# Unsaturated efficient diffusion coefficient in OGS-5

Hi all,

does anyone have some further insights into how OGS5 computats the effective Diffusion coefficient? In the keywords it says

 Fick’s law in the porous media (effective diffusion coefficient):

$D^* = \omega \cdot D_d$

where omega is a coefficient that is related to the tortuosity (Empirical coefficient, lab: 0.5~0.01)


Is omega simply the tortuosity given in $TORTUOSITY ? And is there a possibility to account for unsaturated cases, i.e. to make tortuosity a function of water content as e.g. in the Millington-Quirk relation: tortuosity = (water content)^n / (porosity)^m or is saturation accounted for once the mass dispersion tensor is computed? (It seems so from the code in rf_mmp_new.cpp under MassDispersionTensorNew, but saturation is only a linear factor there… ) Cheers, Erik Hi Erik, yes the effective diffusion coefficient (D_e = omega \cdot D_d) is computed using the value for tortuosity (omega) given by$TORTUOSITY.

Unfortunately, I do not know about a possibility to account for water saturation to compute the diffusion coefficient.

Best,

Thanks for the clarification, Johannes!