tailieunhanh - INTRODUCTION TO URBAN WATER DISTRIBUTION - CHAPTER 3

CHÍNH KHÁI NIỆM VÀ ĐỊNH NGHĨA Các nguyên tắc thủy lực cơ bản áp dụng trong vận chuyển nước và thực hành phân phối xuất hiện từ ba giả định chính: 1 Hệ thống này được chứa đầy nước dưới áp lực, 2 nước không nén được, 3 rằng nước có một dòng chảy ổn định và thống nhất. Ngoài ra, nó được giả định rằng sự biến dạng của ranh giới hệ thống là không đáng kể, có nghĩa là nước chảy qua một đàn hồi không Giảm Q Lưu lượng dòng chảy (m 3 / s) thông. | CHAPTER 3 Steady Flows in Pressurised Networks MAIN CONCEPTS AND DEFINITIONS The basic hydraulic principles applied in water transport and distribution practice emerge from three main assumptions 1 The system is filled with water under pressure 2 that water is incompressible 3 that water has a steady and uniform flow. In addition it is assumed that the deformation of the system boundaries is negligible meaning that the water flows through a non-elastic Steady flow Flow Q m3 s through a pipe cross-section of area A m2 is determined as Q V X A where n m s is the mean velocity in the cross-section. This flow is steady if the mean velocity remains constant over a period of time At. Uniform flow If the mean velocities of two consecutive cross-sections are equal at a particular moment the flow is uniform. The earlier definitions written in the form of equations for two close moments t1 and t2 and in the pipe cross-sections 1 and 2 Figure yield v1t1 v1t2 A v-y V U for a steady flow and v t1 1 t1L 1 t2 1 t2 J O V1 v2 A V1 v2 for a uniform flow. A steady flow in a pipe with a constant diameter is at the same time uniform. Thus t1 t1 t2 t2 33 v1 v2 v1 v2 1 The foundations of steady state hydraulics are described in detail in various references of Fluid Mechanics and Engineering Hydraulics. See for instance Streeter and Wylie 1985 . 2006 Taylor Francis Group London UK 56 Introduction to Urban Water Distribution Transient flow Figure . Steady and uniform flow. The earlier simplifications help to describe the general hydraulic behaviour of water distribution systems assuming that the time interval between t1 and t2 is sufficiently short. Relatively slow changes of boundary conditions during regular operation of these systems make At of a few minutes acceptably short for the assumptions introduced earlier. At the same time this interval is long enough to simulate changes in pump operation levels in reservoirs diurnal demand patterns etc. without .