Study of air-core vortical flow structure induced by a plughole vortex

Abstract

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 0.45–0.6, 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.