Exit Conditions in Capillary Jets: A Two-Mode Spatial Approach*

Simple recipes are provided for predicting the breakup length of harmonically stimulated axisymmetric capillary jets as a function of the exit deformation and/or mean axial velocity perturbation.They are valid for arbitrary jet velocities thanks to the adoption of a spatial analysis. Among the spatial modes, only the two downstream capillary modes, dominant and subdominant, are sufficient to have good estimates of the linear evolution of the jet. Comparison with numerical determination of the breakup length shows a good agreement, provided that the exit perturbation is not too strong or its wavenumber not far from the one corresponding to the maximum of the Rayleigh curve. Including the subdominant mode extends the range of amplitudes for which the linear model gives accurate predictions. It has been generally assumed that the shortest intact length corresponds to highest growth rate. However, we show that the correlation is not strict because the amplitude of the dominant mode has a role in the breakup process and it depends on the stimulation frequency.