【作者】 陈小莉；

【导师】 马吉明；

【作者基本信息】 清华大学 ， 水利工程， 2008， 博士

【摘要】 局部绕流冲刷是丁坝和桥墩等水工建筑物周围常见的水力现象,是造成该类工程破坏的重要原因之一。局部绕流流态呈显著三维特性,绕流形成的紊动漩涡与局部冲刷有密切联系。本文采用数值模拟方法对建筑物周围的绕流流场、局部冲刷及抛石防冲体的稳定性进行了研究,取得了如下主要成果:在三维水流模拟中引入修正的非平衡k-ε紊流模型,计算了未冲刷前的平整床面和形成冲坑后的斜坡床面两种地形条件下的桥台绕流,结果与实验吻合良好,较好地反映了建筑物周围的流速分布和漩涡发展规律。以实测的局部冲坑为已知边界,模拟了冲刷发展过程中的水流变化,初步揭示了局部冲刷的机理。模拟结果表明初始时桥墩附近的水流径向流速沿水深方向呈现近均匀分布,该区域水体紊动剧烈。出现冲坑后,冲坑底部形成主漩涡,漩涡中心流速比边缘处大,为强迫涡。随着冲深加大,主漩涡尺度增大,冲坑底部出现新的紊动涡核,紊动动能在冲坑内充分扩散并消耗,使得底部水流流速下降,冲刷速率逐渐降低,直至冲刷停止。提出了一个修正斜坡上输沙率公式的新方法,建立了基于该方法的泥沙数学模型。新提出的修正方法只需做一次修正就能同时反映重力对输沙率大小和方向的影响,与实验资料的对比证明其比以前的方法有更好的修正效果,且能够避免在数值计算过程中出现奇点。建立的泥沙模型中应用了上述改进方法,并在推移质离散插值过程中考虑了重力作用对输沙率插值方向的影响。模型应用于对桥台和丁坝的局部冲刷模拟,取得了与实测冲坑形态较一致的结果,分析表明竖轴漩涡对尾部冲坑形态有重要影响。采用朗肯涡模型对旋涡流场进行了简化,推导了受竖轴旋涡作用的块石起动流速公式,结果表明旋涡作用能使块石起动流速显著降低,导致块石在低速水流中仍可能被卷走。结合丁坝实测的旋涡尺度和强度推导了以颗粒粒径、丁坝长、水深及束窄率表示的临界起动公式,研究对实际工程有参考价值。更多还原

【Abstract】 Local scour is a common occurrence around hydraulic structures like spur-dikes and bridge foundations. This affair is a main reason of failure of bank protections and bridges. The flow structure around structures is in strength three-dimensional character and the formed vortex is the chief attribution of local scour. In this dissertation, the flow field and the equilibrium scour hole are simulated with numerical model. And the stability of the riprap at foundations is studied. The main achievements are as follows:A non-equilibrium k-εturbulent model is introduced to flow model to improve the simulating precision of the circumfluence. The flows around an abutment in a flat bed and scoured bed are simulated respectively. The computed results are in good agreement with the experimental data; and the whirl flow velocity field distribution and development of vortex around the abutment are well reflected.The flow around an experimental cylindrical pier in the measured developing scour beds is simulated and the mechanism of local scour is explored simply. The simulated results show that the flow becomes almost uniform in the vertical direction near the nose of the cylinder, and the turbulence is fierce in this region. When the scour hole occurs, the horseshoe vortex is formed at the scoured bed. The horseshoe vortex is a forced vortex type of flow, as the swirl velocity increases in the outward direction from the center of the vortex. Along with the increase of the scour hole and the horseshoe vortex falling into the scour hole, a new turbulent core appears in the scour hole and the turbulence disperses into the scour hole under the transport of the horseshoe vortex. The turbulent energy exhausts in the scour hole and the scouring power of the flow becomes weaker and weaker, until the scour ceases.A new method is promoted to correct the sediment transport rate on slopes, and a sediment transport model is developed. The new modification method can consider both the quantitative and directional influence of gravity on the sediment transport at the same time. The result shows good performance than former correction methods when compared with experimental data. The new method has an advantage that it can avoid the occurrence of infinite value in numerical computation. In the sediment transport model, the bed load transport rate is interpolated separated in the x and y directions rather than interpolated as an integer scalar. This change can improve the interpolation precision. The model is applied in simulating of local scour around an abutment and spur-dike. The computed scour holes are in good agreement with the experimental results. Analysis of the scour hole shows the tail vortex has important effect on the scour-hole shape downstream of the structures.The Rankine vortex model is used to simplify the flow with vertical vortex. The adsorption affinity of the vortex towards the rocks around a spur dike, The resulting stress on the rocks was used to derive the critical incipient velocity formula for rocks. The critical incipient velocity of the rock is lower than that in the classical formula, which will lead to the rock being rolled away in slow-speed flow. According to the vortex dimension and strength around the experimental spur-dike, a new critical incipient velocity formula is obtained. In this formula, the sediment diameter, length of spur-dike, flow depth and restriction ratio of the flow width are considered. This research has consulting value for the realistic projects.更多还原