In this paper, air entrainment by a liquid jet is studied.
The size of bubbles entrained
by jets plunging into a liquid can be consistently decreased
to the 50–100 μm range,
and their number increased in a highly controllable fashion, by surrounding
a mm-size
jet by a hollow cap with a slightly larger inner diameter. When the right
amount
of air is supplied to the cap, small air bubbles detach from a steady annular
cavity
that forms around the jet and are entrained into the liquid. The fluid
mechanical
principles underlying this interesting and useful effect are investigated
experimentally
and theoretically in this paper. It is shown that a key aspect of the process
is the
jet surface roughness, which is studied quantitatively and explained in
terms of the
boundary layer instability inside the nozzle. The maximum bubble size is
found to
be nearly equal to one quarter of the wavelength of the jet surface disturbances,
consistent with a breakup process of relatively large air pockets around
the jet,
as suggested by close-up pictures. The average bubble size downstream of
the cap
increases proportionally to the air to water flow ratio. Boundary integral
simulations
of the air pocket formation are carried out. The results are found to be
useful
in explaining important characteristics of the experiment such as the threshold
for
entrainment and cavity size.