Can Dispersive Pressure Cause Inverse Grading in Grain Flows?: Discussion
Can Dispersive Pressure Cause Inverse Grading in Grain Flows?: Discussion
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分散压力会导致谷物流的逆级配吗？：讨论
DOI：
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发表时间：
2003
期刊：
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通讯作者：
J. P. Roux
中科院分区：
文献类型：
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作者：
J. P. Roux
分散压力的概念最初是由Bagnold（1954）提出的，他通过实验表明，剪切颗粒材料产生与剪切应力和颗粒浓度成比例的分散压力。Leberg（2002年）提出了一项理论分析，表明分散压力不能解释在颗粒流、改性颗粒流或牵引毯中常见的反级配，这与许多沉积学家几十年来所接受的观点相反。他指出，如果在剪切颗粒物质的分散压力增加，并超过所施加的法向应力或静压，作用在床上的净向上的力会导致它膨胀，事实上已经在实验中观察到。床膨胀立即导致颗粒浓度降低，从而导致分散压力降低，直到后者平衡静压和膨胀停止。如果分散压力变得小于静态压力，则作用在颗粒床上的净向下力导致压实，从而增加颗粒浓度和分散压力。这是一个重要的概念，加强了我们对一个迄今知之甚少的进程的理解。然而，Leiden（2002）进一步指出，“分散压力必须始终等于通过剪切颗粒床在每个水平（强调我）施加的法向剪切应力。“这意味着整个剪切床处于恒定平衡状态，但情况可能并非如此。在本讨论中，提出了一种改进的工艺，该工艺考虑了Sohn（1997）提出的二分牵引地毯结构。
牵引毯是在下方形成并由上覆湍流驱动的高度集中的推移质材料的移动层（Dzulynski和Sanders 1962）。Sohn（1997），基于数值分析、计算机模拟、现场测量和实验研究（Bagnold 1955，1963; Hanes和Bowen 1985; Wan...
The concept of dispersive pressure was originally proposed by Bagnold (1954), who showed experimentally that sheared granular material develops a dispersive pressure that is proportional to the shear stress and the particle concentration. Legros (2002) presented a theoretical analysis suggesting that dispersive pressure cannot account for the inverse grading commonly observed in grain flows, modified grain flows or traction carpets, contrary to what has been accepted for decades by many sedimentologists. He pointed out that, if the dispersive pressure in a sheared granular mass increases and exceeds the applied normal stress or static pressure, the net upward force acting upon the bed causes it to expand, as has in fact been observed in experiments. Bed expansion immediately causes a decrease in particle concentration and consequently in dispersive pressure, until the latter balances the static pressure and expansion stops. If the dispersive pressure becomes less than the static pressure, the net downward force acting upon the granular bed causes compaction, thus increasing the particle concentration and dispersive pressure. This is an important concept enhancing our understanding of an as yet poorly understood process. However, Legros (2002) went further to state that "the dispersive pressure must always equal the applied normal shear stress at every level (emphasis mine) through the shearing granular bed." This implies that the whole sheared bed is in constant equilibrium, which is probably not the case. In this discussion a modified process is proposed that accounts for the bipartite traction carpet structure proposed by Sohn (1997).
Traction carpets are moving layers of highly concentrated bedload material developed beneath and driven by an overlying turbulent flow (Dzulynski and Sanders 1962). Sohn (1997), on the basis of numerical analyses, computer simulations, field measurements, and experimental studies (Bagnold 1955, 1963; Hanes and Bowen 1985; Wan …