Comment 7 for bug 1734653

Q1 - I think it is possible to define a relevant geometry for multiple cracks; it is just the intersection of one tetrahedron with multiple polygonal faces. I think the problem of multiple fracture planes crossing one edge is very special, I would escape it on the basis that you can't break another time something that is already broken, so 2 planes = 1 crack locally if the two planes intersect.

> Q2. [...] area should be "weighted" by the number of fractured edges

As explained before it should be enough to accumulate the conductivities from different cracks. In pseudo code:
for e in crackedEdges:
    for facet incident to e:
        facet->conductivity += trickPermeability(edge) #currently the '+=' is a '='
The triangular surface you mention also appeared in Robert's proposal. It makes sense to use it in trickPermeability().
Problems of your equation are that 1/ it assumes the four points are in the same plane, not the case in practice, they define two triangles in two different planes and 2/ it cannot be used for 3 broken edges.
If 2 triangles are coplanar the above loops will produce your equation for 2 broken edges (it implies that the broken edges are parallel, which can only happen at a boundary of the problem).

Q3: I would say yes, but I don't really see the problem.
Except that V(tetrahedron)-V(spheres) doesn't make much sense when the spheres are just computational nodes.

Q4. How do we define matrixPorosity?
I would leave that to the user.

Q5. No. Why would we do that? The total volume of a cell is changing and it must be balanced by some fluid fluxes regardless of the cracks.

Bruno