Effect of lifting force on bed topography and bed-surface sediment size in channel bend
Date Issued
2005
Date
2005
Author(s)
Jang, Jiun-Huei
DOI
en-US
Abstract
In sediment transport, the lifting force plays an important role in reducing the friction between sediment particle and bed surface, and thus makes particles easier to be transported by shear force. Non-uniform distribution of lifting force in channel bend may cause serious errors in describing sediment transportation if the lifting force effect is not taken into account. In the present study, relevant theories and numerical models are investigated and employed to study the lifting force effect on the bed topography and sediment gradation in channel bend.
Since the lifting force, , is related to vorticity, a 3D numerical model incorporating large eddy simulation is applied to simulate the vorticity field in channel bend. The results of simulation show that is larger around the outer bank and smaller around the inner one in a bend. As bed topography develops in a bend, this phenomenon is enhanced. Unlike the shear stress regarded as being independent of sediment size, the lifting force effect is smaller for a larger sediment particle.
Through a series of physical considerations, existing theories are reviewed and modified to find that the friction of particle in motion, , becomes smaller than that at incipient motion, . By replacing with in the expression of effective shear stress, the lifting force effect can be taken into consideration in predicting the sediment transport rate.
In this research, a total numerical model composed of the flow model and bed model is employed to simulate the bed topography and bed-surface sediment size in a channel bend. Comparing the simulated results with and without consideration of the lifting force effect, it shows that the errors can be reduced by about with the lifting force effect considered. For practical engineering applications, further efforts are made to develop an approximate model for bed-surface sediment size distribution to shorten the computation time. Although errors inherited in the approximate model increase by about , it consumes only about of the computation times of the total model.
Finally, various numerical experiments are conducted under various conditions to study the effects of flow and channel parameters on bed topography and sediment gradation in channel bend. The fully developed transverse bed-slope, ratio of the erosion area to the total channel bed area, maximum erosion depth of the developed bed, and maximum median diameter of bed-surface sediment of the developed bed, are expressed as regression functions composed of non-dimensional parameters. The regressed results show that densimetric Froude number and initial sediment gradation are the major factors affecting the variations of bed topography and bed-surface sediment size. In order to verify the applicability of these regression equations, more experimental data in channel bend are employed to testify their validity. The results show that the predicted values agree quite well with the experimental results and the correlation coefficients are all greater than . This verifies the applicability of these regression equations in practical engineering use.
Subjects
昇力
彎道
沈滓運移
床形
床質
lifting force
bend
sediment transport
bed topography
bed-surface sediment size
SDGs
Type
thesis
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