The boundary between absolute and convective (linear) instability of two-dimensional inertial jets and wakes is determined as a function of the ratio of jet/wake to ambient density, as well as the ratio of mixing layer thickness to jet/wake width, the velocity ratio, and the Reynolds number. For this, a viscous, heat-conducting ideal gas is taken as the fluid, a zero Mach number, no buoyancy and a parallel basic flow are assumed, and the density variation is achieved by specifying a mean temperature profile similar to the velocity profile. Considering both ‘‘varicose’’ and ‘‘sinuous’’ disturbances, results are obtained for the inviscid top-hat jet/wake bounded by two vortex sheets, the inviscid jet with continuous velocity and density profiles, and the viscous wake. For the latter, both constant and temperature-dependent viscosity are investigated. In all the cases it is found that low density of the high-speed fluid promotes absolute instability, while low density of the low-speed fluid has the opposite effect. By comparison with experiments it is shown that the present results provide useful information about the parameter range in which flow oscillations are self-excited.

A two-dimensional hot-air jet is investigated experimentally in the transitional regime. The density effect on the near-field behaviour of the initially laminar jet is explored by flow visualization, mean flow measurements and spectral analysis of fluctuating data. It is shown that the broadband amplitude spectra which characterize cold jets become line-dominated for hot jets when the ratio of the jet-exit to the ambient density is below approximately 0.9. Below this critical density ratio the oscillations in the hot jet are shown to be self-excited. That is, the onset of the global oscillations is identified as a Hopf bifurcation and the critical parameter is determined from amplitude spectra and autobicoherence, with the latter proving to be more reliable. Furthermore, the development of three-dimensional structures, which contribute to the jet spreading, is revealed by flow visualization. It is found that, for the parameters investigated, the spreading of the two-dimensional hot jet is not as spectacular as in the axisymmetry case.

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