| As a discharging building, tunnel is used commonly in hydraulic engineering. Understanding the variation of flow field and flow pattern in a tunnel provide a designing basis for size and structure of the tunnel. Especially the knowing of the variation law of free surface in a free-flow tunnel is the key of the designing. Combining with the hydraulic model experiment of the discharging free-flow tunnel of a reservoir in Xinjiang, the characteristics of flow field in the tunnel are researched in detail, and the VOF(Volume Of Fluid) method was chosen for simulating free surface in the tunnel after comparing deferent methods. A water-air two-phase numerical model is developed by using VOF and K-ε turbulence model together. Based on deferent operating conditions, the fluid-mechanics-software FLUENT is used in the two-dimensional numerical modeling and the tree-dimensional numerical modeling. The non-structure grid is applied to deal with the irregular boundaries. PISO (Pressure-Implicit with Splitting of Operators) method which is based upon the close relation between velocity and corrected pressure is used to improve the calculating efficiency by reducing iterative time for convergence. The free surface and velocity in the tunnel obtained from numerical model are in agreement with ones from physical model. The flow pattern in the tunnel in maximal water depth is also obtained, and these parameters can be used for tunnel design.
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