tailieunhanh - Environmental Fluid Mechanics - Chapter 5

Kết quả bất ổn từ một sự cố trong sự ổn định của một dòng chất lỏng và thường liên quan với số lượng lớn của Reynolds. Sự bất ổn có thể được tạo ra bởi một xáo trộn vô cùng trong một dòng chảy khác thành lớp, và sự ổn định dòng chảy phụ thuộc vào liệu rối loạn phát triển hoặc chết ra, như được thảo luận Sec. . Hầu như tất cả các dòng chảy tự nhiên là hỗn loạn đến mức độ nào đó, và nó là quan trọng để hiểu và có thể đại diện cho sự hỗn loạn khi. | 5 Introduction to Turbulence INTRODUCTION Turbulence results from a breakdown in stability of a fluid flow and is normally associated with large Reynolds numbers. The instability may be generated by an infinitesimal disturbance in an otherwise laminar flow and the stability of the flow depends on whether the disturbance grows or dies out as discussed in Sec. . Almost all natural flows are turbulent to some extent and it is important to understand and be able to represent turbulence effects when modeling a given system. Turbulence itself is difficult to define though certain of its characteristics may be summarized as follows Turbulence is generally three-dimensional and is thought of as consisting of eddies superimposed on the mean flow these eddies are represented as fluctuations in the flow field properties. The eddy motions are irregular and vortical they have vorticity . The motions are mostly random in nature and are usually described in statistical terms. In fully developed turbulence there is a continuous spectrum of eddy sizes with a cascade of energy from larger eddies to successively smaller ones until the kinetic energy of the eddies is dissipated by viscosity into heat by the smallest eddies. From an environmental point of view one of the most important effects of turbulence is to enhance greatly the mixing of fluid properties. This is accomplished through the action of the eddies which are much more effective than molecular motions in redistributing fluid particles within a given flow field see Chap. 10 for further discussion of molecular and turbulent diffusivities . A full analysis of turbulence is made difficult by a number of factors primarily because of its random nature and wide range of scales of motion from the largest eddies which scale with the mean flow geometry to the smallest eddies at which viscosity dissipates the kinetic energy of the Copyright 2001 by Marcel Dekker Inc. All Rights Reserved. turbulent motions. This means for .