Including acoustic modes in the vortex-sheet eigenbasis of a jet.

Vortex-sheet (V-S) models of jets are widely used to describe the dynamics of modes, such as the Kelvin-Helmholtz instability and guided acoustic waves. However, the absence of the free-stream acoustic modes in the V-S eigenspectrum is seldom pointed out in the literature. This family of modes is important if, for example, one is interested into the problems of sound emission or flow-acoustic interactions.