Vortex Formation Process of a Starting Square Jet

This paper presents results of an experimental study on starting square jets at Reynolds numbers ranged from 2000 to 5000 (based on the jet diameter and the bulk mean velocity on the jet exit plane) using planar laser induced fluorescence for flow visualization and particle image velocimetry for velocity measurements, respectively. Similar to starting a circular jet at Reynolds numbers below 3000, vortex leapfrogging was found to appear, whereas pinch-off for the leading vortex ring was observed at higher Reynolds numbers. The "imperfect" leapfrogging between the third and the fourth vortex rings promoted growth of instability, which had led to the transition to turbulence for the entire jet. The maximum circulation that a vortex ring can attain during its formation is reached at the nondimensional time (t* = tW 0 /D e ) within the range of 1.6 to 2.4. The pinch-off of the leading vortex ring appeared shortly after the jet initiation for Reynolds numbers above 3000. Axis switching was found to play an important role for the vortex development in starting square jets. The interaction between the axis-switched vortex ring and the square-shaped jet stem facilitated the generation of contrarotating streamwise vortices at the corners of the jet stem. They were actually rotating in the opposite sense as those responsible for axis switching, and their existence had facilitated the thinning of the jet stem, causing a fast pinch-off for the leading vortex ring.