Vortex formation process in gravity-driven starting jets

Experiments were conducted to investigate the vortex formation process in gravity-driven starting jets at three Reynolds numbers (\({Re = D\overline{W} _{0} /\nu = 2,358,}\) 3,528 and 4,716, where D is the nozzle diameter, \({\overline{W} _{0}}\) the average discharging velocity and ν the kinematic viscosity). Planar laser induced fluorescence (PLIF) was used to visualize the flow while particle image velocimetry (PIV) was used to quantify the 2D velocity and vorticity fields. Vortex leapfrogging was found to appear at Re = 2,358, while pinch-off for the leading vortex was observed in the other two cases. Conditions for the vortex pinch-off appeared to be different from those found in the piston-driven starting jets. Although the leading vortex attained the maximum circulation and energy level very quickly after the jet was initiated, its detachment from the main stem may not be achieved necessarily. Attempt had been made to elaborate the different flow characteristics for respective cases from the related distribution of circulation and energy level via the measured velocity field by PIV.