. These experiments enabled effective use of the equipment, techniques and expertise available at both institutions. The principal objective of this research was to use experimental measurements to answer questions of importance regarding these impinging flows which remain unresolved in the literature. Using experimental techniques unavailable to earlier researchers, the work presented herein makes a substantial contribution to the existing knowledge of these flows. Free and impinging plumes and the dynamics of the filling-box flow are studied in detail. Notably, the data gathered are of very high spatial resolution and provide a resource for those interested by not only the plume dynamics, but also radial gravity currents and the filling-box.