| Due to the over-exploitation of sand and gravel materials in the Changjiang River,the riverbed topography is seriously damaged,and the flow pattern near the sand pit is disordered,which poses a certain threat to the riverbed evolution and ship navigation.At present,most of the research on sand mining pits focuses on the numerical simulation and physical model test of the evolution of sand mining rivers,and there are few studies on the local flow movement characteristics and sediment deposition of sand mining pits.This paper mainly studies the flow characteristics and deposition distribution in sand mining pits,which can provide theoretical support for the evolution analysis of sand mining pits and provide reference for subsequent studies.This paper mainly studies the flow structure characteristics and sediment deposition pattern of rectangular sand pit.The generalized physical model test was carried out,and the test was carried out under different flow conditions.The acoustic Doppler velocimeter(ADV)was used to measure the three-dimensional flow velocity of the measuring point in the test section.The test results were processed,and the distribution characteristics of three-dimensional flow structure characteristics such as the time-averaged flow velocity,turbulence intensity and Reynolds stress in the sand pit were analyzed.The experiment also studied the influence of sediment on the flow characteristics in sand pit,and observed and analyzed the deposition pattern of sediment-containing flow in sand pit in detail.The main findings of the thesis are as follows:(1)The flow movement inside the sand pit shows obvious three-dimensional velocity characteristics.Under different flow conditions,the distribution of time-averaged velocity in the sand pit is roughly similar.There is an obvious flow velocity partition in the sand pit,and there is backflow near the bottom wall of the sand pit.The combination of circulation around the upper wall and vertical velocity makes the water flow spiral.With the increase of water depth under different conditions,the flow velocity decreases,and the reflux intensity will also decrease accordingly.(2)The distribution of turbulence intensity and Reynolds stress of water flow in sand mining pits in various directions shows certain similarity,indicating that there is a certain correlation between them.The relationship between turbulence intensity and Reynolds stress in x,y and z directions is:>>,>>.Turbulence intensity and Reynolds stress are larger in the severe energy exchange area,mainly concentrated in the water mixing area near the bottom edge of the sand pit mouth and the vertical surface,which is easy to cause erosion of the edge of the sand pit.(3)Compared with the clear water condition,when the flow through the sand pit contains sediment particles,the distribution of each flow characteristic quantity along the water depth direction is roughly the same,and the flow velocity changes.The presence of sediment has a certain inhibitory effect on the flow turbulence,so the turbulence intensity and Reynolds stress are reduced to varying degrees.(4)Sediment deposition occurs at the bottom of the sand pit.Under different conditions of water and sediment,the deposition process of the sand pit is basically similar:the initial stage of the test changes rapidly,and then slows down until equilibrium.The bottom sediment deposition pattern is related to the velocity distribution,mainly concentrated in the upstream area of the bottom of the sand pit,and there is no sediment deposition at the downstream.The final deposition body formed in the test is divided into two parts:zonal deposition in the recirculation zone and transverse circulation deposition,and there is a clear boundary between the two. |
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