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.

实验研究了超音速膨胀不足的空气射流在空气-氦气和空气-六氟化硫混合物中的排放。空气射流从收敛的圆形喷嘴中逸出，射流的射流范围为中等的雷诺数，覆盖射流的层流状态和过渡状态。实验是在低压射流装置中对“冷”射流和加热射流进行的，这可以独立保持喷嘴的射流雷诺数，射流压力比和围绕射流的气体密度。皮托管用于测量空气射流的超音速核心长度，该长度是射流所排放的周围介质的密度的函数。此外，射流在周围空间中产生的声振荡的振幅-频率特性的空间分布是通过压力波动的移动传感器来测量的。证明了膨胀不足的空气喷射密度与周围介质密度之比对超音速磁芯长度的影响。结果表明，环境介质密度的影响机理与空气不足膨胀的整体不稳定性条件的变化有关，尤其是与产生尖叫声的条件有关。