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