When a fluid pours from an outlet into the air, it forms a
free-falling stable jet that accelerates, stretches, and narrows
under the influence of gravity.1 The jet flow behavior is of
considerable interest in fluid mechanics and engineering
practice and has found a wide variety of applications such as
the sol-gel process in the production of small fluid particles,
the spinning processes in fabrication of polymer fibers, and
biomedical devices. Recently, a liquid microjet has been produced2
that can be used in spacecraft propulsion, fuel injection,
mass spectroscopy, and ink-jet printing.
The key challenge when analyzing a jet flow is to find the
jet shape function (JSF);3–8 that is, the relationship between
the jet radius r and the axial distance z from the exit orifice.
For laminar flow of an isothermal liquid with a density q,
issuing from a circular orifice of radius R0 with exit velocity
t0 in a gravitational field g, dimensional analysis predicts the
following functional dependence for the JSF: