Global instabilities in countercurrent jets

Self-excited instabilities in supersonic, countercurrent (CC) jets are demonstrated based on numerical simulations, and it is argued that they can be used to explain unresolved discrepancies between laboratory and theoretical analysis. Self-excitation is based on upstream feedback mechanisms acting on the subsonic outer jet regions, and associated with underexpanded initial conditions or partial confinement effects introduced by a collar surrounding the CC jet shear layer. Recognition of these instabilities provides new insights on the role played by the collar in the laboratory CC jet systems, suggesting that practical approaches to the active control of CC jets might be based on suitable direct excitation of the shear layer within the region of influence of the collar.