In an underground tunnel with longitudinal ventilation, the stratification of the buoyant contaminants is crucial for the safety of the workers and passengers. A series of experiments are conducted in a water tank to investigate the density stratification in the downstream of the buoyancy source. Highly lazy plumes are produced by releasing coloured brine solution into a longitudinally ventilated tunnel which is placed in a large reservoir filled with fresh water. The transient distribution of the reduced gravity is measured using a light attenuation technique. The results suggest that the vertical distribution of the reduced gravity in the buoyant layer is approximately linear. The gradient Richardson number demonstrates that the downstream stratification can be very unstable under the effect of forced longitudinal ventilation. The difference between the reduced gravity at the ceiling and that at the floor $\Delta g^{\prime }$, the average reduced gravity $\overline{g^{\prime }}$ and the thickness of the buoyant layer $h$ are chosen to quantify the stratification. With the increase in the longitudinal ventilation velocity, both $\Delta g^{\prime }$ and $\overline{g^{\prime }}$ decrease but $h$ tends to increase. As the source buoyancy flux increases, all the three parameters become larger. Based on the experimental data and dimensional analyses, quantitative models are proposed to estimate the above three parameters. It is hoped that the models may provide useful references for the downstream distribution of the buoyant contaminants produced by highly lazy plumes in a longitudinally ventilated tunnel.

在具有纵向通风的地下隧道中，漂浮污染物的分层对于工人和乘客的安全至关重要。在水箱中进行了一系列实验，以研究浮力源下游的密度分层。高度惰性的羽流是通过将有色盐溶液释放到纵向通风的通道中而产生的，该通道放置在装满淡水的大型水库中。重力降低的瞬态分布是使用光衰减技术测量的。结果表明，浮力层中重力降低的垂直分布近似线性。梯度Richardson数表明，在强制纵向通风的作用下，下游分层可能非常不稳定。$\Delta G^{\prime }$，平均重力降低 $\stackrel{〜}{G^{\prime }}$ 和浮力层的厚度 $H$选择以量化分层。随着纵向通风速度的增加，两者$\Delta G^{\prime }$ 和 $\stackrel{〜}{G^{\prime }}$ 减少但 $H$倾向于增加。随着源浮力通量的增加，所有三个参数都变大。基于实验数据和尺寸分析，提出了定量模型来估计上述三个参数。希望这些模型可以为在纵向通风隧道中由高度惰性羽流产生的浮性污染物的下游分布提供有用的参考。