Axial stability of Taylor bubbles

Long gas bubbles rising in a vertical tube are observed to lose axial symmetry and become unstable in a downward liquid flow. In this paper an approximate linear stability analysis of this phenomenon is presented. It is found that, under the combined effect of gravity and the pressure gradient which drives the liquid flow, the relative velocity between the bubble and the liquid decreases with increasing downflow, which diminishes the stabilizing effect of convection. The decrease of the relative velocity is accompanied by a flattening of the bubble nose, which also has a destabilizing effect by strengthening the Rayleigh-Taylor instability at the bubble nose.