This paper presents the results of an experimental study on starting square jets at three Reynolds numbers (R_e = 2358, 3528, 4716) utilizing Planar Laser Induced Fluorescence (PLIF). Starting circular jets under the same initial R_e were also investigated as a basis for comparison. Observations showed that for both circular and square jets, the rate of penetration is almost constant within the first four diameters from the exit plane. Beyond that, the rate reaches an asymptotic behavior that is inversely proportional to the square root of time (t~(0.5)). The expansion rate of the head vortex core is proportional to t~(1.5). It was also found that the jet front travels slower in square jets. The interaction and deformation between the spanwise and streamwise vortices in square jets lead to an enhanced entrainment. The phenomena of axis-switching, vortices leapfrogging and coalescence were observed in the experiments and their formation mechanisms are discussed in details.
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