  A three-dimensional round liquid jet within a low-speed coaxial gas flow is
numerically simulated and explained via vortex dynamics ($\lambda_2$ method).
The instabilities on the liquid-gas interface reflect well the vortex
interactions around the interface. Certain key features are identified for the
first time. Two types of surface deformations are distinguished, which are
separated by a large indentation on the jet stem: First, those near the jet
start-up cap are encapsulated inside the recirculation zone behind the cap.
These deformations are directly related to the dynamics of the growing cap and
well explained by the vortices generated there. Second, deformations occurring
farther upstream of the cap are mainly driven by the Kelvin-Helmholtz (KH)
instability at the interface. Three-dimensional deformations occur in the
vortex structures first, and the initially axisymmetric KH vortices deform and
lead to several liquid lobes, which stretch first as thinning sheets and then
either continue stretching directly into elongated ligaments - at lower
relative velocity - or perforate to create liquid bridges and holes - at higher
relative velocity. The different scenarios depend on Weber and Reynolds numbers
based on the relative gas-liquid velocity as was found in the temporal studies.
The deformations in the upstream region are well portrayed in a frame moving
with the convective velocity of the liquid jet. The usefulness of the temporal
analyses are now established.
