Chapter 8 Convection in External urbulent Flow8章在外部的湍流对流.docVIP

Chapter 8: Convection in External Turbulent Flow Introduction Turbulent flow is disordered, with random and unsteady velocity fluctuations; hence, exact predictions cannot be determined. Turbulence affects local velocity distribution, drag force, and heat transfer in both natural and industrial processes. Understanding of turbulent flow leads to the ability to make design improvements to either reduce or enhance turbulent effects Our understanding still relies on empirical data and rudimentary conceptual drawings, and, more recently, computer simulations. Exact solutions are not possible. Chapter Focus: Wall-bounded shear flows Examples of Turbulent Flows (a) Mixing Processes Combustion processes – proper mix of fuel and air is one of the requirements for combustion efficiency Chemical processing, such as the production of polymers Laminar mixing occurs when liquids are viscous and/or slowly mixed, and can be problematic (b) Free Shear Flow Jet Flows (refer to fig. 8.1a): jet energy is dissipated to the surrounding fluid Turbulent wake (refer to fig. 8.1b): transfers energy between an object and the ambient flow, contributes to an object’s drag Smoke stack exhaust is dispersed by turbulence (refer to figure 8.c) (c) Wall-Bounded Flows Flow of air over a flat plate or airfoil Flow of fluid in a pipe Irregular or random motions cause the shape of the velocity profile and the boundary layer edge location to change with time Instantaneous velocity profiles are time-averaged for simplicity: Turbulent velocity can be decomposed into steady (mean, ) and unsteady (fluctuating, ) components Note: Refer to fig. 8.2 for details on the velocity profiles and unsteady components The mixing of velocity fluctuations in turbulent flow creates a steeper profile than that of a laminar flow, with a larger boundary layer and higher wall shear stress. Turbulent fluctuations enhance momentum transfer between the surface and the flowing fluid, resulting in higher skin friction; this sugge