Filling and Emptying of a Water Tank Using the VOF Multiphase Model

A siphon can empty a tank with no pump and no power, but only once it has primed, and that depends on how the water level, the tube and the air all interact over time. It is a moving free-surface problem, and exactly the kind of behaviour that is hard to judge without seeing it.

In this project we modelled the filling and emptying of a water tank as a transient multiphase problem in Ansys Fluent, using the volume of fluid (VOF) model for air and water. The tank starts with some water in it, is fed for the first second of the run, and is then left alone. As the level rises over the crest of a U-tube the siphon starts, and from then on it keeps drawing water out on its own, with the supply already switched off.

Each boundary was named on the model (inlet, outlet, ambient, wall and side wall) so it could be given the right condition. Getting these named selections right is what lets the water enter, leave and interact with the tank correctly once the solver runs, and lets air flow in and out of the openings as the water moves.

With the supply on, water comes down the inlet channel and the level in the tank starts to climb, with waves and a splash where the incoming water lands. The U-tube is still full of air: nothing leaves the tank until the level reaches the crest.

This stage sets up everything after it, because the tank has to fill far enough for the water to reach the top of the U-tube.

Once the water climbs over the crest, it runs down the far side of the U-tube and pulls the water behind it: the siphon has primed. The tube fills completely and water pours into the lower box, while air bubbles stirred up by the filling still rise through the tank.

The animation shows the whole run, from the starting level and the inflow down the inlet channel to the siphon priming and the level in the tank falling as it drains:

Once primed, a siphon keeps running on its own, because the water in the long outer leg keeps pulling water over the crest. It stops only when the level in the tank falls to the tube's intake and air gets in, breaking the siphon.

The simulation captures the full cycle of a self-draining tank in one transient run: a short fill, the water reaching the U-tube, the siphon starting and the tank draining on its own. The VOF model follows the air and the water the whole way through, which is what makes the level and the siphon easy to read. It is a compact example of how multiphase CFD predicts free-surface behaviour that would be hard to judge any other way. We used the same VOF approach to capture the air core in a cyclone separator.