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[vangenuchten] Introduce pore-connectivity parameter L

Timo Koch requested to merge feature/introduce-vangenuchtem-mualem-param-l into master

What this MR does / why does DuMux need it: Introduces the pore-connectivity parameter l. The parameter is used in the relative permeability curve in the Mualem model. Mualem (1976) suggested that 0.5 is a good value for many soils. Sometimes a value different from 0.5 is provided, which is why this patch implements it as additional variable.

Special notes for your reviewer:

  • There was something weird in krn. The function uses the factor (1-S_{we})^{1/3} but according to the van Genuchten Mualem model this should be (1-S_{we})^{1/2} or (1-S_{we})^l. See Mualem 1976 ( and e.g. Dury et all (1999) ( @holle The 1/3 appears e.g. in your habilitation but no other sources than Mualem and van Genuchten are given. Can you comment on where it might come from?

  • I believe the derivative \frac{\partial k_{rn}}{\partial S_{we}} was wrong. Please recheck the new result before merging.

These two points change the van genuchten material law (not backwards compatible) and could also result in differences in the reference solutions. If the changes get accepted we should

  • Add note in changelog about the changes in vangenuchten
  • Check tests and possibly update references
Edited by Timo Koch

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