# GfsBubbleFraction

### From Gerris

**GfsBubbleFraction** defines the concentration of GfsBubbles
as a projection of the bubble volume onto the grid using a user-defined kernel function.

The syntax is

[ GfsParticulateField ] { kernel = [ GfsFunction ] rkernel = RAD }

where

- kernel
- is a GfsFunction that defines the kernel used to project the bubble volume onto the grid,
- rkernel
- is the nondimensional radius of the region of kernel projection around the bubble.

Both variables are defined in nondimensional units, where the characteristic length is the bubble radius. That is, x represents x/R.

The kernel function is internally multiplied by a constant to satisfy that the discrete integral of the kernel over the volume of projection is 1.