An air core is often formed during liquid draining from cylindrical tanks. An understanding of the mechanism behind its formation and the parameters that accentuate its growth is central to the development of an air-core vortex suppression strategy. In the present study, liquid draining from a cylindrical tank is investigated with the aid of computational fluid dynamics tools. A qualitative and a quantitative comparison of the temporal variation of critical height against available experiments is reported. A systematic investigation has revealed that, drain port shape, size, pressurization, initial rotation, etc, play a vital role in the formation of an air core vortex and its growth. These variables were also observed to influence critical height and total drain time, both of which are of engineering interest. Towards the development of a gas-core suppression strategy, the circular drain port is modified to either a stepped or a bell mouth shape. Although the new drain port shapes have delayed the gas-core from entering the drain-port, they were found to be only marginally advantageous. © 2014 The Japan Society of Fluid Mechanics and IOP Publishing Ltd.