离心泵特性曲线-turbcomphydr.pdf

7. TURBULENCE AUTUMN 2004 Part (a) - Introduction to turbulence 7.1 What is turbulence? 7.2 Turbulence notation 7.3 Effect of turbulence on the mean flow equations 7.4 Turbulence transport 7.5 Particular shear flows Part (b) -Turbulence modelling 7.6 Obj ectives in turbulence modelling 7.7 Eddy-viscosity models 7.8 Advanced turbulence models 7.9 Wall boundary conditions PART (a) - INTRODUCTION TO TURBULENCE 7.1 What is Turbulence? • A “random”, 3-d, time-dependent eddying motion with many scales, superposed on an often drastically simpler mean flow. Instantaneous Mean • A solution of the Navier-Stokes equations. • The natural state at high Reynolds numbers (i.e. most civil-engineering flows). • An efficient mixer ... of mass, momentum, energy, constituents. • A maj or source of energy loss. • “The last great unsolved problem of fluid mechanics”. CFD 7-1 David Apsley Laminar, Turbulent, Transitional Laminar flow is smooth, with adj acent layers of fluid sliding past each other without intermingling. Cross-stream transfer of momentum occurs because viscous forces act between adj acent layers moving at different speeds. Turbulent flow is “chaotic”, with adj acent layers continually distorting and intermingling. A net transport of momentum occurs because of the mixing of fluid elements from different layers with different mean velocity. Transition from laminar to turbulent flow occurs at sufficiently high Reynolds number: UL UL Re ≡ = through a process of local bursts of turbulence coalescing to fill the flow. The value at which transition occurs is called the critical Reynolds number, Recrit. It is only approximate,