Porosity under deformation
In material/fluidmatrixinteractions/porositydeformation.hh
, we calculate the effective porosity as
\phi_\text{eff} = \phi_0 ( 1 + \text{div}\,u ).
In the last release's geomechanics/el2p/elementvolumevariables.hh
, we use
\phi_\text{eff} = \frac{\phi_0 + \text{div}\,u}{1 + \text{div}\,u},
justified by the following comment:
// this equation would be correct if the bulk volume could change (Vol_new = Vol_init *(1+div u)),
// however, we have a constant bulk volume therefore we should apply phi_eff = phi_init + div u
// but this causes convergence problems. Since div u is very small here the chosen relation is
// assumed to be a good approximation
The comment suggests that the truth should be
\phi_\text{eff} = \phi_0 + \text{div}\,u.
@martinb, @DennisGlaeser, @holle: What's your opinion? What should we implement? It actually makes a rather big difference: In the setting discussed in #546 (closed), I get the following effective porosities:
Red corresponds to the first formula (master), black to the second (release) and green to the third (suggested by the comment).