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A research program was carried out to investigate turbulent mixing in stably stratified shear flows with the hope of gaining an improved understanding of stably stratified nocturnal boundary layers. The program was mainly laboratory experimental, supplemented by theoretical and numerical developments. The flow configuration consisted of a three-layer system, with upper turbulent layer driven over the lower stratified, quiescent, layer while an intermediate (inversion) layer sandwiched between these two layers. The studies included the nature of instabilities, intermittent generation of turbulence, sustenance and decay of turbulence under varying background conditions (essentially determined by the Richardson number) and ensuing turbulent mixing in the inversion layer. An unprecedented volume of laboratory data were gathered during the program, which enabled to delve into the mechanics and energetics of mixing in stable boundary layers. The laboratory results were compared with, and was used to gain insights on, field observations. Also, the parameterizations developed were compared with those currently used in numerical models. A meso-scale numerical model also was used to check the efficacy of some of the laboratory-based parameterizations.
Develops a physical theory from the mass of experimental results, with revisions to reflect advances of recent years.
Turbulence, mixing and the mutual interaction of turbulence and chemistry continue to remain perplexing and impregnable in the fron tiers of fluid mechanics. The past ten years have brought enormous advances in computers and computational techniques on the one hand and in measurements and data processing on the other. The impact of such capabilities has led to a revolution both in the understanding of the structure of turbulence as well as in the predictive methods for application in technology. The early ideas on turbulence being an array of complicated phenomena and having some form of reasonably strong coherent struc ture have become well substantiated in recent experimental work. We are still at the very beginning of understanding all of the aspects of such coherence and of the possibilities of incorporating such structure into the analytical models for even those cases where the thin shear layer approximation may be valid. Nevertheless a distinguished body of "eddy chasers" has come into existence. The structure of mixing layers which has been studied for some years in terms of correlations and spectral analysis is also getting better understood. Both probability concepts such as intermittency and conditional sampling as well as the concept of large scale structure and the associated strain seem to indicate possibilities of distinguishing and synthesizing 'engulfment' and molecular mixing.
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 book allows readers to tackle the challenges of turbulent flow problems with confidence. It covers the fundamentals of turbulence, various modeling approaches, and experimental studies. The fundamentals section includes isotropic turbulence and anistropic turbulence, turbulent flow dynamics, free shear layers, turbulent boundary layers and plumes. The modeling section focuses on topics such as eddy viscosity models, standard K-E Models, Direct Numerical Stimulation, Large Eddy Simulation, and their applications. The measurement of turbulent fluctuations experiments in isothermal and stratified turbulent flows are explored in the experimental methods section. Special topics include modeling of near wall turbulent flows, compressible turbulent flows, and more.