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Stratified flow in an environmental chamber has been investigated. The chamber of dimensions (7.5m long, 5.6m wide and 3.0m) at the University of Hertfordshire has been used. Sets of experiments investigating the effect of the major flow parameters such as airflow rate, jet momentum, flow conditions and height of the air supply device have been conducted. Results have been obtained to evaluate the flow characteristics and thermal stratification mechanism. The study has demonstrated the validity of using smoke visualization to evaluate the stratified flow characteristics such as interface level height, stratified layer thickness, and degree of stratification. The effects of both hot and cold airflow rates in the ranges of (0.0 to 8.0 m3 /min) were investigated. The flow characteristics vary depending on the flow parameters and the experimental conditions. The effect of supply terminal and extract terminal at various airflow rates on the flow characteristics is experimentally investigated. It has been found that relative influence of inertia and buoyancy forces resolves the stratified flow characteristics. The stratification interface level height and the ventilation flow rates are two main factors in the design of natural ventilation system. The results can be used to obtain a good estimation of the effectiveness of a ventilation system at design stage. Experimental work was carried out using ceiling jet to supply hot and cold air to a confined space, to investigate the effect of jet momentum in breaking and mixing the stratified layer. The flow of high momentum was supplied downward from the ceiling. The magnitude of momentum needed depends on the degree of stratification, stratified layer interface level height and the stratification conditions. It can be seen that the jet momentum has significant influence on the mixing of the stratified flow characteristics. The results indicated that once the momentum was initiated a mixed flow grew in the occupied zone above the floor. The height of this zone depends on the stratified flow characteristics, and the temperature and momentum of the ceiling jet. Another area of experimentation was the inversion of input airflow supplies. In this case, the flow of high buoyancy was supplied upward, whilst the flow of high momentum was supplied downward from the ceiling. The stratified layer lost its stability and broke down due to the drag and tearing of cold air penetrated downward from higher levels. The compound effect of these two conditions will circulate the air in the whole space and disturb the stability of the stratified layer to reach fully mixed flow A comprehensive definition of the degree of stratification was formulated. Analytical solutions were developed for the stratified layer thickness and location as a function of temperature gradient and airflow ratios. These expressions were calibrated using the experimental results. The critical momentum needed to breakdown the stratified layer also evaluated. Comparisons with previous studies where also carried out. It was found that the stratified layer interface level height is dependent on the ratio of airflow rate and geometrical effects. If mixed flow is desired then the cold inflow aperture should be located higher than the hot inflow aperture, whiles the interface level height is not located at the exhaust aperture height. The critical vertical momentum necessary in order to break down a stratified layer has been found to depend on the stratified layer interface level height. A semi-empirical formula based on the present experimental results has been developed to predict the critical vertical momentum for given stratified conditions. Based on the present experimental results, the effect of momentum is greater than the effect of buoyancy and the time needed to break down the stratified layer is considerable less than the time it takes to stratify. Experimental data also demonstrate a ventilation method for increasing the occupied zone height without breaking down the stratified layer.

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In this work the physics of transitional separated-reattched flow with and without free-stream turbulence on a blunt leading edge plate have been studied numerically employing the Large Eddy Simulation approach. One of the fundamental features of 'turbulent' separated-reattached flows is two basic modes of characteristic frequencies. The higher-frequency mode is associated with the usual large scale motions in the shear layer while the lower-frequency mode reflects overall separation bubble growth/decay dynamics or shear layer flapping as it is frequently called in the literature. It has been drawn from the current study that the low-frequency mode will not occur in low-Reynolds number transitional separated-reattached flows and the phenomena appears to be an integral feature of a fully turbulent separation. The numerical data have been comprehensively analysed to elucidate the entire transition process. Coherent structures have been visualised in the different stages of transition. In the case with no-free-stream turbulence, the 2D Kelvin - Holmholtz rolls are the dominant structures in the early stage of transition and the well known A-shaped vortices commonly associated with flat plate boundary layer transition are the common features in the late transition stages. Many experimental studies have indicated that the separated shear layer on a blunt plate is unstable owing to the Kelvin-Holmholtz instability. However, sufficient and detailed evidence has not been given in separated boundary layer transition studies to show that the instability mechanism at work is indeed the Kelvin-Holmholtz instability in this particular geometry. In the current study, it has been shown that the primary instability is indeed of the Kelvin-Holmholtz type. The results also strongly support the idea that 'helical-pairing' instability could be the secondary instability responsible for the breakdown to turbulence in the late stages of transition. The addition of free-stream turbulence result in the transition OCCllring earlier leading to a short mean reattachment bubble length. The coherent structures which are clearly observed in the no-free-stream turbulence case have been barely visible. The primary instability was found to be the same as in the no-free-stream turbulence case, i.e., Kelvin - Helmholtz instability.

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This thesis describes the development of a large eddy simulation code for low-Mach number variable density non-reacting and reacting flows. Governing equations for large eddy simulation of variable density flows have been derived based on low-Mach number approximation. Mixture fraction conserved scalar method has been adopted for variable density non-reacting flows and turbulent non-premixed combustion flows, with a subgrid PDF method for turbulence-mixing and turbulence-chemistry interactions. Reaction progress variable method with an algebraic flame surface density model has been used for turbulent premixed combustion flows. Explicit numerical solution procedure has been developed, with modified convective outflow boundary condition to ensure global mass conservation and feedback method to generate fully developed turbulent inflow field. For turbulent reacting flows with large density ratio, relaxation method has been adopted to remove unphysical. high-frequency fluctuations and to maintain numerical stability. The code has been validated against a number of experimental test cases, including: non-reacting variable density confined jets over a large range of non-isothermal Craya-Curtet number and initial density ratio; turbulent non-premixed combustion in a simplified axisymmetric combustor geometry; and turbulent premixed combustion in the ORACLES dump combustor. The good results obtained in these simulations have demonstrated the robustness and accuracy of the large eddy simulation code developed in present study.

This thesis is devoted to the study of some difficulties of practical implementation of finite element solution of differential equations within the context of multi-scale engineering flow problems. In particular, stabilized finite elements and issues associated with computer implementation of these schemes are discussed and a novel technique towards practical implementation of such schemes is presented. The idea behind this novel technique is to introduce elemental shape functions of the polynomial forms that acquire higher degrees and are optimized at the element level, using the least squares minimization of the residual. This technique provides a practical scheme that improves the accuracy of the finite element solution while using crude discretization. The method of residual free bubble functions is the point of our departure.