| Researches focusing on some critical problems in flood discharging and energy dissipation of high velocity flow in spillway tunnels are canied out by numerical simulation in this dissertation.I. In the first part, as a new and effective energy dissipator with complex bodily profile discharging whirling current with variable free surface, and a critical technology in the reconstruction of diversion tunnel to spillway tunnel, the hydraulic characteristics of vortex-flow shaft spillway are simulated by VOF method in three dimensions. T he vortex flow regime, velocity, pressure, air volume and energy dissipation ratio are obtained and the results, compared with the model test data, are reasonable and satisfied. On this basis, some detailed understandings of the flow characteristics are gleaned as follows:1. Area ratio between the section of the throat and its corresponding shaft section, defined as Aa/A, is put forward as a criteria of safe and fluent discharge;2. Particular analyses of the tangential velocity, radial velocity, and axial velocity of the flow and air in the shaft show that the vortex flow acts as a quasi-free vortex, while the air in the core follows the characteristics of forced vortex;3. Affected by the vortex, except some particular part, there exists highresidual pressure on the shaft wall under different fluxes, while there exists minus pressure in the air core; distributions of the pressure gradient, tangential velocity and vortex reveal a good correlation among them;4. The radical and axial distributions of two important parameters, vorticity and helicity, tokens of the vortex intensity of the flow, are given on typical sections of the shaft;5. High energy dissipation ratio validated by the simulation further demonstrates the shaft's great advantage as a new type of dissipator;6. Axial distributions of the minimum pressure and minimum cavitation number around the shaft wall of the proposed bodily profile under several fluxes are analyzed in detail, and specific measures to reduce occurrence of cavitation are brought forward.II. There exists great potential of cavitation damages near the end of the concave section in many high water-head discharging tunnels, and the reasons leading to it are studied in the second part of the paper:1. An existing project is simulated in three dimensions with two-phase flow theory. According to the distributions of velocity, pressure, and air concentration near the concave section, there exist obvious negative or low pressure areas and zones with zero or low air concentration along the side wall and the floor, while both the pressure gradients and velocities on the corresponding locations are great, which easily lead to cavitation damages;2. The completion bodily profile of the project researched is simulated thereafter and the damage reason is concluded as the influence of constructional defect, the upper edge of the air vent higher than the bucket lip, which causes the so-called "gate slot effect", anddeteriorates the flow pattern and pressure field near the air vent, so raised the possibility of cavitation further.III. The research of air-water flow simulation, which is of great importance for correctly and reasonably calculating high velocity flow, is carried out in the last part of the dissertation. Based on the model test of aerated flow, the selection of the numerical model for the air-water two phase flow and some important influence factors, including the diameter of air bubble, simulation dimension, wall function, and single or multiple bubble diameter scheme, are carefully discussed and analyzed. The simulation of the distribution of air concentration is adjusted and improved by using User-defined Function of drag coefficient. And the results fit the model data well.The right selection of turbulence model is also important for correct simulation. Therefore much work of validation and comparison on the numerical model is done for each simulation in this paper to ensure the precision and reasonableness of the results.
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