How can I measure a flux through a fracture?

Hello everyone;

 I'm trying to measure the permeability of a fracture contained inside a cubic rock sample and, for that, I need to relate the flux inside the fracture to the pressure difference along its extension.

I do the following steps along my code as provided in [1]

1 - Set up a cubic rock matrix cut by a fracture plane along the Y direction
2 - I "force" the flow only to happen in the fracture cells, i.e, the other cells are assumed impermeable
3 - A pressure difference is applied at the bottom of the sample (on the ymin boundary) willing to generate flow inside the fracture only (again, all other walls are impermeables).
4 - Measure the flux at ymin and ymax, Qin and Qout respectively, with the getBoundaryFlux() function.

The problem is that I can't use getBoundaryFlux() function because the flow isn't occurring along all thw facet, but only in the fracture cells. So here I got some questions:

1. How can I measure the flux through a fracture? Is there a more suitable function or any restriction to be applied to the getBoundaryFlux() function?

2. How can I measure the velocity of the flow in a particular point of this fracture?

3. Is there any relation between flow velocity and the flux through the fracture?

Thanks in advance!

[1] https://answers.launchpad.net/yade/+question/691351/+index