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.

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.

The first four symposia in the series on turbulent shear flows have been held alternately in the United States and Europe with the first and third being held at universities in eastern and western States, respectively. Continuing this pattern, the Fifth Symposium on Turbulent Shear Flows was held at Cornell University, Ithaca, New York, in August 1985. The meeting brought together more than 250 participants from around the world to present the results of new research on turbulent shear flows. It also provided a forum for lively discussions on the implications (practical or academic) of some of the papers. Nearly 100 formal papers and about 20 shorter communications in open forums were presented. In all the areas covered, the meeting helped to underline the vitality of current research into turbulent shear flows whether in experimental, theoretical or numerical studies. The present volume contains 25 of the original symposium presentations. All have been further reviewed and edited and several have been considerably extended since their first presentation. The editors believe that the selection provides papers of archival value that, at the same time, give a representative statement of current research in the four areas covered by this book: - Homogeneous and Simple Flows - Free Flows - Wall Flows - Reacting Flows Each of these sections begins with an introductory article by a distinguished worker in the field.