The steady-state flow generated by a rotating bottom in a closed
cylindrical container
and the resulting vortex breakdown bubbles have been studied experimentally.
By
comparing the flow inside two different container geometries, one with
a rigid cover
and the other with a free surface, we examined the way in which the formation
and
structure of the breakdown bubbles depend on the surrounding flow. Details
of the
flow were visualized by means of the electrolytic precipitation technique,
whereas a
particle tracking technique was used to characterize the whole flow field.
We found that
the breakdown bubbles inside the container flow are in many ways similar
to those in
vortex tubes. First, the bubbles are open with in- and outflow and second,
their
structure is, like in the case of vortex breakdown in pipe flows, highly
axisymmetric on
the upstream side of the bubble and asymmetric on their rear side. However,
and
surprisingly, we observed bubbles which are open and stationary at the
same time. This
shows that open breakdown bubbles are not necessarily the result of periodic
oscillations of the recirculation zone. The asymmetry of the flow structure
is found to
be related to the existence of asymmetric flow separations on the container
wall. If the
angular velocity of the rotating bottom is increased the evolution of the
breakdown
bubbles is different in both configurations: in the rigid cover case the
breakdown
bubbles disappear but persist in the free surface case.
Dual vortex breakdown in a two-fluid confined flow
