Influence of Liquid Discharge Rate on the Genesis of Bathtub Vortices with Air-Entrained Structures During Draining of a Water Pool Through Multiple Outlets

This paper presents the experimental investigations on free surface vortex and associated air entrainment during the drainage of water using two outlets from a water pool. The spacing (pitch) between the two outlets and the ratio of liquid discharge rates between the two are considered the controlling parameters in the present experiments. When the outlets are placed close to each other, a single surface dip has originated at the free surface and further extended into two branches once it reached close to the outlets. This resulted in asymmetric entry of entrained air structures into the drainpipes. Upon maintaining the sufficiently large spacing (pitch) between the pipes, two separate non-interacting vortices are observed with symmetric air entrainment into the drainpipes. The vortex structures obtained are almost the mirror image of each other when the liquid discharge rates are kept same in both the pipes. The entrainment of air resulted in transition from bubbly flow to annular flow at high liquid discharge rates where the air has entrained directly in the form of annular core at low liquid discharge rates.