双重障碍物对开闸式异重流运动特性的影响

Effects of Two Consecutive Obstacles on Gravity Currents Dynamics

摘要:

异重流现象广泛存在于自然环境和水利工程中。现实工况中，底床设置障碍物是抑制异重流侵袭的有效手段，因此研究障碍物对异重流运动特性的影响具有实际工程价值。采用开闸式异重流实验，对比不同障碍物间距及高度对异重流最大扩散高度、头部速度等方面的影响，从而得到双重障碍物最优工程布置方案；分析异重流在障碍物上下游区域的速度剖面及其越过障碍物时的涡度场；对比无障、单一障碍物、双重障碍物工况对异重流掺混系数的影响。结果表明，双重障碍物最优布置方案为第一障碍物高于第二障碍物，障碍物间距要尽可能大，但应保证异重流遇到第二障碍物之前未恢复典型头部形态；在双重障碍物上游区域，异重流速度剖面出现畸变，畸变范围大致与障碍物等高，主流速度衰减明显，在障碍物下游区域，完成形态重塑的异重流，其速度剖面均存在明显的壁面区和射流区；相对于无障和单一障碍物工况，流经双重障碍物的异重流厚度明显变小；越障前后，异重流掺混系数呈现“M”型分布，越过第二障碍物后异重流的掺混速率明显大于第一障碍物。结果可为防治异重流灾害及保证水利工程安全等领域提供参考。

关键词:异重流;双重障碍物;工程最优布置;掺混效应;速度剖面

Abstract:

The phenomenon of gravity current exists widely in natural environments and hydraulic engineering. In practical conditions, the obstacle is an effective mean to prevent the invasion of gravity current, so it is valuable and worthwhile to study the effects of obstacles on the dynamic characteristics of gravity current. In this paper, the lock-exchange experiments were carried out to compare the maximum diffusion height and head velocity of gravity current with different obstacle conditions, and the optimal obstacle layout for the engineering application can be therefore obtained. In addition, the velocity profiles in the upstream and downstream of the obstacles, the vorticity field of gravity current over the obstacles, and changes in the entrainment coefficient of gravity current along the channel were analyzed. The results show that for the optimal engineering layout scheme of consecutive obstacles is as follows: the first obstacle needs to be higher than the second obstacle, and the distance between two obstacles should be as large as possible, but the typical head shape of gravity current should not be restored before it meet the second obstacle. In the upstream of the first obstacle, the velocity profiles is disturbed and decreased, and the disturbed range is approximately equal to the height of the obstacle. In the downstream of the second obstacle, there are obvious wall region and jet region in the velocity profiles of the gravity current. Compared with the condition of no obstacle and single obstacle, the thickness of the gravity current flowing through the two obstacles decreases obviously. When the gravity current flows over obstacles, the current entrainment coefficient presents an "M" type distribution. The entrainment coefficient of gravity current over the second obstacle is greater than that in the first obstacle. The conclusion of the study can provide scientific basis for mitigating the hazards induced by gravity current and ensuring the safety of hydraulic infrastructures.

Key words:gravity currents;two consecutive obstacles;optimal engineering layout;entrainment effect;velocity profile