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

摘要

Experiments conducted in helium axisymmetric jets with an annular coflowing air stream yield critical values of the velocity ratio U-2/U-1 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 U-2/U-1 and D/theta. 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. (c) 2007 American Institute of Physics.