Abstract
An underexpanded supersonic circular jet was studied experimentally in a pressure ratio range of 2–15, yielding an equivalent Mach number range of 1.1–2.4 downstream of the nozzle. Spark Schlieren photography and near-field pressure measurements showed large changes in the jet structure for different Mach numbers. For nearly sonic exit velocity, the jet had a dominant symmetric structure and spreading rate similar to subsonic conditions. For an exit velocity equivalent to M>1.2, the jet switched mode to a helical structure with an accompanying substantial increase in the spreading rate. A transitional region was determined for 1.12<M<1.18, in which the jet switched modes intermittently from axisymmetric to helical. For Mach numbers higher than 1.6, another structural change was observed, related to the emergence of a normal shock in the first shock cell. The structural changes affected the near-field pressure fluctuations of the jet. The jet with the helical structure had a strong pressure field component in the upstream direction that could be related to the higher spreading rate of the jet.
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