The dynamics of different classes of turbulent and miscible fountains are stud- ied: from classic axisymmetric fountains issuing from round sources, to confined fountains propagating in a quasi-two-dimensional environment, to line fountains which form when release conditions are approximately two-dimensional at the source. Each class is characterised by distinct dynamical behaviour, which this the- sis analyses both through theoretical arguments and experimental measurements. A model for the entrainment of ambient fluid into a fluctuating fountain top is developed and implemented into a first complete description for round fountains. The solutions of the resulting 'three-region-model' lie in improved agreement with available data and, uniquely, do not diverge near the top of the fountain. Next, con- fined fountains (unexplored to date) are classified into four flow regimes and their behaviour collapsed according to a single governing parameter that captures the severity of confinement. Finally, new experiments on line foutains shed light on the quasi-steady structure of these flows, revealing (and motivating) a strong con- nection between their motion in the vertical and lateral planes. Round, confined and line fountains are then contrasted in the conclusions, where we reflect on what is required to progress towards a unified theory of turbulent fountains.

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A novel gas channel experiment was constructed to study the development of high Atwood number Rayleigh-Taylor mixing. Two gas streams, one containing air and the other containing helium-air mixture, flow parallel to each other separated by a thin splitter plate. The streams meet at the end of a splitter plate leading to the formation of an unstable interface and of buoyancy driven mixing. This buoyancy driven mixing experiment allows for long data collection times, short transients and was statistically steady. The facility was designed to be capable of large Atwood number studies of ABtB ~ 0.75. We describe work to measure the self similar evolution of mixing at density differences corresponding to 0.035 < ABtB < 0.25. Diagnostics include a constant temperature hot-wire anemometer, and high resolution digital image analysis. The hot-wire probe gives velocity, density and velocity-density statistics of the mixing layer. Two different multi-position single-wire techniques were used to measure the velocity fluctuations in three mutually perpendicular directions. Analysis of the measured data was used to explain the mixing as it develops to a self-similar regime in this flow. These measurements are to our knowledge, the first use of hot-wire anemometry in the Rayleigh-Taylor community. Since the measurement involved extensive calibration of the probes in a binary gas mixture of air and helium, a new convective heat transfer correlation was formulated to account for variable-density low Reynolds number flows past a heated cylinder. In addition to the hot-wire measurements, a digital image analysis procedure was used to characterize various properties of the flow and also to validate the hot-wire measurements. A test of statistical convergence was performed and the study revealed that the statistical convergence was a direct consequence of the number of different large three-dimensional structures that were averaged over the duration of the run.

A development of high resolution NFT model for simulation of incompressible flows is presented. The model uses finite volume spatial discretisation with edge based data structure and operates on unstructured meshes with arbitrary shaped cells. The key features of the model include non-oscillatory advection scheme Multidimensional Positive Definite Advection Transport Algorithm (MPDATA) and non-symmetric Krylov-subspace elliptic solver. The NFT MPDATA model integrates the Reynolds Average Navier Stokes (RANS) equations. The implementation of the Spalart-Allmaras one equations turbulence model extends the development further to turbulent flows. An efficient non-staggered mesh arrangement for pressure and velocity is employed and provides smooth solutions without a need of artificial dissipation. In contrast to commonly used schemes, a collocated arrangement for flow variables is possible as the stabilisation of the NFT MPDATA scheme arises naturally from the design of MPDATA. Other benefits of MPDATA include: second order accuracy, strict sign-preserving and full multidimensionality. The flexibility and robustness of the new approach is studied and validated for laminar and turbulent flows. Theoretical developments are supported by numerical testing. Successful quantitative and qualitative comparisons with the numerical and experimental results available from literature confirm the validity and accuracy of the NFT MPDATA scheme and open the avenue for its exploitation for engineering problems with complex geometries requiring flexible representation using unstructured meshes.

El objetivo principal de esta tesis es la mejora y la aplicación de la técnica PIV para analizar diferentes flujos de fluidos complejos en los sistemas que contienen partes sólidos móviles o estacionarias. Las partículas de alginato con/sin compuesto de fluoresceína incrustada se han desarrollado como una serie de elementos alternativos de siembra de nuevas aplicaciones de flujo para el PIV. Se ha encontrado que las micro-partículas de alginato tienen muchas ventajas sobre los tradicionales utilizados en sistemas de PIV. Un estudio amplio sobre el análisis del flujo en tres dimensiones de la bomba de engranajes externa se ha hecho por la técnica tiempo-resuelva PIV para investigar la dinámica y las estadísticas de flujo turbulento. Vorticidad y zonas turbulentas de alta energía cinética se han identificado dentro de la bomba de engranajes externa. La técnica PIV se ha aplicado de manera eficiente para analizar un intercambiador de calor de motor de flujo de bypass en un túnel del viento de bajo velocidad. Las partículas de alginato que contienen fluoresceína se han utilizado eficientemente en los experimentos que conducen a mejorar la calidad de la imagen PIV y analizar las estructuras de pequeña escala de los flujos. / The main objective of the thesis is the improvement and application of the PIV technique to analyse different complex fluid flows in systems that contain solid moving or stationary parts. The alginate particles with/without fluorescein compound imbedded have been developed as a new alternative flow seeding elements for PIV applications. It has been found that the alginate micro particles have many advantages over traditional ones used in PIV systems. A comprehensive study on the three-dimensional flow analysis of the external gear pump has been done by time-resolved PIV technique to investigate its turbulent flow dynamics and statistics. Vorticity and high turbulent kinetic energy areas have been identified inside the external gear pump. The PIV technique has been efficiently applied to analyse an engine bypass flow heat exchanger in a low speed wind tunnel. The alginate particles containing fluorescein have been efficiently used in the experiments that lead to improve the PIV image quality and analyse the small-scale structures of the flows.

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