A jet entering a counterflow improves the dilution of contaminant discharged from a chemical factory through pipe. The velocity ratio of jet-to-counterflow and the offset distance of the jet from the boundary significantly affect the dilution efficiency of sewage. A free and an offset round jet, in a counterflow, with ranges of jet-to-counterflow velocity ratios and offset ratios, were experimentally investigated using particle image velocimetry. The time-averaged features, including the mean velocity, penetration length, half-width, and jet centerline were studied. There is a large recirculation region on each side of a free jet in a counterflow. For an offset jet in a counterflow, there is only one large recirculation region on the upper side when the offset ratio is small, due to the wall hinders the entrainment of the counterflow into the jet. As the offset ratio increased, the gap between the wall and jet increased, which resulted in more counterflow entrainment into the jet and the large recirculation region gradually appeared. Accordingly, the penetration length became shorter, as the offset ratio increased for the same jet-to-counterflow ratio. The maximum axial velocity of a free jet in a counterflow decayed more rapidly than that in a quiescent ambient or an offset jet in a counterflow. The profile of excess velocity in the wall normal direction exhibited self-similarity within one half-width of the excess velocity in the established flow region. The half-width of the excess velocity and zero-velocity increased with downstream distance, for a free or an offset jet entering a counterflow. The mixing efficiency of an offset jet entering a counterflow was verified through the distribution of velocity and spreading rate at different streamwise stations.

中文翻译：

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不同偏置比的圆形偏置射流在逆流中的速度特性

进入逆流的射流改善了从化工厂通过管道排放的污染物的稀释。射流与逆流的速度比和射流与边界的偏移距离显着影响污水的稀释效率。使用粒子图像测速法对具有射流与逆流速度比和偏移比范围的逆流中的自由和偏移圆形射流进行了实验研究。研究了时间平均特征，包括平均速度、穿透长度、半宽和射流中心线。在逆流中自由射流的每一侧都有一个大的再循环区域。对于逆流中的偏移射流，当偏移比很小时，由于壁阻碍了逆流进入射流，因此在上侧只有一个大的再循环区域。随着偏移比的增加，壁与射流之间的间隙增加，导致更多的逆流夹带进入射流，并逐渐出现大的再循环区域。因此，穿透长度变得更短，因为对于相同的射流与逆流比的偏移比增加。逆流中自由射流的最大轴向速度比静止环境或逆流中的偏移射流衰减得更快。壁法线方向上的超速分布在已建立的流动区域中超速的一半宽度内表现出自相似性。对于进入逆流的自由或偏移射流，超速和零速度的半宽度随着下游距离的增加而增加。