Studies On The Three-dimensional Numerical Model For Free-surface Flow And Suspended-sediment Transport

Three-dimensional numerical model is an important approach to cope with the free-surface flow, sediment transport and the river bed deformation, especially in the complex situations, on which the study is valuable from the academic and practical views. A 3-D numerical model for free-surface flow and suspended-sediment transport is developed in this paper, and the typical 3-D flow and sediment transport in a reservoir are simulated. The main work and results are as follows.On the basis of the former work, a 3-D non-hydrostatic pressure model for free surface flows on the C-D unstructured, z-level grid is built. The details and properties of model formulation are described, and the model is validated by benchmarks. The model is based on the newly developed pressure-split mode, and it augments the solution of the tangential momentum equation comparing to models on the C grid. Hence, the comparison and discussion about the non-hydrostatic pressure models on the C-D and C grid is done. Based on practice, simulations by using the former are free of noisy, but the model produce unphysical solutions and becomes unstable when a strong nonlinear distribution of non-hydrostatic pressure exists. Based on analysis, a combination of the two is proposed to avoid the instability of the C-D grid, which gives the model the stability of the C grid and the exactness of the C-D grid. By using the improved method, the lock-exchange salty density flow is simulated successfully.By combining numerical experiments and formal analysis we investigate the artificial resistance of the Eulerian-Lagrangian Method (ELM) which is used in our model. The phenomena are explained and an improved method is proposed based on the illustrations and analysis. The validation indicates that though the new method does not eliminate the ELM interpolation error absolutely, it does improve the exactness for simulations at small time steps. More over, a 3-D hydrostatic pressure model for free surface flows on the C-D unstructured,σgrid is built. The artificial resistance is revealed to be ubiquitous for the ELM, though theσgrid is found to be able to reduce its influence. A suspended-load transport model is built on the basis of the hydrodynamic model, and validations indicate that the simulations agree well with the experiment data. Then a method for tracking moveable, irregular river bank is proposed, the qualitative check indicate that it is applicable and effective. More over, the sub-grid technique is used in the model to make simulations more exact, and it is found that this technique may introduce instability due to the bed degradation and aggradation. And a grid-union method is proposed to make the sub-grid technique robust.Based on the analysis of the physical process of the flow around an orifice, new experiential methods are proposed to determine the boundary conditions of discharge and hydrodynamic pressure at the orifice. More over, a module for simulating the block sliding under water is built. After that, the following projects are simulated: the orifice associated flow, the density flow after a hydraulic jump, the simplified reservoir density flow, the development of the filler just before the bottom orifice at different topography and water levels. The simulation results agree well with the experiment data.