Suppression of global modes in low-density axisymmetric jets using coflow

Experiments conducted in helium axisymmetric jets with an annular coflowing air stream yield critical values of the velocity ratio U₂/U₁ needed to suppress global instability inherent in these low-density flows. Global mode suppression was achieved for coflowing velocities less than approximately 20% of the jet centerline velocity, though the critical velocity ratio displayed a nonmonotonic relationship with the initial shear layer momentum thickness. The experiments are supported by spatio-temporal inviscid stability theory, where the convective-absolute transition was tracked in an operating domain including U₂/U₁ and D/θ. For initially thick shear layers, the experimental observations are in good agreement with linear theory, but deviate considerably as the separating shear layer thickness is reduced.