OSCILLATIONS IN THE NEAR-FIELD OF A HEATED 2-DIMENSIONAL JET

A two-dimensional hot-air jet is investigated experimentally in the tr ansition regime. The density effect on the near-field behaviour of the initially laminar jet is explored by flow visualization, mean flow me asurements and spectral analysis of fluctuating data. It is shown that the broadband amplitude spectra which characterize cold jets become l ine-dominated for hot jets when the ratio of the jet-exit to the ambie nt density is below approximately 0.9. Below this critical density rat io 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 bifur cation and the critical parameter is determined from amplitude spectra and autobicoherence, with the latter proving to be more reliable. Fur thermore, the development of three-dimensional structures, which contr ibute to the jet spreading, is revealed by flow visualization. It is f ound that, for the parameters investigated, the spreading of the two-d imensional hot jet is not as spectacular as in the axisymmetry case.