【作者】 朱德军；

【导师】 陈永灿；

【作者基本信息】 清华大学 ， 水利工程， 2007， 博士

【摘要】 南水北调中线明渠段一旦发生水污染突发事故,必须快速揭示不同运行条件下污染物的输移扩散规律,提出事故处置对策。论文通过建立纵向一维和水深平均二维水流-水质数值模型,对事故污染特性的模拟方法进行了深入研究,并开展了针对南水北调中线典型明渠段突发事故污染的模拟分析。论文建立了高精度的一维水流-水质数值模型,模型中分别采用Preissman和Preissman-Holly格式离散水流及水质方程,保证了数值离散具有良好的精度和稳定性。首次将最大信息熵原理应用于梯形明渠,确定了纵向流速的断面分布公式,基于Fischer的三重积分方法,提出了精确的纵向离散系数计算公式。论文还建立了水深平均二维水流-水质模型,选用通量差分裂格式求解方程组的对流项,能有效处理各种复杂流态,并高分辨率模拟污染物的输移扩散。通过构造离散单元内部流动变量分布,间接求解底坡项,考虑了底坡项作用的迎风特性并简化了边界条件。提出了淹没节点法和守恒型人工渗透法,用以处理动边界问题;采用网格局部加密技术和当地时间步长法,提高模型的精度和效率。论文通过多个典型算例对建立的二维模型进行了验证,表明模型可以处理溃坝波、急流和缓流过渡、间断浓度场等流场和浓度场突变的问题。论文利用一维和二维模型研究了正常运行情况下南水北调中线典型明渠段水体流动和污染物输移的规律。两种模型都能准确地模拟污染云团的平均移动速度,一维模拟具有快速、及时的优点,但是,由于忽略了射流核心区和扩散区的影响,模拟结果中将污染云团的偏态分布形式近似为正态分布,错估了最大浓度点的移动速度,并夸大了污染云团的纵向长度;二维模型不仅能更准确地模拟污染云团的纵向分布特征,还能反映污染物进入水体的横向位置对浓度场的影响,污染物进入水体越快、横向混合系数越小,越有必要采用二维模型准确模拟污染物的输移规律。在恒定流模拟的基础上,论文利用二维模型对事故处置过程中,采用闸前定水位控制运行方式下的非恒定流场和浓度场进行了模拟研究,研究表明上游节制闸门的关闭历时应大于两倍渠道长度与渠道中浅水长波波速的比值,关闭闸门不仅会减缓污染云团的平均运动速度,还将改变云团的浓度分布情况,关闭速度越快影响越大。论文成果为深入分析南水北调中线明渠段水污染事故的特性和进一步研究事故处理措施,提供了坚实的理论和技术基础。更多还原

【Abstract】 Once a water pollution accident breaks out in the middle route open channel of the South-to-North Water Transfer(SNWT), the characteristics and developing mechanism of the flow fields and pollutant concentration distributions caused by the accident must be quickly determined and effective measures are eventually taken against the accident. In this dissertation a 1-D and a 2-D numerical models are developed. Assumed accidents in a typical segment of the middle route main canal of SNWT are studied by the models.A 1-D numerical model with high accuracy and stability is developed in which the hydrodynamic and water quality equations are solved based on Preissman and Pressman-Holly schemes respectively. Based on the maximum entropy principle, a formula is originally derived to describe the longitudinal velocity distribution in the cross-section of a trapezoidal open channel. From the formula an equation to predict the longitudinal dispersion coefficient is proposed based on Fischer’s triple integral relation for dispersion coefficient.A depth-averaged 2-D model is also developed. Employing a high-resolution scheme of flux difference splitting scheme (FDS) to deal with the advective terms of hydrodynamic and water quality equations, the model can handle various complicated flow fields and pollutant concentration distributions accruately. A novel upwind method by means of reconstructing vaiables in cells is designed to avoid handling the bed slope term directly. The boundary conditions can easily be specified. To model wetting and drying of 2-D shallow-water flows two novel methods called the submerged node method(SNM) and the conservational artificial porosity method are proposed. Hierarchy meshes and local time-step scheme are combined in the 2-D model to improve the model’s acurracy and efficiency. Several typical problems are employed to verify the 2-D model and excellent results indicate that the model can accurately and efficiently simulate discontinuous flows such as dambreak flood, transcritical flow, or discontinuous pollutant concentration distributions. Assumed water pollution accidents in steady flows in a typical segment of the middle route main canal of SNWT are simulated using the two models. It is found that either 1-D or 2-D model can predict mean velocity of polltant clouds. The 1-D model is pretty suitable for rapid simulation. However, because mixing process in near field is not taken in account, the skewness of section-averaged pollutant concentration distribution can not be reproduced, and the velocity of the point of maxium section-averaged concentration is misvalued by the 1-D model. The 1-D model will also overvalue the length of pollutant clouds. The 2-D model can not only discribe the longitudinal pollutant distributions more exactly but account for the difference of the pollutant distribution resulting from the differennce among tranverse positions where pollutants enter flows. The faster pollutants enter waters and smaller is the transverse mixing coefficient, the more superior is the 2-D model. From the above results the characteristics of flow fields and pollutant transport in unsteady flows with fixed water level before downstream gate during pollution accident treating processes are simulated by the 2-D model. The simulations demonstrate that the upstream controlling gate should be shut during a period longer than twice travelling the channel length at shallow-water wave speed, and not only the mean velocity but the distribution of pollutant clouds are changed during shutting controlling gates. The study carried out in this dissertation has set a concrete basis for analyzing characteristics of water pollution accidents in the middle route open channel of SNWT and pursuing treatment to the accidents.更多还原