Flapping instability of a liquid jet

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.