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Physical Processes in Earth and Environmental Sciences Phần 5

Chương 4 Giá trị ngưỡng là như vậy, một tham số thực tế quan trọng trong kỹ thuật môi trường. Vận tốc cắt Một chất dịch đặc biệt, u *, trên ngưỡng cho chuyển động cũng có thể được thể hiện như là một tỷ lệ đối với vận tốc ngưỡng quan trọng, u * c. | 122 Chapter 4 are given as a critical value of the overall dimensionless boundary shear stress Ớ. The threshold value is thus an important practical parameter in environmental engineering. A particular fluid shear velocity u above the threshold for motion may also be expressed as a ratio with respect to the critical threshold velocity u c. This is the transport stage defined as the ratio u u c. Once that threshold is reached grains may travel Fig. 4.38 by 1 rolling or intermittent sliding 2 repeated jumps or saltations 3 carried aloft in suspension. Modes 1 and Suspended load Bedload Bed Fig. 4.34 Stresses responsible for sediment transport. Note decay of pressure lift force to zero at 3 sphere diameters away from surface as the Bernoulli effect is neutralized by symmetrical flow above and below the sphere z -----x Fig. 4.35 Relative magnitude of shear force drag and pressure lift force acting on spheres by constant air flow at various heights above a solid surface. 2 comprise bedload as defined previously. Suspended motion begins when bursts of fluid turbulence are able to lift saltating grains upward from their regular ballistic trajectories a crude statistical criterion being when the mean upward turbulent velocity fluctuation exceeds the particle fall velocity that is w Vp 1. 4.8.2 Fluids as transporting machines Bagnold s approach It is axiomatic that sediment transport by moving fluid must be due to momentum transfer between fluid and sediment and that the resulting forces are set up by the Fig. 4.36 left W.S. Chepil made the quantitative measurements of lift and drag used as a basis for Fig. 4.35. Here he is pictured adjusting the test section of his wind tunnel in the 1950s. Much research into wind blown transport in the United States was stimulated by the Midwest dust bowl experiences of the 1930s. Fig. 4.37 Variation of dimensionless shear stress threshold 0 for sediment motion in water flows as a function of grain Reynolds number. 0 is known as the .