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

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 approximately 10 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.