This work utilizes various computational techniques to study the turbulent mechanisms found in stratified shear flows. Three-dimensional DNS was used to investigate the influence of stratification on turbulence and mixing within a shear layer between two currents. Similarities in the development of secondary instabilities during transition to turbulence and discrepancies in flow evolution are seen between the case of uniform stratification considered here and the two-layer density profile of prior works. Vertical contraction of the shear layer is identified in cases with low Richardson number and determined to be the result of the flattening of Kelvin-Helmholtz billows before the flow becomes fully turbulent. Transition layers with enhanced shear and stratification form at the periphery of the shear layer and are found to support turbulent mixing. In an effort to find a less computationally costly tool than DNS, the Dynamic Smagorinsky, Ducros, and WALE subgrid-scale models were chosen for an LES study of the stratified shear layer. This investigation revealed the Ducros model to the least computationally costly LES option and the most reliable with coarsening grid resolution. A subgrid analysis revealed the LES models to be largely unsuccessful in capturing convective turbulence though the mean flow and turbulent kinetic energy were well-captured. To address the limitations of DNS and LES, a hybrid spatially-evolving DNS model was developed. The wake of a sphere towed in a stratified background was selected for validation. The hybrid model involves extracting planes from a spatially-evolving, body-inclusive simulation and feeding the planes as inflow into a body-exclusive simulation thereby eliminating the need for a highly resolved grid to capture flow near the body. This study revealed that particular attention should be paid to the extraction location, grid resolution, and time step between extractions. Planes must be extracted downstream of the recirculation region behind the body and sufficient grid resolution is required in the body-exclusive simulation to capture small-scale turbulence. Results show the hybrid DNS model to be an effective tool in the study of the stratified turbulent wake. The combination of results presented herein offer computational techniques and cost-saving options for future studies of shear flows.

This volume contains the papers presented at the workshop on Statistical The ories and Computational Approaches to Turbulence: Modern Perspectives and Applications to Global-Scale Flows, held October 10-13, 2001, at Nagoya Uni versity, Nagoya, Japan. Because of recent developments in computational capabilities, the compu tational approach is showing the potential to resolve a much wider range of length and time scales in turbulent physical systems. Nevertheless, even with the largest supercomputers of the foreseeable future, development of adequate modeling techniques for at least some scales of motion will be necessary for practical computations of important problems such as weather forecasting and the prediction and control of global pollution. The more powerful the available machines become, the more demand there will be for precise prediction of the systems. This means that more precise and reliable knowledge of the underlying dynamics will become important, and that more efficient and precise numerical methods best adapted to the new generation of computers will be necessary. The understanding of the nature of unresolved scales then will playa key role in the modeling of turbulent motion. The challenge to turbulence theory here is to elucidate the physics or dynamics of those scales, in particular their sta tistical aspects, and thereby develop models on sound bases to reduce modeling ambiguity. The challenge to the computational method is to develop efficient algorithms suitable for the problems, the machines, and the developed models.

This book presents the latest research in one of the most challenging, yet most universally applicable areas of technology. Multiphase flows are found in all areas of technology, at all length scales and flow regimes, involving compressible or incompressible linear or nonlinear fluids. The range of related problems of interest is vast, including astrophysics, biology, geophysics, atmospheric process, and many areas of engineering. The solution of the equations that describe such complex problems often requires a combination of advanced computational and experimental methods. For example, any models developed must be validated through the application of expensive and difficult experimental techniques. Numerous problems in the area thus remain as yet unsolved, including modelling nonlinear fluids, modelling and tracking interfaces, dealing with multiple length scales, characterising phase structures, and treating drop break-up and coalescence. The papers contained in the book were presented at the eighth in a well established series of biennial conferences that began in 2001. They represent close interaction between numerical modellers and other researchers working to gradually resolve the many outstanding issues in understanding of multiphase flow. The papers in the book cover such topics as: Multiphase Flow Simulation; Bubble and Drop Dynamics; Interface Behaviour; Experimental Measurements; Energy Applications; Compressible Flows; Flow in Porous Media; Turbulent Flow; Image Processing; Heat Transfer; Atomization; Hydromagnetics; Plasma; Fluidised Beds; Cavitation.

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