Structural and dynamical characteristics of intermittent structures in homogeneous turbulent shear flow.

摘要

The emphasis of this study is on the quantification and analyses of loosely held concepts such as "structure," "scale" and "dynamic significance" of intermittent regions within turbulent flows in general, shear flows in particular. The approach is to develop a robust algorithm which "extracts" regions of concentrated activity in a fluctuating turbulent variable and labels each region individually for quantitative and graphical analysis. A detailed examination of the structural characteristics of intermittent regions in the vorticity, strain-rate, and Reynolds stress fields is presented. The significance of these intermittent structures to the kinematics and dynamics of shear turbulence has been investigated.;We found the intermittent regions to be dominant contributors to the turbulence statistics. In general, these events are structurally distinct from the low-level fluctuations surrounding them. In particular, the vorticity and strain-rate fields display structural similarity in the highest intensity events. The structural and kinematic characteristics of intermittent structures make up most of general characteristics of turbulent shear flow. Furthermore, the coherent fluctuations within intermittent structures are associated to a large extent with the local dynamic processes which lead to the global characteristics of turbulent shear flow. The intermittent structures are clearly important in the structure and dynamics of fully developed turbulence.;A second part of this study focuses on the local dynamical processes which contribute to global evolution in the transition from initially isotropic to shear-dominated turbulence. The dynamic interactions among small-scale regions of intermittent vorticity and strain-rate are analyzed quantitatively and graphically to understand the effects of shear on local dynamics underlying evolutionary changes within a fully developed turbulence. An intermittent structure of vorticity which appears as a hair-pin vortex during the application of mean shear was followed in time. This vortical structure evolves in three stages. The topology of this vortical structure evolves in a manner correlated with the local dynamic processes.