The flow patterns in confluence channel and the simulation of confluence flow are more complex than that in straight channel. Additional terms in the momentum equations, i.e. dissipation terms, denoting the impact of turbulence, and dispersion terms, denoting the vertical non-uniformity of velocity, show great impacts on the accuracy of numerical simulations. The dissipation terms, i.e. the product of eddy viscosity coefficient and velocity gradient, are much larger than those of the flow in straight channel. In this study, the zero equation model and the depth-averaged k-e model are used to analyse the impact of eddy viscosity. Meanwhile, the dispersion terms in the momentum equation, depending on the vertical non-uniformity of velocity, are usually neglected in routine simulation. With the use of detailed experimental data for verification, this study presents the distribution of parameters of vertical non-uniformity and the intimated connection between non-uniformity parameters and accuracy of numerical simulations of confluence flow with depth-averaged models. The results present that simulation accuracy of confluence flow is very sensitive to the turbulence modes, which cannot be handled by normal, simple turbulence model. On the contrary, the impact of dispersion terms is both flow-condition-dependent and place-dependent, and such impact is negligible when secondary circulation is weak. The results indicate the key elements in modelling confluence flow and are helpful for selecting suitable numerical model and solving engineering problems encountered in confluence channel.
To alleviate the problem that the water level drops more and more seriously in recent years, Jiangxi province proposes to set up regulation gates at each estuaries of Ganjiang river to raise the water level in Nanchang city during the dry season. The operation of this project will raise the upstream water level, which means the sediment transport rate will decrease and lead to deposition problems. In order to find out the deposition area and development trend, we carried out the suspended sediment deposition test on an existing physical model. The study shows that the upstream sediment is mainly deposited in the reaches near the two deltas. Some special topographic features are more likely to cause siltation, such as the wide and deep thalweg and the backflow areas at the downstream of convex banks. After the operation of the project, the erosion trend of the upper reaches of the central and southern branches will change to siltation, and the siltation of the upper reaches of the Western branch will be further strengthened. 15 years later, with the decrease of the upstream sediment intensity, the sediment siltation phenomenon will gradually move to downstream, however this will be a long process.
The gradual characteristics of variations in transverse width often occur in mountain river because of the effect of geological conditions and river evolutions. This will influence flow and sediment transport due to the variations of flow structures resulted from the river width changes. A flume experiment that was conducted to explore how flow characteristics perform with the varied channel width, showing the connection of the flow parameters, flow rate and channel width. For example, the longitudinal water depth and velocity change along with the transition width of flume. On the other hand, a depth-averaged two-dimension model was proposed according to series of methods such as body-fitted coordinate, finite volume method, SIMPLER algorithm. The numerical results agreed well with the experimental observation in a flume, which showed that the model could reasonably simulate gradual channel transition flow. In addition, the validated numerical model was applied to simulate flow with different flow rates with the same imposed boundary condition in order to further analyse the effects of channel width variation on flow characteristics.
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