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[Bruno Chareyre <1734653@xxxxxxxxxxxxxxxxxx>]

> we should use directly crackAperture*crackArea as Vo in dV/Vo, no?

That's more or less what I wanted to suggest. I can't explain why I wrote that strange formula.
However I also wanted to skip the calculation of crackAperture*crackArea, namely because there can be multiple cracks, as you point out.
So my idea was to store the initial value of the numerical porosity in one cell (call it V(t=0)) and to define the cracked volume as the difference V(t)-V(t=0). It makes multiple cracks less a problem I think.

However, an obvious problem of this is that it result in a reference
equal to zero when the crack occurs, and we need to divide by this
volume... that was the reason to introduce an initial amount of liquid
via matrixPorosity (could also depend on slotInitialAperture). Just an
idea anyway. The problem of fluid flow inside an advancing crack tip
seems fundamentally tough... I can't imagine a perfect answer yet.

Bruno

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https://bugs.launchpad.net/bugs/1734653

Title:
  DFN+fluid compressibility not using the correct reference volume

Status in Yade:
  New

Bug description:
  In the basic PFV scheme the incremental change of density of a pore
  fluid after a change of pore volume is dependent on dV/Vo where Vo is
  the reference pore space within a tetrahedral cell [1], i.e.
  V(tetrahedron)-V(spheres).

  In DFNFlow "V" should reflect the fact that porosity may be mainly due to a crack an its opening.
  Typically V=V(tetrahedron)-opening*area-matrixPorosity.
  Currently it is using the same formula, hence overestimating the compressibility effect (because the DEM porosity is larger than a typical rock porosity). What should be the reference "opening" in above formula is to be clarified though, it has physical as well as numerical implications. Maybe slotInitialAperture is a candidate? Let you DFN people tell what it should be.

  Note that the "dV" is less a problem because it is a difference
  (independent on the choice of the reference volume), so the
  incompressible scheme is not affected.

  Bruno

  [1]
  https://github.com/yade/trunk/blob/master/pkg/pfv/FlowEngine.ipp.in#L439

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