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The flow field of an impulsively started round, confined nitrogen jet was investigated using combined high speed schlieren imaging and particle velocimetry (PIV) measurements. PIV measurements were carried out at five different, normalized times (55 ≤t*≤ 392) relative to jet intrusion into a constant volume chamber. Between 100 <t*< 250, the NPR linearly increased to that for a moderately underexpanded jet (NPR ≈ 3.5). Distributions of the mean flow and Reynolds normal and shear stresses revealed two different stages in jet development. In stage I (t* = 55–103), prior to clear shock cell appearance, the jet was characterized by a leading, toroidal vortex whose induced recirculatory motion inhibited the growth of the trailing jet's shear layer instabilities and radial spreading. In stage II (t* = 196 and 392), the jet became moderately underexpanded (NPR ≥ 2) and close to the nozzle exit, flow characteristics resembled those of a “co-annular” jet. The co-annular region did not extend beyond 15 D. An analysis of instantaneous vortex numbers and strengths further supported the two identified stages in jet development and their connection to shear layer instability growth. Based on the distributions of mean flow and Reynolds stresses, it was shown that the static pressure gradient along the jet's centerline is mainly governed by the dynamic pressure gradient. Gradients of the Reynolds normal and shear stresses play a minor role. Important for gaseous fuel injection at high injection pressures, results point at limited mixing during stage I and enhanced mixing during stage II.

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