Three regimes of air-core vortex formation are demonstrated to exist for the free draining of liquids containing angular momentum. These comprise of a rapidly intensifying air-core vortex, a quasisteady vortex and a decaying vortex. The regimes are shown to be strongly influenced by the size of the drain-port used in relation to the tank diameter. The influence of Froude number on air-core vortex is discussed.