Mode switching in supersonic circular jets

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