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The value given here is directly connected to the project's scene scale settings. With RealFlow's standard scale of 1.0, one square of the viewport's grid measures exactly 1.0 m x 1.0 m. With the parameter's default value of 0.3, the smallest volume element of the domain is 0.3 m x 0.3 x 0.3 m. A value of 0.01 represents a voxel size of 0.01 m x 0.01 m x 0.01 m – that is just 1 cm per side. With such a tiny voxel size, the number of particles can easily reach several millions – depending on the size of the emitter. "Cell size" is measured in metres [ m ].
When you work with “Cell size” and objects you also have to take the object's “Volume” panel into consideration. There you will find another “Cell size” value, but this time for the rasterization of the colliding/interacting object.
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