| The problem of sedimentation is an urgent problem to be solved in the operation and management of reservoirs on sediment-rich rivers.The use of natural conditions and reservoir scheduling to shape different flows for reservoir sedimentation is limited by natural and spatiotemporal conditions,and the ability to handle sediment is also limited.Since its application in the 1990 s,hydraulic dredging has been vigorously developed due to its high efficiency and low cost.This paper studied the physical process and mechanism of using jets to erode the sand bed,and the flow law of density current and its formation.The feasibility of using jets to shape density current for reservoir sediment treatment is discussed.The main research work and conclusions of this paper are as follows:Firstly,by studying the experiments on sand bed erosion with underwater plane stationary jets,the process and erosion of sand bed by underwater stationary plane oblique jets under different moving jet speeds,jet angles and slope gradients were observed and analyzed.Morphology and muddy water flow characteristics outside the crater.The results showed that: There were geometric similarities of the scour holes with asymmetrical geometry and downstream-toward pro-files;The development processes of the scour holes can be divided into three stages: initial stage,development stage and stable stage;The depth of the scour holes developed with time as a logarithmic relationship,and the erosion rate reached the largest when the crater depth reaches half of the maximum depth;With the increase of jet angle,the areas of two-dimensional crater decreased firstly and then increased.It reached its minimum value when the jet angle was at 45;The volumes of sediment erosion and the speed of density current outside the crater increased with the slopes of the sand bed.However,the growth trend slowed down after the slope was greater than 10.00 ‰.Secondly,based on the underwater static jet test,a series of experiments on two-dimensional plane jet erosion sand bed were carried out to study the erosion effects of sand bed by underwater moving jets.The jet movement speed and sand bed slopes were changed.The sediment transportation and density flow under different jet conditions were recorded.According to experimental results,the variation law of sediment transportation and density flow movement with jet movement speed and sand bed slope were analyzed.The physical mechanisms of the two-phase flow movement and sedimentation morphology were discussed.Results showed that the moving jet erosion of sand bed formed density current,which could induce sediment incipience and transport a certain distance.The sediment deposition profiles after the erosion were similar,and the averaged transport distances in all conditions were approximately equal.There was similar relationship between accumulated relative sediment volume distribution and sand particle size distribution.The relative amount of sediment transport was positively correlated with the jet moving speed and the sand bed slope,but when the slope was greater than 10 ‰,the slope effect was no longer significant.Finally,according to the basic characteristics and equations of density current,a model for estimating the motion parameters of muddy water gravity flow generated by jet erosion and the amount of jet erosion sediment was established.The Richardson number of the test conditions was far less than one,and the jet factor was the main factor affecting the flow of the density current formed by the jet eroding the sand bed.The calculation results of the model showed that the weighted flow formed by the erosion of the underwater jet had a strong ability to carry and transport sand.Under Under moving jet working condition 2,the relative density of the density current reached 30.3%,and the calculation result of the theoretical model was basically consistent with the test result.Therefore,this method can form an artificial gravity flow and has the ability to achieve efficient reservoir dredging. |
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