明渠紊流中涡结构的运动规律

Experimental study on the movement of vortices in turbulent open-channel flows

明渠紊流中随水流迁移的涡结构在传质过程中具有重要作用。为观测涡结构在整个生存期内的运动过程,将两套高频粒子图像测速系统的测量范围沿水流方向拼接,实现流场的大范围测量。将上述方法用于测量摩阻雷诺数为490~870的明渠紊流的纵垂面流场,并结合涡结构跟踪算法,得到了涡的生存时间、运动速度、位移及强度变化等参数。分析表明,涡结构的水平运动与水流同步,但整体有向水面抬升的趋势,在低雷诺数条件下,最大生存时间约10倍涡周转时间,最大垂向位移约0.4倍水深;随着雷诺数的增大,涡结构的量纲一运动速度未发生明显变化,但极限强度和强度变化速率均增大,且极限强度的增加幅度相对较小,使得涡结构的生存时间缩短,纵、垂向位移相应减小。研究结果揭示了生存时间是影响涡结构运动规律的关键变量,初步得到了涡结构运动规律随水流条件的变化趋势。

Vortices convecting with the local flow play an important role in mass transfer in a turbulent open-channel flow. To capture the movements of vortices throughout their life spans,two time-resolved particle image velocimetry systems are placed side by side in the streamwise direction to achieve a wider field of view. This experimental setup is used to measure the two-dimensional velocity fields in the streamwise—wall-normal plane of uniform open-channel flows with friction Reynolds number ranging from 490 to 870. By tracking the spanwise vortices in the measured velocity fields,their lifetime,displacements,convection velocities,maximum strengths,and rates of change in strength are obtained. The vortices are found to lift away from the wall while convecting with the local flow in the streamwise direction. For the case of the lowest Reynolds number,the maximum lifetime and vertical displacement are approximately10 eddy turnover times and 0. 4 water depth,respectively. As the Reynolds number increases,the maximum strength of the vortices increases at a smaller speed than the rate of change in strength,causing a shortening of their lifetime.Because the dimensionless convection velocities remain unchanged in magnitude,the vortex displacements shorten accordingly. The investigation reveals the lifetime to be the critical parameter for characterizing the movement of vortices and yields a preliminary estimate of the trend of variation in the movement of vortices under different flow conditions.