This study aimed to investigate the formation of a plughole vortex during water drainage from a stationary rectangular tank. Significant observations regarding the vortex behavior and its impact on the drainage process have been made. As the water drains, a surface dip is formed on the free surface resulting in a swirling motion. As the dip approaches the inlet, the shape gradually transforms from an inverted cone to a droplet like air bubble and finally take an appearance of a donut-shaped bubble ring, establishing bubbly flow within the pipe. With decreasing water head, both the bubble and ring sizes increased, transitioning from bubbly flow to slug flow within the pipe. The slug flow further transformed into annular flow. Standard computational fluid dynamics (CFD) software is employed to utilize a three-dimensional numerical approach for the analysis and validation is done using obtained experimental results. The numerical case set up was done using VOF method with Realizable k-ε model.
The comparison was made for the varied spacing (pitch) between drainpipes and varied diameters of the two outlets. The study revealed that the non-interacting vortices are formed when the drainpipes were far apart, while interacting vortices are formed when drainpipes were close. Through the combination of experimental observations and numerical simulations, this study provides comprehensive insights into plughole vortex dynamics, including droplet-to-ring transformation and flow patterns within the drainpipe.