Narrow Band Width

– Too many particles are needed to fill the entire volume of flow objects. So the big computational times are required.
– The majority of the FLIP particles filling the fluid objects don’t contribute to the fluid’s appearance (especially for larger volumes).

– Narrow Band Width was introduced to reduce computational times as lets the solver assign the Flip particles only to the voxels in a fixed band (inside the liquid surface).
– Of course Narrow Band makes more sense for big volumes of fluids, as small volumes are always filled by particles so there is no advantage in using it.
– Narrow Band Width=3 seems to be the lower value you can use to avoid volume lost.

– In this example I rendered a viewport section so you can see the narrow band of the cube set as liquid. The bigger the Narrow Band Width value, the thicker the band.

Blender Domain Liquid Narrow Band Width