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Environmental Fluid Mechanics - Chapter 13

Chủ đề trong dòng phân tầng 13,1 nổi VÀ CÂN NHẮC ỔN ĐỊNH Gần như tất cả các cơ quan mặt nước tự nhiên được phân tầng ít nhất là một phần của thời gian. Điều này có nghĩa là có sự thay đổi mật độ, thường là theo chiều đứng. Biến thể ngang cũng có thể tồn tại, nhưng không phải trong trạng thái ổn định nếu không có những lực lượng khác như hiệu ứng Coriolis có mặt để cân bằng sự khác biệt áp lực kết quả (xem Chap 9). Biến thể của mật độ tồn tại phổ biến. | 13 Topics in Stratified Flow 13.1 BUOYANCY AND STABILITY CONSIDERATIONS Nearly all natural surface water bodies are stratified at least part of the time. This means there are density variations usually in the vertical direction. Horizontal variations also may exist but not in steady state unless there are other forces such as Coriolis effects present to balance the resulting pressure differences see Chap. 9 . Density variations exist most commonly because of temperature and or salinity gradients. Salinity is the main contributor to density in the oceans and in some inland lakes such as the Great Salt Lake in Utah or the Dead Sea in Israel though these water bodies also may have temperature gradients. In freshwater lakes temperature stratification is most important. In fact from a water quality point of view there is usually great interest in modeling the temperature structure of water bodies. Most biological and chemical reactions depend on temperature and fish choose habitats based partly on this parameter. As discussed in Chap. 12 gas transfer across the air water interface also depends on temperature. A closely related parameter to density is buoyancy defined as Po p b g g 13.1.1 P0 where p is the density of a fluid parcel and P0 is the reference density. Buoyancy is also known as reduced gravity g and is defined so that a fluid parcel tends to rise when its buoyancy is positive i.e. its density is less than the reference value and a particle with higher buoyancy has a greater tendency to rise. Buoyancy may be treated like other state variables such as temperature or concentration and a conservation equation can be defined db C V Vb V kbVb source sink 13.1.2 dt Copyright 2001 by Marcel Dekker Inc. All Rights Reserved. where kb is diffusivity for buoyancy and the source sink terms are defined depending on which component is contributing to the buoyancy. For example a solar heating term might generate buoyancy in a temperature-stratified system Sect. 12.2 while .