Global stability analysis of underexpanded screeching jets

This article deals with a global stability analysis of the screech phenomenon. We have shown that a laminar underexpanded supersonic cold jet can exhibit globally unstable modes. A closer look at the structure of these modes shows that they present upstream propagating waves, which is known to be a major component of the screech phenomenon. Furthermore, we find a good agreement between the frequency of the eigenmodes and existing empirical formulas for the prediction of screech frequency. We have then studied the influence of two key parameters on the linear stability of the flow, the jet pressure ratio (JPR) and the nozzle lip thickness, which are known to play an important role in the screech phenomenon. Finally, a careful study of the structure of the unstable modes shows that the upstream propagating acoustic waves of those modes are generated by supersonic phase velocity disturbances, a well-known sound generation mechanism.