[Paraview] integration of a variable

[Berend van Wachem]

Hi,

I would like to integrate a variable over a surface; is this possible in 
  paraview, and so, how do I do this?

An example of this is, when I have the variables velocity and density 
defined, and I want to calculate the massflow going through a surface. I 
need to calculate the cell face velocity times the cell face density 
times the cell face area times the cell face normal and then sum these 
values for all cell faces lying on a surface. Can this be done in Paraview?

Thanks,

Berend van Wachem.