– Too many particles are needed to fill the entire volume of flow objects. So that 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.