【作者】 张晓东；

【导师】 高季章；

【作者基本信息】 中国水利水电科学研究院 ， 水力学及河流动力学， 2004， 博士

【摘要】 目前在高山峡谷所建高坝，其泄水建筑物多具有水头高、落差大、泄洪量大等特点，高速水流问题严重。针对泄洪洞高速水流的几个关键问题，本文应用数值模拟技术，紧密结合工程实践，进行了如下工作： 1．导流洞改建泄洪洞的关键技术之一——竖井旋流式泄洪洞的水力学问题是本文研究的第一项内容。针对其体型复杂，水面变化剧烈的特点，本文运用自由表面跟踪技术VOF方法进行了三维数值模拟，得到了螺旋水流的流态、流速、压强、通气量、消能率等各项水力参数，很好的模拟和再现了模型试验成果。并在此基础上： 1．提出以喉管处空腔与竖井横断面的面积比A_a／A作为判别竖井泄流是否安全顺畅的参数； 2．详细分析了竖井中水流与空气运动的切向、径向和轴向速度，指出竖井中的螺旋水流是一种准自由涡运动，空腔中的空气受其带动而做的旋转为强迫涡运动； 3．计算分析了竖井压强的分布和特性，受螺旋流作用，不同泄量下，大部分竖井边壁上有较大的剩余压力，某些局部压力较小；内空腔则有一定的负压存在；压强梯度的变化与切向速度的变化以及涡的分布一致； 4．给出了表征水流旋转强度的两个重要参数旋度和螺旋度在竖井典型断面的径向及轴向变化规律；中国水利水电科学研究院博士学位论文5.验证了竖井旋流式泄洪洞的高消能率，进一步证明了其作为新型消能工 的巨大优势;6.分析了推荐竖井体型在不同流量下井身边壁上最小压力与最小空化数 的轴向分布，提出了减免空化空蚀的具体措施。”.龙抬头式泄洪洞反弧末端易发生空化空蚀破坏的问题，是本文研究的另一个 重点: 文中结合工程实例，应用水气两相流理论，就反弧段附近的流速分布、 压强特性和掺气浓度分布进行了模拟，结果表明反弧末端附近的边墙和 底板上存在明显的负压、低压区和掺气盲区以及压强梯度剧烈变化区， 而相应的流速却在40耐s左右，因此容易导致空化空蚀的发生:2.所研究工程的竣工体型由于受施工因素影响，通气孔上缘高出挑坎形成 「1槽效应，恶化了通气孔附近的流态和压力场，增大了空化空蚀发生的 可能性。川.对水气两相流数值模拟的探讨是论文最后部分的内容。要实现对高速水流的 合理模拟，水流的掺气问题必须予以考虑。结合挑坎水流模型试验，文中就 水气两相流模型的选取以及影响模拟的几个重要因素，如气泡粒径、计算空 间与壁函数的选取、单颗粒与群颗粒模拟等作了较为细致的验证、对比和分 析，并通过自定义相间曳力系数调整和改善掺气浓度分布。数值模拟的结果 与模型试验数据吻合较好。 紊流模型的合理选取是数值模拟的一项关键工作，为此本文对上述每一个问题都选用不同的模型做了大量的验证和对比分析工作，确保了结果的合理性与精度。关键词:高速水流，竖井旋流式泄洪洞，龙抬头式泄洪洞，数值模拟，水气两相 流更多还原

【Abstract】 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.更多还原