挡水物附近污染带的数值模拟研究

拟建崇海公路大桥位于长江口北支上段，东距北支入海口约70km，该河段河床宽浅，沙洲众多，河势多变，长江口潮型属浅海不规则半日潮，桥位附近青龙港平均涨潮历时3．06h，落潮历时9．19h；涨潮平均流速一般大于落潮平均流速。为了分析公路大桥建设对长江海门段流场和浓度场的影响程度和范围，并考虑到该水域水深较浅。低潮时大量露滩的特点，采用基于无结构网格的具有普遍适用性、守恒性、逆风性等优点的有限体积Osher格式建立二维水量、水质数学模型。在桥墩处理上，改变传统的用网格贴合挡水物的方法，直接将桥墩作为陆地边界生成无结构网络。由建桥前后的流速对比图可以发现靠近桥墩的水域流速减小，而墩间流速增大，这与实际情境十分吻合，说明将桥墩作为陆地边界的处理办法是可行的，对比建桥前后的污染带，发现建桥后桥位下游污染带有所减小，但不明显，这与建桥后流场的变化有很大关系。建桥后桥墩起一定阻水作用，涨落潮时桥位下游流速均存在 减小的趋势，而且岸边正好位于桥墩的密集区域，流速减小更为明显，这一区域流速值的减小范围在0.28m/s以内。由于对时间项的离散采用显格式，受计算稳定性的限制，时间步长必须取得很小，从而需要消耗大量计算时间，这是今后模型值得改进的地方。

Numerical Simulation Study on Pollution Area Close-by Impediment

Chonghai road bridge will be constructed at the northern branch of Yangtze estuary, it is 70 km to the entrance of ocean of northern branch and its riverbed is wide and shallow with many cays, and river-terrain is changeable. The tide model of Yangtze estuary is irregular half-day tide. The average duration of rising tide is 3.0 hours and that of ebb tide is 9.19 hours; and the average flow velocity of rising tide is generally bigger than that of ebb tide. For analyzing the impact degree and range on flow and concentration field, due to water depth near the project is shallow and many cays emerge periodly, the two-dimension water-quantity and water-quality computation model based on unstructured mesh was set up by using Osher's scheme of FVM which have advantages of universality, conservativity and upwindness. The piers are processed as land boundary directly, comparing to the method that mesh is used to fit impediment. According to comparison of flow field before and after bridge building, it found that velocities close by piers were decreased and velocities between piers were increased. It is fit to practical condition, workable, and it is feasible that piers are processed as land boundary. Comparing the pollution area before and after bridge building, it found that pollution area at downstream of bridge site has decreased, but not obviously, it is related to the change of flow field after bridge building directly. As the result of water will be blocked by piers after bridge building, the velocities at downstream of bridge site have decreased tendency and more obviously near the river bank, the range of velocities decreased were less than 0.28m/s. Owing to explicit scheme is used, time step must small to assure the stability of computation, and this is the point needs to be further improved.