Two-Dimensional Numerical Simulation Of Water Flow At90-Degree Open-Channel Junction

Confluences of main stream and tributary stream are common in natural river systems and artificial water channels. Comparing with the straight water flow in the open channel, there are mixed factors characterizing the junction flow’s complex hydrodynamics, including discharge ratio, flow physics, confluent angle, channels’ conditions(like roughness, slope, size, shape) and depth of water of outlet, resulting in a wide rage of potential problems in the fields of river regulation, intake-or-outfall setting, waterway management, flood control, etc.. For example, sediment deposition is likely to make shoals at junctions, the swinging separation zone accumulates pollutants, and flood peaks from main stream and tributary stream overlap. Therefore, research of the law of water movement of confluences is of great theoretical and practical interests. In order to systematically study the effects of different boundary condition on confluence hydrodynamics, this thesis researches two-dimensional confluence hydrodynamics through numerical simulation, with the main contents as follows:(1) According to the control volume method, horizontal mathematical model for two-dimensional flow is established to simulate the equal width rectangular confluence, with SIMPLE algorithm used to solve the equations. The program is written in Fortran language, with its calculation results going well with experimental data, which offers the basis of analysis of the law of flow movement in confluence.(2Considering the advantages of both stream-line method and iso-line method, the mixing method is given in this thesis:to fix the most downstream point on the equal-to-zero velocity of the iso line along the mainstream, and to define the distance from this point to the lower lip of the confluence as the separation zone’s length; to plot a stream line through this point, and to define the maximum distance of the stream line at the y direction as the separation’s width.(3) CFD software Fluent employing the RSM turbulence model numerically simulates the separation zone size of the two-dimensional90°equal-width open channel junction. Comparison among the simulation results, experimental data, and present simulation results from H-L model and the k-ε model shows that the RSM model is better than H-L model and k-ε model.(4) Fluent employing the RSM model is used to research the influences of changing property relations of main channel width and tributary channel width to separation zone’s sizes. According to the simulation results, a series of relations are found, including the relations between separation zone’s length, separation zone’s width, shape factor, coefficient of contraction of main channel and the width ratio, respectively; as well as the relation between separation zone’s nondimensional length and ratio of momentum.(5) Fluent employing the RSM model is used to simulate the mass diffusion in an open channel junction. The results show that the dispersion of pollutants is significantly one-way characteristics, and setting outfall at the side far from tributary channel is conducive to the dispersion of pollutants to downstream.