  This paper describes a study of the generation of a plughole vortex and its
consequences in a drainpipe during drainage of water from a stationary
rectangular tank. The critical and minimum depths of water above the inlet of
the drainpipe, where a surface dip starts to develop for drainpipes of various
diameters, were examined parametrically. This study explored the following
naturally occurring phenomena arising from a plughole vortex. (i) A plughole
vortex initially causes a surface dip to develop towards the inlet of the
drainpipe and as the surface dip approaches the inlet of the drainpipe it
creates a droplet-shaped air bubble. (ii) A unique bubble transformation, i.e.,
from a droplet-shaped to a donut-shaped bubble ring, occurs just after the
separation of the droplet-shaped air bubble from the surface dip. (iii) The
donut-shaped bubble ring flows with the drain water and initially causes bubbly
flow in the drainpipe. (iv) As the water head above the inlet of the drainpipe
decreases, the droplet-shaped bubble size increases, and consequently, the
bubble ring size increases and causes slug flow in the drainpipe. (v) As the
slugs combine, the flow of the draining water eventually becomes annular flow
in the drainpipe. Sounds, such as that of instantaneous fizz and bubble sink
draining, were observed to be produced as a result of the bubble formation
process. Temporal changes in the shape and size of the air bubbles were
studied. Within the range of 0.45 to 0.6, the ratio of the bubble diameter to
the bubble length was found to be linearly proportional to the ratio of the
water depth to the diameter of the drainpipe. Several drainage cases were
simulated numerically to observe the physics of these naturally occurring
phenomena. The shapes and sizes of the vortices induced by plugholes have been
visualised and analysed using the vortex core method.
