ABSOLUTE AND CONVECTIVE INSTABILITY OF AXISYMMETRICAL JETS WITH EXTERNAL FLOW

The stability of an axisymmetric jet was examined in the presence of e xternal co-flow and counterflow. Spatio-temporal theory was used to di stinguish regions of absolute and convective instability in a paramete r space including the velocity ratio, density ratio, Mach number, and the shear layer thickness. The absolute-convective transition was iden tified for two distinct axisymmetric modes. One of these modes became absolutely unstable in the presence of ambient co-flow while the other mode required external counterflow to admit an absolutely unstable so lution. In general, the former mode was most unstable in low density j ets, while the latter became more unstable as the jet density was incr eased relative to the surrounding fluid. In the range of parameters st udied, both modes became increasingly unstable with decreasing jet den sity and for lower Mach numbers. The results of the spatiotemporal the ory are also compared to globally unstable modes identified in laborat ory jets.