Gravity set in incompressible single phase simulation

Does anyone know how to set gravity in incompressible simulation? I learnt that it’s doable in multiphase simulation, but my case is single phase. And it feels very weird if gravity cannot be considered in single phase situation. Has gravity been considered by default along -z axis?


Hi, it’s actually not necessary to define gravity for single phase, incompressible simulations.

For single phase simulations, gravity only plays a role when we have buoyancy effects. Since for incompressible simulations we have the same temperature everywhere (and therefore, the same density), then gravity will make no impact in the results.

This thread contains more insights, in case you are interested in reading more details.


Hi Ricardo, thanks very much for your explanation.

The case I’m interested for example is a water flow from one side to another side through a tank. Such as the below picture shows. Shouldn’t the flow direction encounter different gravity effect? I’m thinking that it’s pure water (closed system) and same temperature, then it’s single phase. And the flow rate is pretty small, thus incompressible.


I think I might get the point… Only when tank is empty the gravity works since there’s density difference between air and water, but if tank has been filled with water, gravity force from injecting water molecular transfers to neighboring water molecular and finally goes to the wall, thus gravity should not affect the flow pattern…

Yup, exactly - this is the logic.

Of course if we have multiphase, then it’s a different topic (and it requires gravity, since the phases typically have different densities).

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