We study the destabilization of a round liquid jet by a fast annular gas stream. We measure the frequency of the shear instability waves for several geometries and air/water velocities. We then carry out a linear stability analysis, and show that there are three competing mechanisms for the destabilization: a convective instability, an absolute instability driven by surface tension, and an absolute instability driven by confinement. We compare the predictions of this analysis with experimental results, and propose scaling laws for wave frequency in each regime. We finally introduce criteria to predict the boundaries between these three regimes.

We measure experimentally the frequency of the large-scale instability developing on a liquid jet incompletely atomized by a parallel fast gas stream. We demonstrate that this "flapping instability" can be triggered by different mechanisms: in a first regime it is synchronized with the shear instability developing upstream, provided the wavelength of this shear instability is larger than the liquid jet diameter HL. When the shear instability exhibits wavelengths shorter than HL, a second regime is observed where the flapping instability becomes independent of the gas stream velocity. This second regime is characterized by a constant Strouhal number, provided the Froude number of the jet is correctly taken into account.