Instability of screech switch for underexpanded free jet

For underexpanded supersonic jets three main components of noise radiation are turbulent mixing noise, broadband shock-associated and discrete tone screech. The screech frequencies show various multimodes. The process of mode switching is instability, while flow visualization using Schlieren photography show the unstable flow structure. The mode switching represents the sound pressure level at one mode frequencies decay and another mode stronger, being main mode. Accompanying discrete tone occurs until the end of mode switch. The characteristic of the screech frequencies are related to the instability of the flow and vortex. Linear instability wave theory indicates that the maximum growth rate of instability wave of the two azimuthal modes agrees well with measurement.