Linear instability waves in supersonic jets confined in circular and non-circular ducts

The linear instability of supersonic jets confined in circular and non-circular ducts is investigated both analytically and numerically. In the case of the non-circular duct, the numerical solution is based on the boundary element method. It is shown that the presence of an outer wall introduces additional instability modes. A highly supersonic unconfined jet possesses many modes of instability. These include the Kelvin-Helmholtz instability and supersonic instabilities. The modifications to these instabilities by a coflowing stream and an outer wall are examined. For the case of a circular jet in a circular duct, both a vortex sheet model and a model that includes the effect of a jinite thickness shear layer are considered. The results of these calculations are compared with those for unconfined supersonic jets with an external flow. Finally, the effects of changes in the duct geometry on the instability modes are examined.