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