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.
Bibliographical noteFunding Information:
The authors would like to thank the National Science Foundation for support of this research under Grant No. CTS-0317429 as well as H. Oertel, Jr. and the University of Karlsruhe for providing support for the research visit of P.G. We have benefited considerably from interactions with our colleagues D. Forliti, J. V. R. Heberlein, E. Pfender, D. Outcalt, B. Tang, and C. Youakim.
Copyright 2019 Elsevier B.V., All rights reserved.
- Boundary layers
- Confined flow
- Flow instability
- Laminar flow
- Shear flow
- Spatiotemporal phenomena