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The concept of autosuspension implies that if, in a flow transporting suspended sediment down a slope, the sediment adds more energy to the flow than it extracts, the concentration can rise to an indefinitely high level. This was tested in a small closed duct, with variable slope, connecting two free-surface reservoirs. The sediment-carrying flow could be recirculated continuously from the downstream end of the duct to the upstream end. For each of five sediment sizes ranging from 14 μm to 96 μm, two flows at the same discharge were run, one with zero slope and one with a slope chosen so that autosuspension should have been in effect. Something is wrong with the autosuspension criterion: in each set of runs, equilibrium suspended-sediment concentrations were virtually identical in the two cases. Analysis of the term in the autosuspension criterion expressing work done by the flow to suspend sediment shows that this quantity is much smaller than is assumed in autosuspension, and so the criterion should have been even more strongly oversatisfied in our tests. Although both the energy-expenditure form and energy-contribution form in the criterion have a physical reality, comparison of the two has no relevance to the equilibrium state of the flow; autosuspension is based on an irrelevant system of energy bookkeeping. Suspension must be viewed as limited by factors involving alterations in turbulence structure leading to changes both in boundary resistance and in the ability of the flow to entrain and suspend sediment. 相似文献
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Pratima Pandey S. Nawaz Ali AL. Ramanathan P.K. Champati ray G. Venkataraman 《地学前缘(英文版)》2017,8(4):841-850
Hamtah and Chhota Shigri are two nearby, well monitored glaciers of western Himalaya, lying in the same climatic zone and driven by the same climatic conditions. In this study, topographical characteristics of both the glacier have been explored to understand the role of topography in controlling the glacier response. Further, their topographical characteristics and possible response towards climatic variations have been compared with each other and also with that of the other glaciers in the basin to find out the suitability of these two glaciers to be considered as representative of the region. Multi sensor and multi temporal remote sensing data have been used to carry out to fulfill the objectives. It is found that being in the same climatic zone, the mean accumulation area ratio of Chhota Shigri is 54% and Hamtah is 11% between 1980 and 2014. In comparison to Hamtah, Chhota Shigri glacier has a small upslope area, low compactness ratio indicating the ability of the glacier to receive direct precipitation and solar radiation. The analysis revealed that the Chhota Shigri glacier has a closer resemblance with the other glaciers in the region than Hamtah glacier. Also, the topographical settings of Chhota Shigri glacier are suitable for recording and reflecting year-to-year climatic variations. 相似文献