It has recently been shown that the instability characteristics of planar jets and wakes change when the flows are confined between two flat plates. This is due to constructive interaction between modes with zero group velocity in the inner and outer flows. In this theoretical study, a linear spatio-temporal analysis is performed on unconfined and confined round jets and wakes in order to discover whether the same effect is observed. There are several similarities between the planar case and the round case as well as some significant differences. Nevertheless, the effect of confinement on round flows is found to be very similar to that on planar flows and to act via the same physical mechanism. This paper examines density ratios from 0.001 to 1000 and has important implications for the design of fuel injectors, which often employ confined shear flows at high Reynolds numbers and large density ratios to generate strong mixing in combustion chambers.
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