Numerical studies on air-core vortex formation during draining of liquids from tanks

The gas-liquid interface dips during the draining of a liquid through a discharge port of a vessel or a tank. The dip develops into a gas-core vortex which subsequently enters the discharge port. This entry can be either gradual or sudden, depending on the intensification of the rotational flow currents during the draining process. The extension of the gas-core into the drain port reduces the flow area and consequently the flow rate. In liquid propellant rocket motors, this phenomenon can have adverse effect on the performance as well as lead to under utilisation of the propellant. In this paper, the authors have tried to find the reason why such an air-core vortex develops in the first place, and the factors that influence its intensification over time, as reported in literature. These investigations have been carried out through simulations done using the commercial ANSYS Fluent code to validate the findings from the CFD results. The simulations are carried out using the volume of fluid (VOF) method, which obtains the volume fraction of each of the fluid throughout the domain and thereby captures the gas-liquid interface motion. Thereafter, the air-core vortex height predictions are validated with results reported in the literature.