Flow visualisation experiments on free draining of a rotating column of liquid using nets and tufts

The characteristics of the air-core vortex formed during free draining of liquids from tanks strongly depend on the size and shape of the drain ports used. In order to understand the role of the drain-port in influencing the air-core vortex, the flow currents induced while draining a rotating column of liquid with different drain-port diameters is assessed. The flow currents are monitored using nets and tufts. It is shown that for small drain ports, for which an air-core vortex is spontaneously formed, the rotational flow currents are concentrated along the central core. With larger drain ports, the rotational flow currents are smaller and are diffused over larger radii leading to weak air-core vortex.