Fluid Modifier
The Fluid modifier is a container for a Fluid Physics simulation. It can be useful for example, to simulate on a low-poly mesh then add a Subdivision Surface Modifier after the Fluid modifier to improve the visual quality of the fluid without drastically increasing simulation times.
Options
As the modifier is only a container its actual options can be configured in the Physics Properties tab. See the Fluid Physics Properties for more information.
Example
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Example of a liquid simulation.