Numerical study on air-core vortex: analysis of generation mechanism

An air-core vortex can be formed wherever water drains or discharges from a container. In some cases, detached bubbles or a complete air core can develop and subsequently enter the downstream equipment causing performance loss or even damage. In the present study, a numerical study and a physical discussion are done on the water draining out of a tank in order to determine why air-core vortex generates and on which factors it depends. The same condition as in the existing experiment was simulated numerically and validated by comparing against it. Subsequently, the numerical simulations are developed to alter physical and geometrical factors, i.e., wall rotation and surface tension both for laminar and turbulent flow regimes. It is realized that the generation of the air core depends on the action of drain and not to the environmental elements such as the effect of the surrounding walls. In fact, these elements can produce or destruct the angular momentum which plays a vital role in process of air-core vortex generation. Meanwhile, the fluid particle moves toward the drain port, and its angular velocity increases that it makes a narrow zone in the water having larger axial velocity and angular velocity with a lower pressure compared to the surrounding water. This narrow zone is the key factor, and when it develops, the free surface deforms and the air core generates.