On the mechanism of the influence of medium density on the supersonic core length of an underexpanded jet

Exhaustion of a supersonic underexpanded air jet into air–helium and air–sulfur hexafluoride mixtures is studied experimentally. The air jet escapes from a convergent circular nozzle in the range of moderate Reynolds numbers covering the laminar and transitional regimes of the jet flow. The experiments are performed with “cold” and heated jets in a low-pressure jet setup, which allows the Reynolds number of the jet exhaustion from the nozzle, the jet pressure ratio, and the density of the gas surrounding the jet to be maintained independently. A Pitot tube is used to measure the supersonic core length of the air jet as a function of the density of the ambient medium whereto the jet exhausts. In addition, the spatial distributions of the amplitude–frequency characteristics of acoustic oscillations generated by the jet in the ambient space are measured by a moving sensor of pressure fluctuations. The effect of the ratio of the underexpanded air jet density to the ambient medium density on the supersonic core length is demonstrated. It is shown that the mechanism of influence of the ambient medium density is associated with a change in the conditions for the global instability of the underexpanded air jet and, in particular, the conditions for generation of the screech tone.